HomeMy WebLinkAboutStormwater Management Manual - January 2026i
JANUARY 2026
LAKE OSWEGO
STORMWATER MANAGEMENT MANUAL
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TABLE OF CONTENTS
1 Introduction ....................................................................................................................................................... 1
1.1 Purpose .......................................................................................................................................................2
1.2 SWMM History ...........................................................................................................................................2
1.3 Climate Change ...........................................................................................................................................2
1.4 Stormwater Effects .....................................................................................................................................3
Water Quantity Effects .......................................................................................................................5
Hydromodification Effects ..................................................................................................................5
Water Quality Effects .........................................................................................................................6
1.5 Impaired Rivers and Streams ......................................................................................................................6
1.6 Document Hierarchy...................................................................................................................................9
2 Development Process ...................................................................................................................................... 10
2.1 Plan Review ............................................................................................................................................. 10
2.2 Construction Inspections ......................................................................................................................... 11
2.3 Post-Construction .................................................................................................................................... 11
2.4 Stormwater Facility Alterations ............................................................................................................... 16
Restorations and Renovations ......................................................................................................... 16
Replacements .................................................................................................................................. 16
Augmentation .................................................................................................................................. 16
3 Stormwater Thresholds .................................................................................................................................... 18
3.1 Project Class............................................................................................................................................. 18
Impervious Area Calculation............................................................................................................ 18
Small Projects .................................................................................................................................. 19
Large Projects .................................................................................................................................. 19
Dredge and Fill Projects ................................................................................................................... 19
3.2 Project Exemptions and Variances .......................................................................................................... 20
Exemptions ...................................................................................................................................... 20
Variances ......................................................................................................................................... 21
4 Submittals ......................................................................................................................................................... 22
4.1 Pre-Construction Submittals ................................................................................................................... 22
Plan Submittals ................................................................................................................................ 22
Report Submittals ............................................................................................................................ 26
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4.2 Stormwater Plan Revisions ...................................................................................................................... 32
4.3 Post-Construction Submittals .................................................................................................................. 32
Stormwater Facility Certification ..................................................................................................... 32
DEQ Approval of Underground Injection Control Systems ............................................................. 33
Recorded Operations and Maintenance Plan ................................................................................. 33
Pipe System Video ........................................................................................................................... 33
As-Built Drawings ............................................................................................................................. 34
Miscellaneous .................................................................................................................................. 34
5 Source Control .................................................................................................................................................. 35
Outside Solid Material or Waste Storage Areas .............................................................................. 35
Liquid Material or Waste Storage Areas .......................................................................................... 35
Covers .............................................................................................................................................. 35
6 Site Assessment ................................................................................................................................................ 36
6.1 Existing Drainage Patterns ....................................................................................................................... 36
6.2 Geotechnical Conditions .......................................................................................................................... 36
Soil Classification ............................................................................................................................. 36
Steep Slopes .................................................................................................................................... 39
Hydraulically Restrictive Layers ....................................................................................................... 39
Soil Fill .............................................................................................................................................. 39
Contaminated Soil ........................................................................................................................... 39
6.3 Hydrology ................................................................................................................................................ 39
Floodplains....................................................................................................................................... 41
Groundwater ................................................................................................................................... 41
Infiltration Tests............................................................................................................................... 42
Groundwater Mounding Analysis .................................................................................................... 44
Post-Construction Infiltration Testing ............................................................................................. 44
6.4 Setbacks ................................................................................................................................................... 44
Onsite Wastewater Systems ............................................................................................................ 44
Constraints ....................................................................................................................................... 44
7 Stormwater Facility Selection ........................................................................................................................... 46
7.1 Project Type ............................................................................................................................................. 46
7.2 Facility Hierarchy ..................................................................................................................................... 47
7.3 Onsite Retention ...................................................................................................................................... 48
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7.4 Extended Filtration .................................................................................................................................. 48
7.5 Water-Quality Limited Waterways .......................................................................................................... 48
Suspended Material ......................................................................................................................... 48
Phosphorous .................................................................................................................................... 49
Copper and Zinc ............................................................................................................................... 49
7.6 Capital Improvements and Public Improvements ................................................................................... 49
7.7 Summary .................................................................................................................................................. 50
8 Stormwater Modeling ...................................................................................................................................... 52
8.1 Approved Models .................................................................................................................................... 52
Single-Event Models Santa Barbara Urban Hydrograph ................................................................. 52
Continuous Simulation Models ....................................................................................................... 53
Prescriptive Sizing ............................................................................................................................ 53
8.2 Model Parameters ................................................................................................................................... 54
Curve Numbers ................................................................................................................................ 54
Time of Concentration ..................................................................................................................... 55
Infiltration rates ............................................................................................................................... 55
Orifices ............................................................................................................................................. 55
8.3 Flow Control ............................................................................................................................................ 55
Exemptions ...................................................................................................................................... 55
Flow Control Procedure ................................................................................................................... 56
8.4 Downstream Capacity Analysis................................................................................................................ 58
9 Stormwater Facility Design ............................................................................................................................... 61
9.1 General Design Standards ....................................................................................................................... 61
Vegetated Facilities ......................................................................................................................... 61
Underground Injection Controls ...................................................................................................... 65
Regional Facilities and Shared Facilities .......................................................................................... 66
Proprietary Stormwater Treatment ................................................................................................ 68
9.2 Specific Design Standards ........................................................................................................................ 68
Drywells (Standard Detail SD1-02) .................................................................................................. 69
Porous Pavement (Standard Detail SD9-09) .................................................................................... 70
Permeable Pavers (Standard Detail SD9-10) ................................................................................... 71
Planters (Standard Details SD9-12 and SD9-13) .............................................................................. 73
Ponds ............................................................................................................................................... 75
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Raingardens (Standard Details SD9-15 and SD9-16) ....................................................................... 76
Sand Filter ........................................................................................................................................ 77
Swale (Standard Details SD9-19 and SD9-20) .................................................................................. 79
Trenches and Galleries (Standard Detail SD9-06) ........................................................................... 80
Vegetated Filter Strips (Standard Detail SD9-05) ............................................................................ 81
Water-Quality Vaults ....................................................................................................................... 83
Wetlands, Constructed .................................................................................................................... 83
10 Conveyance Design ........................................................................................................................................... 85
10.1 Detention ................................................................................................................................................. 85
10.2 Conveyance Structures ............................................................................................................................ 85
Alignment and Location ................................................................................................................... 85
Private and Public Stormwater System Connections ...................................................................... 86
Stormline and Manholes ................................................................................................................. 86
Catch Basins and Inlets .................................................................................................................... 87
Ditches ............................................................................................................................................. 87
Culverts and Bridges ........................................................................................................................ 87
10.3 Outfalls and Offsite Stormwater Discharges ........................................................................................... 88
10.4 Public Conveyance Extension .................................................................................................................. 89
10.5 Easements ............................................................................................................................................... 89
10.6 Encroachments ........................................................................................................................................ 90
11 Maintenance of Stormwater Facilities ............................................................................................................. 91
11.1 Operations and Maintenance Plans ....................................................................................................... 91
11.2 Facility Access .......................................................................................................................................... 91
11.3 Inspections ............................................................................................................................................... 91
Inspection Records .......................................................................................................................... 92
City Inspections................................................................................................................................ 92
Property Sale and Facility Ownership Transfer ............................................................................... 92
11.4 Maintenance ............................................................................................................................................ 92
Catch Basins ..................................................................................................................................... 92
Detention Tanks and Vaults ............................................................................................................. 93
Drywells, Infiltration Trenches, and Infiltration Galleries ............................................................... 93
Porous Pavements and Permeable Pavers ...................................................................................... 93
Planters, Raingardens, and Swales .................................................................................................. 93
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Ponds and Wetlands ........................................................................................................................ 94
Sand Filters ...................................................................................................................................... 94
Vegetated Filter Strips ..................................................................................................................... 95
Wetlands .......................................................................................................................................... 95
12 Erosion and Sediment Control .......................................................................................................................... 96
12.1 Regulatory Overview ............................................................................................................................... 96
Municipal Code ................................................................................................................................ 96
MS4 Permit ...................................................................................................................................... 96
TMDL Implementation Plan ............................................................................................................. 96
12.2 Erosion and Sediment Movement ........................................................................................................... 97
Soil Type ........................................................................................................................................... 97
Slope Gradient ................................................................................................................................. 97
Slope Length .................................................................................................................................... 97
Project Size and Timing.................................................................................................................... 98
12.3 ESC Permits .............................................................................................................................................. 98
ESC Plans .......................................................................................................................................... 98
Certified Professionals ..................................................................................................................... 98
12.4 Erosion and Sediment Control Selection ................................................................................................. 99
Required Erosion and Sediment Controls ....................................................................................... 99
Construction Access and Parking ..................................................................................................... 99
Sediment Control ........................................................................................................................... 100
Run-on Control .............................................................................................................................. 102
Dust Control ................................................................................................................................... 102
Stabilize Slopes and Disturbed Areas ............................................................................................ 102
Stockpile Areas .............................................................................................................................. 103
Source Control ............................................................................................................................... 103
Wet Weather Requirements ......................................................................................................... 104
12.5 Pre-Construction.................................................................................................................................... 104
Schedules and Phasing .................................................................................................................. 104
Weather Forecasts ......................................................................................................................... 105
Training .......................................................................................................................................... 105
12.6 Construction .......................................................................................................................................... 105
Inspections and Inspection Logs .................................................................................................... 106
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Significant Discharges .................................................................................................................... 106
Modifications to Approved ESC Plans ........................................................................................... 107
Enforcement .................................................................................................................................. 107
Post Construction .......................................................................................................................... 107
13 Definitions ...................................................................................................................................................... 108
14 References ...................................................................................................................................................... 122
Appendix A Stormwater Report Template ............................................................................................................. 124
Stormwater Report ............................................................................................................................................ 125
Basic Stormwater Report ................................................................................................................................... 128
Appendix B Infiltration Test Log and Report Template .......................................................................................... 131
Infiltration Test Log ............................................................................................................................................ 132
Infiltration Report .............................................................................................................................................. 133
Appendix C Plant Selection and Approved Plant List ............................................................................................. 134
Appendix D Operations & Maintenance Template ................................................................................................ 154
Appendix E Simple Erosion and Sediment Control Template ................................................................................ 155
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LIST OF TABLES
Table 1. Impaired Streams in Lake Oswego .............................................................................................................. 7
Table 2. City Watersheds affected by TMDLs ............................................................................................................ 8
Table 3. Project Requirements Based on Classification .......................................................................................... 19
Table 4. Performance Standards ............................................................................................................................. 46
Table 5. Stormwater Facility Hierarchy ................................................................................................................... 47
Table 6. Approved Stormwater Facility Types and Applicability ............................................................................. 51
Table 7. Design Storms and Precipitation Depths ................................................................................................... 52
Table 8. Prescriptive Sizing Factors .......................................................................................................................... 53
Table 9. Design Storms for Downstream Analyses .................................................................................................. 60
Table 10. Sand Media Specifications ....................................................................................................................... 78
Table C-1. Invasive and Noxious Weeds in Oregon ............................................................................................... 136
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LIST OF FIGURES
Figure 1. Lake Oswego Watersheds .............................................................................................................. 4
Figure 2. Summary of City Review and Permitting Process......................................................................... 10
Figure 3. Land-Use Process .......................................................................... ...............................................12
Figure 4. Construction Process .................................................................................................................... 13
Figure 5. Public Improvement Process ........................................................................................................ 14
Figure 6. Erosion and Sediment Control Process ........................................................................................ 15
Figure 7. Site Assessment Process ............................................................................................................... 37
Figure 8. NRCS Soil Classification in the City of Lake Oswego ..................................................................... 38
Figure 9. Wetlands and Areas of Historic Landslides .................................................................................. 40
Figure 10. Setbacks from Foundations and Property Lines ......................................................................... 45
Figure 11. Stormwater Design ..................................................................................................................... 57
Figure 12. Downstream Capacity Analysis ................................................................................................59
Figure 13. Planting Requirements for Vegetated Facilities ......................................................................... 62
Figure 14. Soil Classification ........................................................................................................................ 63
Figure 15. Vegetated Filter Strip.................................................................................................................. 82
Figure C-1. Considerations for Plant Selection in a Stormwater Facility ................................................... 138
Figure E-1. Example Simple Erosion and Sediment Control Plan .............................................................. 156
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ACRONYMS
AASHTO American Association of State Highway and Transportation Officials
ADA Americans with Disabilities Act
APWA American Public Works Association
ASTM American Society for Testing and Materials
CFR Code of Federal Regulations
cfs cubic feet per second
CIP Capital Improvement Plan
CN Curve Number
CWA Clean Water Act
DEQ Oregon Department of Environmental Quality
DMA Designated Management Agency
DO Dissolved Oxygen
DOE Washington Department of Ecology
DOGAMI Oregon Department of Geology and Mineral Industries
DSL Oregon Department of State Lands
EPA United States Environmental Protection Agency
ESC Erosion and Sediment Control
FEMA Federal Emergency Management Agency
fps feet per second
GIS Geographic Information System
GULD General Use Level Designation
HDPE High-Density Polyethylene
HSPF Hydrologic Simulation Program-Fortran
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LO Lake Oswego
LOC Lake Oswego Code
LU Land-Use
MODRET Model for Retention Pond Design
MS4 Municipal Separate Storm Sewer System
NGVD National Geodetic Vertical Datum of 1929 (current datum used by City)
NPDES National Pollutant Discharge Elimination System
NRCS Natural Resources Conservation Service
O&M Operation and Maintenance
OAR Oregon Administrative Rule
ODFW Oregon Department of Fish and Wildlife
ODOT Oregon Department of Transportation
OISC Oregon Invasive Species Council
ORS Oregon Revised Statutes
OSBEELS Oregon State Board of Examiners for Engineering & Land Surveying
PE Professional Engineer
PFAs Per- and Polyfluoroalkyl Substances
PICP Permeable Interlocking Concrete Pavement
PIT Pilot Infiltration Test
psi pounds per square inch
PVC Polyvinyl Chloride
Q Discharge, usually in cfs
ROW Right-of-Way
SBUH Santa Barbara Urban Hydrograph
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SFR Single Family Residential
SWMM Stormwater Management Manual
TAPE Technology Assessment Protocol –Ecology
TMDL Total Maximum Daily Load
TSS Total Suspended Solids
UIC Underground Injection Control
UST Underground Storage Tank
VSS Volatile Suspended Solids
WQ Water-Quality
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1 INTRODUCTION
The Clean Water Act (CWA) of 1972 prohibits the discharge of pollutants into waters of the US and was
enacted through amendments of the Federal Pollution Control Act (1948). It established the
requirement for water quality standards and created the basic structure of the National Pollutant
Discharge Elimination System (NPDES) program. Revisions in 1987 created the framework for regulating
stormwater discharges including those from Municipal Separate Storm Sewer Systems (MS4s).
The City of Lake Oswego (City) received its first NPDES MS4 Phase I permit (hereafter “MS4 permit”)
from the Oregon Department of Environmental Quality (DEQ) in 1995 as a co-permittee of the
Clackamas County permit (#101348) which currently includes the jurisdictions of Clackamas County,
Gladstone, Happy Valley, Johnson City, Lake Oswego, Milwaukie, Oak Lodge Sanitary District, Oregon
City, Rivergrove, West Linn, and Wilsonville.
The Clackamas County MS4 permit was re-issued by DEQ in October 2021. While there are many aspects
and programs associated with the MS4 permit, this Manual fulfills the following requirements:
1. Prioritize green infrastructure and low impact development.
2. Require a site performance standard for stormwater management.
3. Require long-term maintenance of stormwater facilities.
4. Require an erosion and sediment control (ESC) program that minimizes construction site
discharges.
The requirements of the MS4 permit are implemented primarily through the Lake Oswego Municipal
Code (LOC) Chapters 38 (Utility Code - Stormwater Management) and 52 (Erosion Control). The City’s
municipal code is implemented through the stormwater requirements of the City’s Stormwater
Management Manual (SWMM), the Erosion Control Manual, the Engineering Design Standards, and the
Standard Detail Drawings.
Oswego Lake
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1.1 PURPOSE
The City’s SWMM contains guidance and standards to implement the City’s stormwater program and
mitigate the impacts of stormwater from the creation and redevelopment of impervious areas. It is a
dynamic document and updated periodically to reflect the current state of stormwater practices and
treatment technologies and to address new water-quality requirements from DEQ. It is intended for use
by developers, stormwater professionals, contractors, and others responsible for managing stormwater
created by impervious surfaces in the city.
1.2 SWMM HISTORY
The City’s SWMM was created in 2016 from the 2003 Surface Water Design Manual. The 2016 SWMM
was subsequently updated in 2020 primarily to address requirements of the City’s relatively new Class V
Underground Injection Control (UIC) permit. This document, an update of the 2020 SWMM, reflects the
requirements of the MS4 permit that was re-issued in 2021 and revised Total Maximum Daily Load
(TMDL) allocations set by DEQ in 2019 and 2025. A list of the major revisions from the 2020 SWMM are
listed prior to the Table of Contents of this SWMM.
1.3 CLIMATE CHANGE
Climate change models predict hydrological changes caused by increases in global temperatures.
Previous climate models have accurately predicted future climate patterns but did not foresee that the
region would experience those changes so rapidly.
The city is currently experiencing more intense storms and prolonged dry periods predicted for the
future by climate models. Storms that used to provide 0.5 inches of rain in two to three days now build
quickly and discharge the same amount of precipitation in less than 24 hours. As these types of storms
occur more frequently, the City expects its stormwater system to become routinely overwhelmed by the
stormwater volume created by the storms.
Paradoxically, the region’s normally dry summers are becoming drier and causing it to experience
regional drought (Bumbaco, et. al, 2024) and higher summer temperatures. October 2022 was the 27th
driest and 3rd warmest since 1895. Snowpack and stream flows in the region are also affected by climate
change. The region’s snowpack during the winter of 2022-2023 was the 4th highest since 1991 due to
several atmospheric rivers. However, a heat wave in May 2023 (the 3rd warmest May since 1895)
resulted in the rapid melting of the snowpack and subsequent low flows in watersheds that are
dependent on snowpack recharge during the summer. While the region was classified as abnormally dry
in the fall of 2022, it was designated as being in a moderate to severe drought by the fall of 2023 due to
the loss of snowpack in May 2023.
These swings in temperature and precipitation can have profound impacts on the aquatic habitat in our
rivers and streams as they experience high flows in the winter, elevated summer temperatures, and
reduced summer stream flows. Infiltrating stormwater in the winter will reduce flooding, erosion, and
hydromodification while augmenting stream flows in the summer with cooler groundwater.
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1.4 STORMWATER EFFECTS
Stormwater is the precipitation (rain and snow) that cannot
infiltrate into the soil. It is collected through a network of
catch basins and inlets before being conveyed by pipes,
culverts, ditches, and streams to the Willamette River,
Tualatin River, and Oswego Lake (See Figure 1). While the
entire system is known as the public surface water
management system, the public stormwater system is the
network of public catch basins, inlets, pipes, culverts, ditches,
and stormwater facilities used to treat and convey
stormwater to receiving waters.
The City collects a stormwater development charge from
development projects connecting to the public stormwater
system. A surface water fee is collected monthly from
property owners as part of the City’s utility bill. These fees
provide funds to:
• Manage and protect natural resources
• Repair, replace, and maintain the public stormwater system
• Comply with DEQ’s requirements for the protection of
water quality as stipulated in the MS4 permit, UIC permit,
and TMDL Implementation Plan.
Expected Climate Change Patterns
•An increase in flooding and overall rainfall intensity including a greater frequency in storm
intensity previously only seen every 25 years.
•Increased rainfall intensity and volume will cause increased hydromodification in streams,
increased stress on the public stormwater system, and increased property loss due to erosion.
•No substantial change in annual rainfall is expected, however storm intensity is expected to
increase, e.g. more atmospheric rivers causing destructive streamflows and increased
flooding.
•The Pacific Northwest is expected to experience hotter and drier summers and warmer
winters. As more precipitation falls as rain in the winter and as the snowpack disappears
before summer because of the warmer temperatures, the availability of potable drinking
water and streamflows to support aquatic species will decrease.
•As streamflows decrease, river levels are expected to decrease which may affect the ability of
hydroelectric plants to provide enough electricity for expected power grid needs in the hotter
summers.
(Adapted from Fleischman, 2023 and Dalton et. al. 2013):
Development can affect streams by:
• Decreasing infiltration of
precipitation.
• Increasing stormwater volumes.
• Decreasing shallow groundwater
(interflow) mixing with streams
during the summer.
• Destabilizing stream
temperatures.
• Decreasing reaction time of
streams to the onset of
precipitation.
• Increasing duration of peak
flows in a stream from a storm.
• Increasing hydromodification
resulting in decreased stream
depths and increased erosion.
• Increased property loss due to
erosion.
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Figure 1. Lake Oswego Watersheds
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Water Quantity Effects
Without treatment or detention, impervious surfaces increase stormwater volumes and decrease the
time between the start of a storm event and the increase of flow in a stream. This can result in flooding
and erosion throughout the city.
Some areas of Lake Oswego have either no public stormwater system or an undersized system. During
periods of heavy rain, local flooding and erosion may occur as the existing drainage pattern or
stormwater system is overwhelmed by stormwater created by increased impervious areas.
Utility conflicts, lack of right-of-way (ROW), and the cost of obtaining property quickly depletes the
stormwater funds available for capital projects. Requiring development to manage stormwater onsite
reduces local flooding, erosion, and property loss during periods of heavy rain. It also increases the
amount of stormwater funds available for capital projects.
Hydromodification Effects
Streams and other waterways are affected by the timing,
velocity, and volume of stormwater. As impervious areas in
a watershed increase, the time between the onset of a
storm and its effect on streams decreases as infiltration is
reduced and a greater percentage of the rain is converted
immediately to stormwater. Hydromodification occurs as
streams adapt to larger peak flows and velocities. Streams
become steeper, wider, and deeper as they experience
erosion from higher stormwater volumes and velocities.
While sandy streambeds experience erosion with small
changes in stream velocity, streambeds composed of clay
soils can also experience erosion with longer durations of
peak streamflows.
Soil from streambeds and streambanks is mobilized and re-deposited downstream as stream velocities
decrease. Sediment deposits result in shallow stream depths and cause elevated stream temperatures
Effects of Hydromodification
•Increased sediment in streams results in shallow stream depths.
•Shallow stream depths from sedimentation result in higher stream temperatures, decreased
stream oxygen, and adverse effects on aquatic life and habitat.
•Changes in stream channel direction or geometry results in additional flooding and erosion as
a stream adapts to new conditions.
•Properties adjacent to streams experience property and tree loss due to erosion.
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(Lane 1955; Leopold and Maddock 1953). The erosion and resulting sedimentation downstream
adversely affect aquatic life and habitat as vegetated streambanks and riparian trees are undercut by
the increase in streamflow and velocities. Requiring projects to infiltrate and matching post-
development flowrates to pre-development flowrates reduces hydromodification.
Water Quality Effects
As streams experience elevated flows for a longer duration, their streambanks erode to accommodate
the increased flow resulting in wider stream channels and decreased stream depths during dry periods.
Stream temperatures increase with decreased stream depth.
Groundwater can improve stream temperatures through cool inflows (50 to 60° F) to streams in the
summer. However, the ability of shallow groundwater to stabilize summer stream temperatures is
decreased as infiltration is reduced by impervious areas.
Stream temperatures higher than 65°F have a deadly
effect on aquatic life and habitat, especially salmon.
Prioritizing infiltration over detention reduces stormwater
volumes to streams in the winter, increases summer
groundwater inflows to streams, and stabilizes stream
temperatures.
Stormwater mixes with many of the materials it
encounters and carries it to the nearest waterway. Dirt,
oil, feces, litter, and microscopic metal particles from tires
and brakes mix with stormwater and can cause a
decreased water-quality when it enters a stream or other
waterway.
Stormwater that enters a traditional catch basin (no sump or water-quality snout) is not treated and is
discharged directly to streams. Stormwater from projects creating or redeveloping 1,000-sq ft or more
of impervious area must be treated by private stormwater facilities before being discharged to the City’s
surface water management system.
1.5 IMPAIRED RIVERS AND STREAMS
Sections 303(d) and 305(b) of the CWA requires DEQ to determine if a waterway contains pollutants at
levels that exceed water-quality standards. The current DEQ Integrated Report, commonly referred to as
the “303(d) list”, was approved by EPA in September 2022. While only Oswego Lake and Oswego Creek
were specifically listed in the 2022 assessment, an impaired segment may include all the tributaries to
the waterway (see Table 1).
DEQ uses water-quality data to evaluate the most common beneficial uses such as aquatic life, drinking
water, and recreation. Waterbodies that exceed the water-quality standards are identified as impaired.
To address impairments identified for a waterway on the 303(d) list, DEQ calculates the total maximum
daily load (TMDL) that can be assimilated by a stream or river while maintaining the most stringent
beneficial use of the impaired waterway.
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Table 1. Impaired Streams in Lake Oswego.
Stream Affected Stream Reach Tributary Stream Parameter Time Period
Fanno
Creek
Carter Creek to Tualatin
River Carter Creek
Dieldrin
TTCE Not Specified
Fanno
Creek
Carter Creek to Tualatin
River Carter Creek
Total Iron
Copper Not Specified
Fanno
Creek
Carter Creek to Tualatin
River Carter Creek Dissolved Oxygen1
Spawning
(Cool Water)
Fanno
Creek
1st through 4th order
streams
Carter Creek
Ball Creek Dissolved Oxygen1
Spawning
(Cool/Cold
Water)
Fanno
Creek
1st through 4th order
streams
Carter Creek
Ball Creek
Biocriteria, Aquatic Weeds
Chromium VI, Zinc Not Specified
Oswego
Lake Lake
Lost Dog Creek
Springbrook Creek
Boones Ferry Algal Blooms Not Specified
Oswego
Creek2
1st through 4th order
streams
Lost Dog Creek
Springbrook Creek
Boones Ferry Temperature1
Spawning
Year Round
Oswego
Creek2
1st through 4th order
streams
Lost Dog Creek
Springbrook Creek
Boones Ferry
Biocriteria
Zinc
Copper
Spawning
Year Round
Oswego
Creek2
1st through 4th order
streams
Lost Dog Creek
Springbrook Creek
Boones Ferry
Benz(a)anthracene
Benzo(a)pyrene
Benzo(b)fluoranthene 3,4
Benzo(k)fluoranthene
Chrysene
Indeno(1,2,3-cd)pyrene Not Specified
Tryon
Creek2
Palantine Hill Creek to
Willamette River Nettle Creek Dissolved Oxygen
Spawning
Oct 15–May 15
Tualatin
River
McFee Creek to
Willamette River
Streams on City’s
south side Dissolved Oxygen
Spawning (Cool
Water)
Tualatin
River
McFee Creek to
Willamette River
Streams on City’s
south side
Biocriteria, Harmful Algal Blooms,
Total Iron Not Specified
Tualatin
River
McFee Creek to
Willamette River
Streams on City’s
south side Dieldrin Not Specified
Willamette
River
Clackamas River to
Johnson Creek
Streams on City’s
east side Biocriteria; Cyanide; Temperature All Year (Temp)
Willamette
River
Clackamas River to
Johnson Creek
Streams on City’s
east side
Ethylbenzene, PAHs, PCBs;
Hexachlorobenzene; Aldrin;
Dieldrin; DDE 4,4; DDT 4,4 Not Specified
1 – See Table 2; 2 – Essential salmonid habitat
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DEQ determines the number and type of jurisdictions currently
discharging to the impaired waterway and identifies them as
Designated Management Agencies (DMAs). A maximum load is
calculated for each DMA, or group of DMAs, that can be
discharged to the waterway on a daily or seasonal basis. This is
known as a TMDL allocation.
After the TMDL allocation is set, DEQ determines which land
management practices the DMA, or group of DMAs, can use to
reduce current loading. The DMAs submit a TMDL
Implementation Plan (Plan) to DEQ using these management
practices or proposing other practices to meet the TMDL
allocation. Once the Plan is approved by DEQ, the DMA
implements the approved practices through municipal code,
the SWMM, and the Engineering Design Standards. The TMDL
allocations are enforced through the MS4 or other NPDES
permit as a wasteload allocation or through the TMDL
program as a load allocation.
The City is a DMA for the Tualatin River and Willamette River TMDLs (see Table 2) which, together,
affect all the watersheds in the city. The SWMM requirements help the City meet these TMDL
allocations.
Table 2. City Watersheds affected by TMDLs
TMDL Basin Watershed1 Issue Date Parameter
Tualatin River
Basin
Carter Creek
Ball Creek
Lower Tualatin1
2001 • Temperature (surrogate – effective shade)
• Bacteria
1988/2012
Revised 1988 Parameters:
• pH
• Chlorophyll a (surrogate – total phosphorous)
• DO (surrogates – TSS and VSS)
Lower
Willamette
River
All watersheds
in the city
2006
• Bacteria
• Mercury
• Temperature (surrogate – effective shade)
20192 Revised mercury (surrogate TSS)
2025 Revised temperature3
1 – Southern area of the city; 2 – Issued by DEQ in December 2019 and revised by EPA in February 2021; 3 – Oswego Creek and
the lower part of Nettle Creek are classified as essential salmonid habitat whereas the mainstem of the Willamette River is
classified as a migration/rearing corridor for salmonids
TSS=total suspended solids; VSS=total volatile suspended solids; DO=dissolved oxygen
Beneficial uses include:
•Domestic water supply
•Fishing
•Industrial water supply
•Boating
•Irrigation
•Water contact recreation
•Livestock watering
•Aesthetic quality
•Fish and aquatic life
•Hydropower
•Wildlife and hunting
•Commercial navigation
(OAR 340-041-0340)
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1.6 DOCUMENT HIERARCHY
Several documents, in addition to the SWMM, govern stormwater management in the city. The
Engineering Design Standards facilitate the planning, design, and installation of conveyance
infrastructure to serve new and future development and to upgrade existing infrastructure. The
Standard Details provide engineers with granular detail of stormwater structures for conveyance and
water quality. The ESC Manual is used to prevent the release of construction site stormwater offsite
during construction.
While these documents and the SWMM complement each other, they are rarely updated at the same
time. In the event of a conflict between their requirements, or within a document, the requirement that
is most protective of water quality retains primacy.
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2 DEVELOPMENT PROCESS
The City’s requirements for stormwater management vary depending on the proposed project size and
type of development. Most projects start with a pre-application conference for the applicant to discuss
the proposed project, potential constraints, and requirements of the approval process with staff (see
Figure 2).
1 – Geotechnical reports are submitted when required by LOC Chapter 38, LOC Chapter 50, or this SWMM.
DEQ – Oregon Dept of Environmental Quality; DSL – Oregon Dept of State Lands; ESC – Erosion and Sediment Control;
O&M – Operations and Maintenance Plan; ODFW – Oregon Dept of Fish and Wildlife; UIC – Underground Injection Control
Figure 2. Summary of City Review and Permitting Process
2.1 PLAN REVIEW
Chapter 50 of the Lake Oswego Code (LOC) determines whether a project is required to follow the land-
use process or can proceed directly to construction plan review. It is strongly suggested that applicants
1 1
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consult with the City’s Planning staff to determine whether their project needs to undergo the land-use
process.
The Engineering Development Review staff review projects and provide conditions of approval at the
land-use stage of a project (see Figure 3). Staff ensure that conditions of approval have been met, if
required by the land-use process, and review compliance with the stormwater documents (see Figure 4).
At the time of land-use application and construction plan submittal, a stormwater report must be
submitted to the Engineering Development Review staff for review and approval.
Projects that are required to provide public improvements must apply for a public improvement permit
and submit construction plans to Engineering Development Review staff for review and approval (see
Figure 5). The City Engineer may require that the public improvement be completed before the
development of private tax lots. The decision of the City Engineer is final.
2.2 CONSTRUCTION INSPECTIONS
When the construction plan set is submitted to the Engineering Development Review staff, the applicant
must also submit an erosion control and sediment (ESC) plan to the City’s ESC Inspector (see Figure 6).
After receiving the ESC permit, the applicant must install the ESCs, schedule the initial ESC inspection,
and wait for approval before starting construction. Additional information is provided in Chapter 12
(Erosion and Sediment Control).
During the construction of public stormwater infrastructure, other construction inspections must be
coordinated with the City’s Construction Inspectors.
2.3 POST-CONSTRUCTION
After construction, the City requires surveyed as-builts as a post-construction submittal. Contractors for
capital improvements (CIPs) projects must provide surveyed as-builts to the City’s CIP Project Manager
for review and approval. Applicants completing public improvements must provide surveyed as-builts to
the Engineering Development Review staff for review and approval. For private projects, a composite
utility plan must be submitted to the Engineering Development Review staff for review and approval.
Public improvements and CIP projects must undergo a warranty period of 1 year before City acceptance
of a facility or structure. In addition, contractors or applicants must post a 1-year maintenance bond.
Applicants constructing private stormwater facilities must provide documents as detailed in Section 4.3
(Submittals – Post-Construction Submittals).
Failure to install ESCs and obtain approval of the first ESC inspection before
clearing, grubbing, or starting construction is a violation of the stormwater code
and the ESC permit. It is subject to citations, fines, and stop-work orders as well
as other enforcement actions allowed by LOC 52 (Erosion Control).
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Figure 3. Land-Use Process (modified from OTAK, 2022)
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Figure 4. Construction Process (modified from OTAK, 2022)
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Figure 5. Public Improvement Process (modified from OTAK, 2022)
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1 – Conducted after erosion and sediment controls have been installed but before construction begins; 2 – Scheduled before concrete work begins (pre-footing for private projects); 3 – Scheduled after soil has been stabilized and stormwater facilities have been
constructed for private projects and prior to closeout for public improvements and capital improvement projects. Post-construction submittals must have been received and approved by the City’s ESC Inspector.
Figure 6. Erosion and Sediment Control Process
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2.4 STORMWATER FACILITY ALTERATIONS
Any alteration of a stormwater facility, after it has been transferred to the facility owner, must be
submitted to the Engineering Development Review staff for review and approval. Alterations include
restorations, renovations, or replacements. They do not include normal maintenance such as replacing
media filters in a water-quality vault or removing sediment in an infiltration gallery.
Restorations and Renovations
Stormwater facilities may need restoration or renovation due to
deferred maintenance. Renovations and restorations differ in the
facility’s condition and the type of work required to bring the
facility back to normal operating capacity.
Non-structural repairs to a failing stormwater facility, such as
replacing soil in a stormwater planter or dredging a detention
pond, constitutes a restoration. Stormwater facility restorations
must adhere to the original approved plans or land-use
requirements. If a conflict exists between the approved plans
and the land-use requirements, then the requirement that is
most protective of water quality will apply for the restoration
Work on a stormwater facility that no longer provides the
designed stormwater treatment or detention is classified as a
renovation. Structural repairs on a failing or failed stormwater
facility, i.e., replacing a perforated pipe or a chamber of an
infiltration gallery are defined as renovations. Renovations must
adhere to the current SWMM requirements.
If a conflict exists between the approved plans and the land-use requirements, then the requirement
that is most protective of water-quality will apply.
Replacements
Stormwater replacements are treated as a new project and must adhere to the current SWMM
requirements.
Augmentation
Projects that discharge to an existing facility must adhere to the current SWMM requirements even if
the amount of impervious area does not exceed the stormwater thresholds. A stormwater report must
be submitted to the Engineering Development Review staff for review and approval. The report must
show that the existing facility has capacity for all the impervious area that will be discharging to it as well
as how the facility meets the current SWMM requirements.
If the existing facility does not provide onsite retention and the site does not meet minimum infiltration
requirements (see Section 7.3 Stormwater Facility Selection – Onsite Retention), then the report must
show that the facility meets the requirements of Section 7.4 (Stormwater Facility Selection – Extended
Restoration vs Renovation
A restoration is a non-
structural repair to a failing
stormwater facility and
requires adherence to the
original stormwater
requirements.
A renovation is a structural
repair to a failing stormwater
facility OR any repair to a
failed stormwater facility. It
requires adherence to the
current SWMM requirements.
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Filtration), Section 7.5 (Stormwater Facility Selection – Water-Quality Limited Streams) and Section 8.4
(Stormwater Modeling – Downstream Capacity Analysis) or a new facility must be constructed to meet
the requirements of this SWMM.
If the existing facility provides onsite retention and the site meets the minimum infiltration
requirements, then the existing facility can be enlarged or a new facility constructed to provide onsite
retention. The stormwater design must also meet the other requirements of this SWMM.
If 3,000 sq ft or more of impervious area will be discharging to the facility after construction, then the
stormwater report must show that the facility meets the Flow Control requirements in Section 7.4
(Stormwater Facility Selection – Flow Control) and Section 8.4 (Stormwater Modeling – Downstream
Capacity Analysis).
If 10,000 sq ft or more of impervious area will be discharging to the facility after construction or there is
a hydraulicly-restrictive layer within 15 feet of the bottom of the facility, then the stormwater report
must provide a groundwater mounding analysis (See Section 6.3.4 Groundwater Mounding Analysis).
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3 STORMWATER THRESHOLDS
The following thresholds for stormwater management apply to all projects in the city. The SWMM and
other documents (see Section 1.6 Introduction – Document Hierarchy) provide design requirements for
stormwater engineers working on private and public projects in the city.
3.1 PROJECT CLASS
For the purpose of determining stormwater management
requirements, projects are categorized based on the amount
of impervious area created or replaced (see Table 3).
Projects using an existing facility must follow the requirements
of Section 2.4.3 (Stormwater Facility Alterations –
Augmentation) even if the following thresholds are not
exceeded for the project.
Impervious Area Calculation
The total amount of impervious area being created or
redeveloped must be calculated when determining the project
class. The total impervious area, as well as the amount of
impervious area treated by each stormwater facility, must be
noted on the stormwater plan sheet and discussed in the
stormwater report.
Permeable pavers and porous pavement are considered
stormwater facilities. The area covered by permeable pavers
and porous pavement must be included in the calculation of
the total impervious area.
Flatwork, pools and pool decks are impervious as well as the roof area delineated by the gutters, eaves,
and drip edges. While the footprint of the pool structure is considered impervious when determining
the size of the project, stormwater management is not required if the pool structure is shown by the
composite utility plan to be connected to the wastewater system.
Transportation projects are a special type of project and include street and road projects, pathways, and
sidewalks. Pavement maintenance projects are redeveloped impervious area however removal of the
pavement to the base course changes the classification to new impervious area.
Stormwater management is required if a project reaches the stormwater thresholds even if a building
permit is not required for the project. Impervious examples that do not require a building permit
include, but are not limited to, sports courts, artificial turf, outdoor kitchens, flatwork, and hardscaping.
Plans for these projects must be submitted to the Engineering Development Review staff for review and
approval of the proposed stormwater management.
Projects that occur on a property
over a 3-year period and which
cumulatively exceed the stormwater
thresholds must manage
stormwater.
Example: In June 2025, a 600-sq ft
patio was built and was exempt from
the stormwater management
because of the 1,000-sq ft threshold.
In September 2027, a 750-sq ft
garage was submitted for plan
review. It would be subject to the
stormwater requirements because
the cumulative 3-year effect of the
projects is greater than the
stormwater threshold (600 sq ft +750
sq ft> 1,000 sq ft).
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Small Projects
Small Projects create 1,000 square feet or more of impervious area but less than 3,000 square feet.
Small Projects which, over the period of three years, cumulatively create and/or replace 3,000 sq ft or
more of impervious area will be considered Large Projects.
Large Projects
A project is identified as a Large Project if the impervious area created and/or replaced is 3,000 sq ft or
more. A project is also identified as a Large Project if the total amount of created and/or replaced
impervious area over three years is 3,000 sq ft or more. The primary difference between Small and Large
Projects is that Large Projects are required to provide Flow Control and a Downstream Capacity Analysis.
Table 3. Project Requirements based on Classification
Project Requirements
Small Project Large Project
Creation and/or replacement of
≥1,000 sq ft but <3,000 sq ft of
impervious area
Creation and/or replacement of
≥3,000 sq ft of impervious area
Design by Licensed Professional1 ✓ ✓
Onsite Stormwater Retention ✓ ✓
Water Quality Treatment2 ✓ ✓
Flow Control3 ✓
Downstream Analysis3 ✓
DEQ approval of UICs4 ✓ ✓
1 – Not required for projects using prescriptive sizing; 2 – For projects with offsite discharges; 3 – For Large Projects with offsite
discharges; 4 – UICs are underground injection controls and include infiltration trenches, infiltration galleries, and drywell
Dredge and Fill Projects
While dredge and fill projects typically do not result in an impervious surface, a permit, waiver, or
exemption from the Department of State Lands (DSL) is required for all projects that include dredge and
fill. The applicant must provide a copy of the DSL decision and requirements for the project to the
Engineering Development Review staff. If a 401 Water Quality Certificate is required, the applicant is
required to provide a copy of the requirements to the Engineering Development Review staff with the
construction plan submittal.
A City ESC permit must be obtained and implemented for all dredging or fill projects within 50 feet of a
waterway or in designated sensitive lands. Dependent on the type of work and affected waterway (see
Section 1.5 Introduction – Impaired Rivers and Streams), the City Engineer may require additional
conditions or constraints.
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3.2 PROJECT EXEMPTIONS AND VARIANCES
Projects may be exempted from specific requirements, or the applicant may apply for a variance for a
requirement under limited circumstances and only with written approval of the City Engineer. Approval
must be obtained during the land-use process or prior to the submittal of construction plans, whichever
occurs first. A copy of the written approval must be submitted to the Engineering Development Review
staff with the construction plan set.
Exemptions
The following developments are exempt from the 2025 SWMM’s requirements:
• Projects that have entered the land-use process or that
have submitted construction plans and whose submittals
have been deemed complete by the Engineering
Development Review staff prior to the implementation
date of the 2025 SWMM. These projects must adhere to
the requirements of the SWMM in effect at the time that
the submittal was deemed complete.
• Projects undertaken during an emergency, such as
when there is an immediate danger of landslides, damage
to public or private property, or failure of a public facility.
The City Engineer must concur that there is an emergency
and may require retroactive compliance with this SWMM.
The City Engineer’s decision is final.
• Re-development in response to a natural disaster or fire
when the redevelopment does not expand the impervious
area beyond the original building’s impervious area.
Confirmation of the impervious area must be provided
through photos and/or original building plans.
• Americans with Disabilities Act (ADA) ramps where
ramp replacement is the only activity.
• Utilities
o Maintenance, repair, or installation of underground infrastructure, e.g. pipes, conduits, and
vaults, that include replacing the ground surface with in-kind material or materials with similar
impervious characteristics.
o Utility work where only trenching is done and where the new surface has the same impervious
characteristics as the original surface.
• Pavement Maintenance
o Pothole repair, crack sealing, and square cut patching.
o Pavement overlays that do not expand the area of coverage.
o Slurry seal that does not expand the area of coverage.
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o Pavement maintenance that does not extend below the top of the road base.
Variances
An applicant may apply for a variance from a requirement of the SWMM during the land-use process or
the submittal of construction plans, whichever occurs first. When a variance is granted, conditions of
approval may be imposed on the applicant by the City Engineer to maintain the water-quality of the
receiving stream. Variance requirements and their approval criteria are found in LOC Subarticle
38.25.145. The decision of the City Engineer is final.
If an “exempt” activity is part of, directly related to, or caused by development or redevelopment
then it is not exempt from stormwater management.
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4 SUBMITTALS
The City requires the following submittals, to provide timely plan review, even if an applicant is not
required to obtain a building permit. Applicants are strongly encouraged to contact the Engineering
Development Review staff for a list of the submittals required during land-use review.
Proposed stormwater systems may change between land-use approval and
submittal of the construction plan set to the City, however the plan set
approved by the Engineering Development Review staff must be
implemented during construction. If changes occur because of field
conditions, the applicant must notify the Engineering Development Review
staff within 24 hours for review and approval of the changes. A revised
stormwater report and plan set may need to be submitted to the
Engineering Development Review staff.
4.1 PRE-CONSTRUCTION SUBMITTALS
The following plan sheets must be submitted to Engineering Development Review staff to facilitate staff
review of construction plans and ensure a timely start date for the applicant. Incomplete applications
will be returned to the applicant and can result in project delays.
Plan Submittals
Each required sheet must be a separate sheet that follows the basic requirements listed below. Final
plan sheets must be signed and stamped by the design engineer.
4.1.1.1 Basic Requirements
All plan sheets must comply with the City’s GIS standards, as published on the City’s website, and use
the local datum and benchmarks. In addition, all construction plan sheets must contain:
• The sheet number, site address, building permit number, LU case number (as applicable), PE
stamp/signature, and revision table in the title block.
• An engineering scale
- Horizontal Scale: 1 inch = 20 ft
- Vertical Scale: 1 inch = 5 ft
• Stationing must increase from left to right with a North arrow on each sheet pointing up (90°),
to the right (0°), or to the left (180°), as appropriate to provide the correct layout.
Each utility structure must be numbered with each structure retaining the same number across all
construction sheets. When existing infrastructure and proposed infrastructure are on the same plan
sheet, the existing infrastructure must be a light gray font to distinguish it from the proposed
infrastructure. Infrastructure to be removed or abandoned must be marked as such.
Profile views must be located under the plan view on each sheet. Stations in the profile view must
correspond to the location of the stations in the plan view.
Not following the
City-approved
construction plans
is a violation of City
municipal code and
subject to citations,
fines, and stop-
work orders.
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All sheets must be a minimum of 22 inches x 34 inches. PDFs are encouraged for submittals, but they
must be legible at the 24”x36” scale. Revisions must be highlighted in red font and encircled with a
“cloud” symbology. Revision dates must be documented in the title block.
4.1.1.2 Title Sheet
Small single family residential (SFR) projects must provide a title sheet containing the project address
and contact information for the applicant, the Project’s ESC inspector, and the stormwater engineer.
The following information is required for all Large Projects and non-SFR projects:
• Index of sheets (if greater than 10) and date of last revision.
• Complete legend of symbols used in construction plans.
• Vicinity map.
• Name, address, and emergency contact information of the applicant.
• Name, address, and emergency contact information for the stormwater engineer.
• Name, address, and emergency contact information for the Project ESC Inspector.
• Approval date of the stormwater report, if available.
4.1.1.3 Pre-Development Sheet
The pre-development sheet must include the following information within 50 ft of the project
boundaries:
• 2-ft contours (1-ft contours for sites containing slopes of 5% or less).
• Location of any existing or decommissioned septic systems and underground storage tanks
including the areal extent of the leach field.
• Location and areal extent of existing soil contamination areas.
• Location of Federal emergency Management Agency (FEMA)-designated 100-yr floodplains,
flood management areas, and sensitive lands.
• Existing stormwater flow direction.
• Existing wastewater, stormwater, and water systems.
• Pre-Development impervious area.
• Infiltration testing locations.
• Existing easements.
• Applicable setbacks.
In addition, the pre-development sheet must note the following information:
• Location of geologic hazards or steep slopes within 200 ft of the proposed stormwater facilities.
• Location of wells within 500 ft of the proposed stormwater facilities.
Pre-development sheets must be submitted with the land-use application as well as the construction
plan submittal.
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4.1.1.4 Erosion and Sediment Control Sheet
The Erosion and Sediment Control (ESC) sheet must include the following information within 50 ft of the
project boundaries:
• 2-ft contours (1-ft contours for sites containing slopes of 5% or less).
• Location of existing or decommissioned septic systems or underground storage tanks.
• Areas of soil contamination.
• Areas of sensitive lands, such as streams or wetlands, and the ESCs to protect them.
• Stormwater flow direction.
• Existing and proposed public stormwater facilities and structures and ESCs to be used.
• Existing and proposed private stormwater facilities and ESCs to be used.
• Primary access point(s) for construction traffic.
• Limits of clearing and limits of construction activities, if different.
• Perimeter controls that will be used to prevent sediment or construction debris from leaving the
site.
• Sediment control measures that will be used on and at toes of slopes.
• Stockpile locations.
• Location of concrete washout.
• Other proposed ESCs.
• General notes for erosion and sediment control.
• Wet weather notes if soil will be disturbed between October 1st and May 31st
The ESC sheet must be submitted with the ESC Permit Application.
4.1.1.5 Grading Sheet
The grading sheet must provide the following information within 50 feet of the project boundaries
• 2-ft contours (1-ft contours for sites containing slopes of 5% or less).
• Limits of activity and limits of construction, if different.
• Proposed areas of cut and fill.
• Location and areal extent of weak soil and landslides.
• Location and areal extent of fill.
• Location of existing wetlands, riparian areas, and floodplains.
• Location of existing roadways and drainage patterns.
• Location of existing and proposed terraces or retaining walls.
• Post-construction plan views of buildings and other structures.
• Location of existing and proposed stormwater facilities.
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4.1.1.6 Composite Utility Plan Sheet
The composite utility plan (scale 1 inch = 100 ft) must provide the following information on the site:
• Plan view of proposed wastewater, water, and stormwater systems including structures such as
manholes, cleanouts, and pipes (including point of connection to public systems) within 100 ft of
the project or property line whichever is closer
• Easements for wastewater, water, and stormwater within 50 ft of the project.
• Sensitive lands (if applicable) and 100-yr floodplains within 50 ft of the project.
• Slopes ≥ 15% within 200 ft of the project.
• ROW limits.
• Impervious areas including but not limited to structures, driveways, and hardscaping
A scale of 1 inch = 50 ft may be used but must be legible on a single 22” X 34” sheet or when viewed as a
24” X 36” sheet in a PDF.
Composite utility plans must be submitted with the land-use application and the construction plans.
4.1.1.7 Stormwater Plan Sheets
The stormwater plan sheet must provide the following within 50 ft of the project boundaries unless
otherwise indicated.
• A table of the impervious area treated by each stormwater facility and the total amount of
impervious area created by the project.
• Location of source controls required by the SWMM (See Chapter 5 – Source Control).
• Location of existing roadways, shoulders, berms, and drainage patterns.
• Location and areal extent of contaminated soil and onsite wastewater leach fields.
• Location of existing wetlands, streams, FEMA-designated 100-yr floodplains, and flood
management areas.
• 2-ft contours (1-ft contours for sites with slopes 5% or less).
• Plan view of the existing and proposed stormwater facilities and the
conveyance system to the facilities (dimensioned and scaled).
• Plan view of the existing and proposed stormwater facilities and
conveyance system from the stormwater facilities to the offsite discharge
point (dimensioned and scaled) including outfall if applicable.
• Profile views of existing (if retained) and proposed conveyance systems,
including outfalls.
• Cross-section views of existing (if retained) and proposed stormwater facilities.
• City detail drawings when available; other detail drawings may be used if not available from the
City but are subject to changes by the City Engineer.
Stormwater plan sheets must be submitted with the land-use application as well as the construction
plan submittal.
Existing systems to
be abandoned or
removed must be
noted as such on
the plan sheet.
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4.1.1.8 Landscape Sheet (see Standard Detail SD9-14)
• Plan view of proposed planting plan for vegetated stormwater facilities.
• Table of proposed plants including species, size, and number of plants.
4.1.1.9 Street Sheet
A street plan sheet is required for all public improvements and when sidewalks, walkways, pathways, or
driveways will be affected by the project. The street plan sheet must adhere to the requirements of the
Engineering Design Standards.
Report Submittals
Reports must be submitted to the Engineering Development Review staff for review and approval. The
reports must document existing geotechnical conditions, infiltration capacity, and support the choice of
the proposed stormwater system.
4.1.2.1 Stormwater Report (see Section 6.3 – Hydrology and Chapter 8 – Stormwater Modeling)
All projects must provide a stormwater report discussing the existing conditions, infiltration rates, and
the proposed stormwater system. A basic stormwater report is required for all projects using
prescriptive sizing and it must discuss the reasons that prescriptive sizing is appropriate.
A preliminary stormwater report must be submitted with the land-use application. A final stormwater
report must be submitted with the construction plan submittal.
4.1.2.1.1 Projects using Models
A stormwater report must be submitted to the Engineering Development Review staff for all Small and
Large Projects at the time of the construction plan set submittal. The report must provide:
• Title Block:
o Report date (and revision dates, as necessary)
o Building permit and land-use case number (if applicable)
o Engineering Company and the Design Engineer’s name and license number
o Property address and tax lot number (current and historical, if appropriate)
o A table with the amount of impervious area and source that is treated by each stormwater
facility as well as the total impervious area created by the project
• Project Overview
o Summary objective, e.g., demolition and re-build of a building
o Development type and property zoning
o Project classification (Large or Small)
o SWMM requirements
• Existing conditions (see Chapter 6 – Site Assessment)
o Predominant soil class on the property
o Presence of steep slopes, weak soil, or landslides. Discuss the risk of landslides on the
property (as defined by DOGAMI)
- 27 -
o Presence or absence of hydraulically restrictive layers on the property and the elevation at
which they were encountered as well as the thickness of the layer if above the infiltration
test depth
o Presence or absence of contaminated soil
o Areal extent of fill on the property and the elevation difference between the applicant’s
property and neighboring properties
o Presence or absence of onsite wastewater system
o A discussion of the hydrology of the property including the presence of waterways and
floodplains on the applicant’s property and within 100-ft of the applicant’s property
o A discussion of the depth to groundwater based on borings from the geotechnical reports
and from infiltration tests
o Infiltration Rate
− Test method used, locations (figure), and depth at which tests were completed
− Table of infiltration results
− Median measured infiltration rate and design infiltration rate
• Infiltration Feasibility (See Chapter 5 – Source Control and Section 6.3 – Hydrology)
o Discuss source controls required by the SWMM
o Discuss final contours and how the final elevation difference between the applicant’s
property and the neighboring properties varies from the existing conditions
o Discuss the results of the groundwater mounding analysis if more than 10,000 sq ft of
impervious area is being infiltrated onsite
o Discuss the design infiltration. Determine whether onsite retention is supported by the
information obtained during the site assessment
• Stormwater Facility Selection (See Chapter 7 – Stormwater Facility Selection)
o Using the facility hierarchy of Section 7.2 (Stormwater Facility Selection – Facility Hierarchy)
and the design infiltration rate, discuss why the proposed facilities are appropriate for use at
the site
o Discuss the groundwater separation appropriate for the proposed facilities as well as
whether it is necessary to control buoyant forces
o Discuss how the stormwater facilities provide the required water-quality treatment in
Section 7.5 (Water-Quality Limited Waterways)
o Discuss the point of approved discharge or, for filtration facilities, the connection location to
the public stormwater system
o Provide any constraints placed on proposed proprietary technologies by the State of
Washington’s (WA) Technology Assessment Protocol – Ecology (TAPE) program
• Model Results (See Section 8.1 – Approved Models and Section 8.2 – Model Parameters)
o Discuss the model used (SBUH vs HSPF) and the inputs to the model such as the curve
number, time of concentration, design infiltration rate, and impervious area treated by each
proposed stormwater facility
o Discuss the design requirement used in the model such as longitudinal slope, storage course
depth, soil and ponding depth (for vegetated facilities), void ratios, etc.
o Discuss the number of facilities required to treat the stormwater and their dimensions
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• Flow Control (see Section 8.3 – Flow Control)
o Discuss whether the project is exempt from flow control and why
o Discuss the pre-development and post-development flowrates at the required design
events. For projects with multiple facilities with one offsite discharge point, discuss the total
peak flow at the offsite discharge point
o Provide a table of the pre- and post-development peak flowrates for each design event
o Discuss the number of orifices or weirs needed, if any, and their type, dimension, location,
and elevation
o An analysis of the proposed system on the existing downstream system including modeled
flows showing the increase on the downstream system’s pipe capacity caused by the project
(see Section 8.4 Downstream Capacity Analyses)
• Downstream Capacity (See Section 8.4 – Downstream Capacity Analysis)
o Discuss the method used for determining the existing flows in the public stormwater system
o Discuss the calculations or assumptions used for determining impervious area and
infiltration rates properties discharging to the public stormwater system
o Using results from the modelling and flow control used, discuss the pre-and post-
development flows in the public stormwater system. Discuss the location at which the
offsite discharge from the applicant’s property becomes less than 10% of the total pre-
existing flow and the length from the offsite discharge to this point
o Discuss the existing capacity of the downstream public stormwater system and whether it
already exceeds 80% capacity or whether the stormwater from the project will cause the
public stormwater system to exceed 80% capacity
o Discuss proposed modifications to the downstream conveyance system, if applicable, and
how it meets the requirements of the SWMM
• Summary
o Summarize the feasibility of the proposed stormwater system and how it meets the SWMM
requirements
• Appendices
o Hydrologic and hydraulic flow calculations for the pre-development and post-development
scenarios (including curve number and time of concentration used, flowrates, hydrographs,
and pipe capacity graphics)
o Infiltration report
o WA TAPE GULD requirements
o Proposed Operations and Maintenance (O&M) plan
• Maps
o Pre-development map showing existing buildings and other structures, infiltration test
locations, and the areal extents of contaminated soil, historical fill, leach fields, hydraulically
restrictive layers, weak soils, historical landslides, steep slopes, waterways, and floodplains
o Post-development map showing the location of required source controls, retained and
proposed stormwater facilities and the conveyance to and from the facilities to the
approved offsite discharge point
o Composite utility plan sheet
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o Stormwater plan sheet
o Catchment areas with the impervious area treated for each stormwater facility
All maps must be scaled and dimensioned with the scale listed on the map. A stormwater report
template is provided in Appendix A.
4.1.2.1.2 Projects using Prescriptive Sizing
A stormwater report must be submitted to the Engineering Development Review staff at the time of the
construction plan submittal for all projects using prescriptive sizing. The report must provide:
• Title Block
o Report date and revision dates, as necessary
o Building permit number and land-use case number (if applicable)
o Design Professional Name and Contact Information
o Property address and tax lot number (current and historical, if appropriate).
o A table with the amount of impervious area and source that is treated by each stormwater
facility and the total impervious area created by the project.
• Project Overview
o Summary objective, e.g., demolition and re-build of a building.
o Development type and property zoning
o Project classification (Large or Small)
o SWMM requirements
• Existing conditions (See Chapter 6 – Site Assessment)
o Predominant soil class on the applicant’s property
o Presence of steep slopes, weak soil, or landslides. Discuss the risk of landslides on the
property (as defined by DOGAMI).
o Presence or absence of hydraulically restrictive layers and the elevation at which they were
encountered as well as the thickness of the layer if above the infiltration test depth
o Presence or absence of contaminated soil
o Areal extent of fill on the property and the elevation difference between the applicant’s
property and neighboring properties
o Presence or absence of an onsite wastewater system.
o A discussion of the hydrology of the property including the presence of waterways and
floodplains on the applicant’s property and within 100-ft of the applicant’s property
o A discussion of the depth to groundwater based on borings from the geotechnical reports
and from infiltration tests.
o Infiltration Rate
− Test method used, test locations (figure), and depth at which the tests were completed
− Table of the infiltration results
− Median measured infiltration rate
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• Infiltration Feasibility (See Section 6.3 – Hydrology)
o Discuss final contours of the site and how the final elevation difference between the
applicant’s property and the neighboring properties varies from the existing condition.
o Discuss the design infiltration rate. Determine if onsite retention is supported by the
information obtained during the site assessment.
• Stormwater Facility Selection (See Chapter 7 – Stormwater Facility Selection)
o Using the facility hierarchy of Section 7.2 (Stormwater Facility Selection – Facility Hierarchy),
discuss why the proposed facilities are appropriate for use at the site.
o Discuss the groundwater separation appropriate for the proposed facilities as well as
whether it is necessary to control buoyant forces.
o Discuss how the stormwater facilities provide the required water-quality treatment in
Section 7.5 (Water-Quality Limited Waterways)
o Discuss the point of approved discharge or, for filtration facilities, the connection location to
the public stormwater system.
• Prescriptive Sizing Results
o Discuss the appropriateness of the use of prescriptive sizing for the project and the total
amount of impervious area that has historically used prescriptive sizing on the applicant’s
property
o Discuss the number and dimensions of the facilities needed to manage stormwater on the
site and the sizing factor used in the analysis.
o Provide a table with the proposed facilities, the impervious area they treat, and the sizing
factor used. Include any facilities on the site that previously used prescriptive sizing and
that will be retained with this project.
• Summary
o Summarize the feasibility of the proposed stormwater system and how it meets the SWMM
requirements.
• Appendices
o Infiltration Report (see Appendix B)
o Proposed Operations and Maintenance Plan (see Appendix D)
• Maps
o Pre-development map showing existing buildings and other structures, infiltration test
locations and the areal extents of contaminated soil, historical fill, leach fields, hydraulically
restrictive layers, weak soils, historical landslides, steep slopes, waterways, and floodplains
o Post-development map showing the location of retained and proposed stormwater facilities
and the conveyance to and from the facilities to the approved offsite discharge point
o Composite utility plan sheet
o Stormwater plan sheet
o Catchment area map delineated with the impervious area treated for each facility
All maps must be scaled and dimensioned with the scale listed on the map. A basic stormwater report
template is provided in Appendix A.
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4.1.2.2 Geotechnical Report
A geotechnical report must be submitted to the Engineering Development Review staff. It must be
signed and stamped by a civil engineer (PE) with a minimum of 4 years of geotechnical experience (see
OAR 820-40-0040) in geotechnical analysis. A preliminary geotechnical report must be submitted with
the land-use application and a final geotechnical report must be submitted with the construction plan
submittal. The report must include:
• Soil classification
o The classification must extend 5 feet below the bottom of the proposed stormwater
facility if completed from November 1st to March 30th. The classification must extend 7
feet below the bottom of the proposed facility if completed from April 1st to October
31st
o Soil mottling must be noted if encountered.
• When applicable, a map or maps showing the extent and depth of
o Existing and proposed fill.
o Weak or unstable soil.
o Contaminated soil and locations of underground storage tanks (USTs).
• Elevation and extent of any hydraulically restrictive layers or bedrock, if applicable.
• Groundwater elevation, if encountered.
• A table of the infiltration test results and a map showing the test locations.
• Signature and stamp of the civil engineer completing or overseeing the work.
When development is proposed for an area within 200 ft of steep slopes, with soil that has a high
erosion potential, or that has other unstable geologic conditions, the report must also evaluate the site’s
potential instability from infiltration if an infiltration facility is being proposed for stormwater
management. The evaluation must include the following:
• Field investigation summary
• Statements regarding the exact nature and extent of the hazard
• Recommendations for site preparation and construction methods to minimize the effects of the
hazard.
• A description of any hazard areas that must not be disturbed by construction activities or post-
construction use
• Whether infiltration, flow spreaders, or other discharges onto the slope will increase instability
• A statement as to whether the proposed development can create conditions conducive to
landslides, subsidence, or further weaken the existing soil conditions.
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4.2 STORMWATER PLAN REVISIONS
The City understands that, during construction, site conditions can result in
changes to the approved stormwater plans. Minor changes include changes
that do not affect facility type, facility design requirements, or water quality.
Major changes such as moving a stormwater facility’s location down-
gradient or more than 50ft from the original location, increasing the amount
of impervious area, changing the facility type, or changing design
requirements require that the applicant provide a revised stormwater
report and stormwater site plan to the Engineering Development Review
staff for review and approval prior to implementation of the change. Failure
to do so is a violation of the City’s stormwater municipal code and will result
in fines, stop work orders, and/or reconstruction of the approved design at the approved location.
The revised stormwater report must show the revision date on the cover page of the report. Revisions
within the report must be shown as redlines/strikeouts, however when most of the report changes or
the proposed stormwater facility or system changes, the report must be resubmitted as a new
application and marked as such.
4.3 POST-CONSTRUCTION SUBMITTALS
The City requires that the following documents and video of
newly installed pipe be submitted, reviewed, and approved by
the Engineering Development Review staff prior to approval of
the final ESC inspection. For example, the stormwater facility
certification can only be submitted after the facility has been
constructed but does not have to wait for the project to be
completed before it is submitted to the Engineering
Development Review staff for review and approval.
Stormwater Facility Certification
To ensure that facilities are constructed as designed and provide
the intended treatment, including facilities designed with prescriptive sizing, the City requires that the
stormwater professional inspect and certify that the stormwater facilities have been constructed in
accordance with the design specifications and facility location in the City-approved stormwater report.
The certification must include 1) a photo-log of the construction process, e.g. photos showing the
excavation and the pipe layout and 2) documentation of the soil used in vegetated facilities or
documentation of treatment media used in other facilities.
Post-construction submittals
must be provided to the
Engineering Development
Review staff prior to scheduling
the final ESC inspection. Delays
in providing the documents will
result in a delay of the
approval for the final ESC
inspection.
Changes to
approved
stormwater plans
must be reviewed
and approved by the
City’s Engineering
Development Review
staff.
- 33 -
The cover page required for the certification will be provided by the Engineering Development Review
staff when the ESC permit is issued.
DEQ Approval of Underground Injection Control Systems
The City allows drywells, infiltration trenches, and infiltration galleries for managing stormwater. These
facilities are considered UIC systems and are regulated by DEQ. The applicant must provide a copy of the
DEQ approval to Engineering Development Review staff for all private systems prior to approval of the
final erosion control inspection. UICs receiving only SFR roof runoff are exempt from this requirement.
Sand filters, planters, and raingardens with an underground inlet are considered UICs and must follow
the requirements of this section.
Public UICs can only be constructed through the City’s capital improvements program. They are not
allowed for public improvements required as a condition of development.
Recorded Operations and Maintenance Plan
An O&M plan is required for stormwater facilities. The O&M Plan must include:
• Maintenance requirements and maintenance frequency,
• The stormwater site plan (as required by Section 4.1.1 Submittals – Pre-Construction
Submittals),
• The detail drawings of the stormwater facilities, and
• A copy of the DEQ approval for private UICs.
The City’s ESC Inspector will provide the required cover page of the O&M Plan at the time of the ESC
permit issuance. The O&M Plan must be recorded in the County of Record for private stormwater
systems. A copy of the recordation, including the County stamp, must be received by the Engineering
Development Review staff prior to the final ESC inspection.
Pipe System Video
Video of new and replaced pipe for public systems must be submitted electronically to the Engineering
Development Review staff prior to approval of the final ESC Inspection. Pipe inspections and video must
comply with Section 00415 in the current Oregon Standard Specifications for Construction
(ODOT/APWA, 2024 or later).
Designers, even those using prescriptive sizing, must certify that stormwater facilities were
built according to the City-approved design. The certification must be provided to the
Engineering Development Review staff prior to the final erosion control inspection.
Private development cannot use UICs to fulfill public improvement requirements.
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As-Built Drawings
As-builts are required for all public projects and all private projects with public improvements and must
provide accurately-surveyed data. Specifications for the as-builts and GIS requirements can be found on
the City’s website.
As-builts must be submitted to the City’s project manager for capital improvement projects or to the
Engineering Development Review staff for public improvements required as a condition of private
development. The as-builts must be approved by the City project manager or Engineering Development
Review staff prior to the facilities entering the warranty period.
For private systems, an accurately dimensioned and scaled composite utility plan must show the private
water, wastewater, and stormwater systems as they were constructed. Changes must be noted in red
font and the date of the changes must be provided in the revision block of the composite utility plan
sheet. The as-builts must be submitted and approved by the Engineering Development Review staff
prior to approval of the final ESC inspection.
Miscellaneous
Post-construction infiltration testing, due to location or facility change or when more than 10,000 sq ft
of impervious area is infiltrated on a site, must be summarized and submitted to the Engineering
Development Review staff for review as part of the post-construction submittals. Format must follow
the template in Appendix B.
When using a sand filter, the sieve analysis for the sand bed must be submitted to the Engineering
Development Review staff for review as part of the post-construction submittals.
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5 SOURCE CONTROL
Source control is an important component of stormwater management for non-SFR projects. The type of
source control depends on the type of activities that are expected to occur at the site. The following
source controls must be implemented for all non-SFR projects.
Outside Solid Material or Waste Storage Areas
Storage areas must be covered, bermed on three sides, and drain to the wastewater system. Waste and
recyclable materials must be in leak-proof dumpsters.
Hazardous material or waste storage areas must adhere to the same constraints as non-hazardous
waste and materials but be separate from them, be placed off the ground, and be clearly labeled as to
the contents.
Liquid Material or Waste Storage Areas
Outside liquid storage areas must be in a covered area that is bermed on all sides and that drains to the
wastewater system. The storage areas must be able to contain 10% of the total volume of the material
stored or 110% of the largest container, whichever is greater, through a containment system.
Liquid hazardous materials and waste must be separate from non-hazardous waste and materials. They
must be clearly labeled as to the contents.
Stationary fuel pumps must be located under cover and a minimum of 10 ft from the edge of the cover .
The area must drain into an oil-water separator that is appropriately sized for the facility. The minimum
capacity of an oil-water separator is 1,000 gallons.
Covers
Loading docks must provide a cover that extends a minimum of 3 feet either side of the loading area or
be able to accommodate a truck backing into the dock a minimum of 5 feet. When the covers are 10 ft
or higher, they must have a 5-ft overhang relative to the edges of the item they will be covering.
Stormwater must be diverted from areas used for outside storage of liquid and solid material
or waste.
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6 SITE ASSESSMENT
Selection of the appropriate technique for stormwater management must start early in the planning and
design process. Applicants must complete a site assessment (See Figure 7) to evaluate existing drainage
patterns, geotechnical conditions, hydrology, and setbacks.
6.1 EXISTING DRAINAGE PATTERNS
Oregon has adopted a civil law doctrine for drainage issues which
states that adjacent landowners shall maintain the normal course
and volume of natural drainage. The downhill or downstream
owner shall accept water that has historically flowed onto their land
from upstream properties and which is not significantly altered as
to the amount and pattern. The downhill landowner may not
obstruct runoff from the upper property if the uphill landowner is
properly discharging water onto their land.
For the uphill property owner, “properly discharging the water”
means maintaining the location of the historical discharge point,
avoiding substantial increases in the velocity and volume of
stormwater, and preserving the existing drainage pattern.
6.2 GEOTECHNICAL CONDITIONS
A cup of soil can hold millions of microorganisms that are critical to recycling soil nutrients, maintaining
soil structure, processing pollutants from runoff, and aiding plants in nutrient and water uptake. The
ability of a site to infiltrate is dependent on soil health and other factors such as soil classification,
topography, hydraulically restrictive soil layers, the extent and depth of fill, and areas of soil
contamination. The City understands the importance of the soil-infiltration relationship and requires a
civil engineer (PE) with a minimum of 4 years of geotechnical experience (see OAR 820-40-0040) to
complete a report on the geotechnical conditions (see Section 4.1.2 Report Submittals).
Soil Classification
Classifying a soil includes determining the type of soil present at a site. As noted in the Clean Streams
Plan (OTAK, 2009), nearly half of the soils in Lake Oswego are classified as belonging to Natural
Resources Conservation Service (NRCS) Hydrologic Soil Group C (see Figure 8). Another 30 percent of the
soils are classified as Group D, and the remainder of the soils are categorized as Group B.
Oregon drainage law, which
originates from common law
or case law, has developed
without legislative action, and
it is embodied in court
decisions. Therefore, there are
no Oregon Revised Statues to
cite pertaining to Oregon
drainage law. For more
information on this topic,
consult the ODOT Hydraulics
Manual.
- 37 -
Figure 7. Site Assessment Process
*Thresholds are the categories used to determine if stormwater management is required for a project including whether
the 3-year cumulative threshold was exceeded.
- 38 -
Soil is classified by the NRCS as follows:
• Group A soils have a high infiltration rate. They are predominately gravelly or sandy soil with
less than 10% clay. Infiltration rates are greater than 5.67 inches per hour and groundwater is
generally 40 inches or more from ground surface.
• Group B soils have a moderate infiltration rate with 10% to 20% clay. Infiltration rates range
between 1.42 inches and 5.67 inches per hour. Depth to groundwater is generally 20 to 40
inches.
• Group C soils are typically fine-textured soils with 20% to 40% clay. Infiltration rates vary
between 0.14 inches per hour and 1.42 inches per hour. Depth to groundwater can vary
between 20 to 40 inches.
• Group D soils have a clay fraction of 40% or more. Infiltration rates are less than 0.14 inches per
hour and the depth to groundwater is generally less than 24 inches. They are not suitable for
infiltration facilities.
Figure 8. NRCS Soil Classification in the City of Lake Oswego
The NRCS maps were completed at a much larger scale and, while informative, they are not indicative of
the soil type at the project scale and cannot be used for the stormwater report or sizing of the facilities
(see warning on the NRCS website).
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Steep Slopes
Steep slopes are defined as topographic relief that is 15% or greater. They can become unstable when
subject to stormwater discharged from impervious areas. For a project to include infiltration facilities on
a property with steep slopes (See Section 6.4 Site Assessment – Setbacks), the geotechnical report must
show that infiltration will not create slope instability, increase current slope instability, nor create an
erosion hazard.
Hydraulically Restrictive Layers
Hydraulically restrictive layers impede infiltration. If a geotechnical report was completed or is being
used for the project, it must provide information on the elevation at which a hydraulically restrictive
layer is encountered and the depth of the layer and be submitted to Engineering staff for review.
Soil Fill
Infiltration tests completed in areas of existing fill can artificially inflate the soil’s capacity for infiltration.
Fill can also create preferential pathways for groundwater and for infiltrated stormwater. If a
preferential channel is created through fill or through grading from the project, it can create an Oregon
Drainage Law issue for neighboring properties.
The Grading Sheet (See Section 4.1 Submittals Pre-Construction Submittals) submitted with the
construction plan set must provide locations of existing and proposed fill and its extent (vertically and
horizontally). The stormwater report, or geotechnical report if required, must evaluate whether the
existence of fill will create preferential pathways for stormwater or groundwater towards neighboring
properties and the likelihood of the pathways to create or exacerbate existing flooding or drainage
problems.
Contaminated Soil
Infiltration facilities cannot be located upgradient from nor over contaminated soil. A Phase II site
assessment, completed in accordance with ASTM E1903 (ASTM, 2019), must be provided to the City
with the stormwater report and evaluate the extent of the contamination (vertically and horizontally).
The report must be stamped and signed by a civil engineer (PE) with at least 4 years of geotechnical
experience (see OAR 820-40-0040).
6.3 HYDROLOGY
The city contains floodplains and areas with high groundwater tables, wetlands, and unstable soil (see
Figure 9). These areas impact the ability of a property to infiltrate stormwater and create groundwater
mounding if the design does not consider them. Infiltration tests assist the designer in determining
whether an infiltration facility is feasible at the site given the project’s soil properties and the elevation
of the groundwater table. All elevations in this section are based on the NGVD 29 datum.
- 40 -
Figure 9. Wetlands and Areas of Historic Landslides
- 41 -
Floodplains
The city contains many areas that are in a FEMA-
designated 100-yr floodplain. The Lake Corporation
manages the 100-yr flood elevation of Oswego Lake
through a spillway on the east side of the lake. For
stormwater facilities influenced by Oswego Lake, the top
of the facility must be above 99.7 ft. If the bottom of the
facility is below 97.4 ft, the structure must be
constructed to counteract buoyant forces.
For properties affected by the Willamette River, the top
of stormwater facilities must be at an elevation that is 2.3
ft above the highest base flood elevation reached on the
property during the 1996 flood (as shown in LO Maps). If
the bottom of a stormwater facility is below the base
flood elevation, it must be designed to counteract
buoyant forces.
For areas with sensitive lands or within 50 ft of a waterway, stormwater facilities cannot be constructed
in the area inundated during the 25-yr 24-hour design storm. The top of stormwater facilities must be at
least 2.3 feet above the highest elevation inundated by the 25-yr 24-hour design storm. Stormwater
facilities must counteract buoyancy forces if the bottom is below the highest elevation inundated during
the 25-yr 24-hour storm.
Projects are exempt from these floodplain requirements if they are only repairing or maintaining
existing structures or existing infrastructure.
Groundwater
Groundwater interferes with a stormwater facility’s ability to infiltrate stormwater. A separation from
groundwater is required by the City for all infiltration facilities.
6.3.2.1 Groundwater Separation
Groundwater may be encountered during geotechnical investigations or during infiltration tests. If
encountered, the elevation at which it was encountered must be shown on the infiltration test log and
in the geotechnical report.
Infiltration testing from April 1st to October 31st is unreliable in the city due to seasonal high
groundwater. Soil characterization must be completed to a depth that is 5 feet below the proposed
If a deeper stormwater facility is constructed than what was approved, the applicant must
submit revised boring logs showing that the facility meets the groundwater separation
requirements and a revised infiltration test showing that the actual depth has similar infiltration
capabilities.
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depth of the stormwater facility if completed between Nov 1st and March 30th and 7 feet beyond the
depth of the proposed stormwater facility if completed between April 1st and October 31st.
When determining groundwater separation for properties located in areas influenced the Oswego Lake,
the groundwater elevation must be assumed to be 97.4 ft. For properties located in areas influenced by
the Willamette River, the groundwater elevation must be assumed to be the highest base flood
elevation of the property reached during the 1996 flood as shown in LO Maps. For properties with a
sensitive lands overlay or that are within 50 ft of a waterway, the groundwater elevation must be
assumed to be the highest elevation inundated by the 25-yr 24-hour design storm, the groundwater
elevation encountered during the infiltration test, or the groundwater elevation encountered during the
geotechnical investigation, whichever elevation is highest.
6.3.2.2 Groundwater Collection and Disposal
Projects on sites with high groundwater present a special challenge for development. The volume of
groundwater to be discharged offsite can be difficult to determine and is highest during the winter when
stormwater volumes place the highest demands on the stormwater facilities. Because of the competing
demands that would be placed on a stormwater facility and the difficulty in determining groundwater
volumes, structures that collect groundwater must not discharge to a stormwater facility. Sump pumps
cannot be used to discharge stormwater to stormwater facilities.
Infiltration Tests
Infiltration tests must be conducted by, or under the supervision of, a professional hydrogeologist or
civil engineer (PE) with geotechnical experience (see OAR 840-40-0040). A signed and stamped
infiltration report must be completed by the supervising professional. A template for the infiltration test
and report is available in Appendix B.
6.3.3.1 Infiltration Test Methods
A minimum of three infiltration tests must be completed for all Large Projects and one infiltration test
must be completed for all Small Projects. Large Projects resulting in more than 3 lots must provide at
least one test per lot. Projects designing only roadways, pathways, sidewalks, or retaining walls must
provide 1 infiltration test per 1,000 linear feet of impervious area OR a minimum of three evenly-spaced
tests for projects with lengths greater than 1,000 linear feet.
The median infiltration rate of the tests must be used to determine the design infiltration rate. While
the measured infiltration rates must be included in the stormwater report, the design infiltration rate
must be used in the stormwater model and design.
Infiltration tests must be completed using one of the following methods and under the constraints as
noted:
Discharging groundwater to stormwater facilities is prohibited.
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• Double-Ring Infiltrometer (ASTM D3385).
o All projects
o Medium to fine-grained soil with no expansive (high plasticity) clay
• Large Scale PIT (Washington DOE, 2024, Volume V-5.4)
o Large Projects, Public Improvements, and Capital Improvement Projects
o Surface area of pit
— 12 to 32 sq ft for projects disturbing less than 1 acre
— 100 sq ft for projects that will disturb 1 acre or more
o Presoak at least 6 hours
• Encased Falling Head (ASTM D8152)
o All projects
o Medium to coarse-grained soil
• Open PIT (Gresham, 2025)
o Small Projects Only
o All soil types
o Test area must be at least 4 sq ft.
o Pre-soak at least 4 hours
6.3.3.2 Infiltration Test Depth
The depth at which the test is completed must be noted on the infiltration log and be at the proposed
bottom elevation of the facility. Based on the requirements of the City’s design standards (see Chapter 9
– Stormwater Facility Design) and standard details, the depths at which the infiltration tests must be
conducted are:
• Vegetated Filter Strips: 1 to 2 ft bgs
• Porous Pavement and Permeable Pavers: 3 to 4 ft bgs
• Planters, Raingardens, Swales, and Infiltration Trenches: 4 to 5 ft bgs
• Infiltration Galleries and Ponds: 6 to 7 ft bgs
• Drywells: 16 ft bgs
Post-construction infiltration testing (see Section 6.3.5 Hydrology – Post-Construction Infiltration
Testing) is required for all constructed facilities not meeting these requirements. A revised stormwater
report using the post-construction infiltration test results must be submitted to, and approved by, the
Engineering Development Review staff prior to approval of the final ESC inspection.
The default testing depth is 16 ft if the facility type is unknown at the time of the infiltration test.
- 44 -
Groundwater Mounding Analysis
A groundwater mounding analysis is required if stormwater from 10,000 sq ft or more of impervious
area is infiltrated onsite or if a hydraulically-restrictive layer was encountered during the soil
characterization completed as part of the infiltration test (see Section 6.3.2 – Groundwater). The
analysis must show that groundwater separation requirements will be met even if groundwater
mounding occurs below the facility.
MODRET or an equivalent model must be used to complete the analysis.
Post-Construction Infiltration Testing
At the time of construction, at least one of the infiltration test locations must be within the construction
footprint of the stormwater facility. Otherwise, post-construction infiltration testing of the facility must
be completed.
Post-construction testing of an infiltration facility is also required when the facility’s bottom elevation
was changed during construction, the location is more than 50 ft (10 ft for SFR projects) from the
proposed location, or when 10,000 sq ft or more of impervious area is infiltrated onsite. The test results
must be provided in a revised infiltration report and submitted to the Engineering Development Review
staff as part of the post-construction submittals.
6.4 SETBACKS
Certain site conditions may determine whether infiltration facilities can be considered for a project.
Infiltration facilities cannot be located over utilities nor in areas of known fill, contaminated soil, or areas
where exposure is likely from hazardous waste or materials such as fuel or solvents.
The infiltrating layer of a stormwater facility must be below areas of new fill to minimize preferential
pathways.
Onsite Wastewater Systems
Infiltration-based stormwater treatment is rarely compatible with existing septic systems and must meet
County-required setbacks. Infiltration facilities proposed for stormwater management must be located
downgradient from leach fields. A groundwater mounding analysis (see Section 6.3.4 Hydrology –
Groundwater Mounding) must be completed if the two systems are within 10 feet of each other.
Applicants may be required to connect to the public wastewater system, whether available within 200
feet of an existing line or not, to alleviate conflicts.
Constraints
Stormwater facilities that will provide onsite retention must meet the following horizontal setback
requirements, as measured from the outside top edge of the stormwater facility (See Figure 10):
• 500 feet from drinking water supply wells or springs (for UICs only)
• 200 feet from contaminated soil. (See Section 6.2 Site Assessments – Geotechnical Conditions)
• 200 feet from steep slopes (See Section 6.2 Site Assessments – Geotechnical Conditions) or
- 45 -
historic landslide areas (see Figure 9 and LO Maps)
• 20 feet from the maximum ponding depth for ponds and wetlands
• 10 feet from a building foundation
• 10 feet from septic systems or drain fields
• 10 feet from underground storage tanks
• 7.5 feet from utility trenches
• 5 feet from property lines
• 5 feet from ROW for private facilities
Figure 10. Setbacks from Foundations and Property Lines
5 10
Infiltration Zone
- 46 -
7 STORMWATER FACILITY SELECTION
The project type, project size, infiltration rate, receiving water, and setbacks will often determine the
type and location of an appropriate stormwater facility. All projects must provide onsite retention if the
soil meets minimum infiltration rates (see Table 4). If the minimum infiltration rate cannot be met, the
project must provide stormwater treatment for the water-quality storm event. All projects with an
offsite discharge must provide extended filtration and specific treatment if the receiving water is a
water-quality limited stream, as designated by DEQ (see Section 1.5 Introduction – Impaired Rivers and
Streams). Non-SFR projects must provide treatment for copper and zinc prior to offsite discharge. Flow
control is required for Large Projects unless they meet the criteria for exemption (See Section 8.3 Flow
Control).
Table 4. Performance Standards
Requirement Project Type Performance Standard1
Onsite
Retention
All projects meeting
minimum infiltration rates
Infiltrate the
10-year, 24-hour design storm
Water Quality
Projects that cannot meet
minimum infiltration rates
Capture and treat the water-quality storm
(80% of the average annual runoff)
Extended
Filtration
All projects with an offsite
discharge
Reduce Suspended Solids by 80% (all watersheds)
Reduce Phosphorous by 65% (Tualatin River basin
and Oswego Lake watersheds only)
Metals
Non-SFR projects with an
offsite discharge
Reduce copper by 30% and zinc by 60% (all
watersheds)
Flow Control Large Projects
Do not exceed pre-development peak flows for
24-hour design storms with 2-year, 5-year, and 10-
year recurrence intervals
1 – Precipitation depths for design storms are found in Chapter 8 – Stormwater Modeling
7.1 PROJECT TYPE
The stormwater facilities appropriate for a project depend on the type of project being proposed by the
applicant. SFR projects can use raingardens, planters, swales, infiltration trenches/galleries, drywells,
permeable pavers, porous pavement, vegetated filter strips, wetlands, and ponds.
Public improvements completed as a condition of development cannot use porous pavement,
permeable pavers, vegetated filter strips, detention tanks, detention vaults, water-quality vaults, or UICs
to satisfy stormwater requirements. Proprietary treatment systems and sand filters can only be used for
public improvements if they are pre-approved in writing during the land-use process by the City
Engineer and the Public Works Director.
- 47 -
Middle housing projects cannot use proprietary treatment, sand filters, water-quality vaults, detention
tanks, or detention vaults.
Capital improvement and non-SFR projects (excluding middle housing projects) can use any of the
available stormwater facilities approved for use by the SWMM.
7.2 FACILITY HIERARCHY
The stormwater professional must provide a feasibility analysis describing why the proposed
stormwater facilities provide the best treatment and management of stormwater created by the
project. Starting with the facilities in Tier 1 (see Table 5), applicants must provide a technical analysis
showing that the proposed stormwater facility will provide the most appropriate stormwater
management for the property. When using a Tier 2 facility, the applicant must show why onsite
retention is not technically feasible at the site. When proposing a Tier 3 facility, the applicant must show
why Tier 1 and Tier 2 facilities are not technically feasible. The feasibility analysis must be submitted
with the stormwater report.
The feasibility analysis must use the site assessment results of Chapter 6 (Site Assessment) and the
constraints of Section 7.1 to provide documentation of the infiltration capability and feasibility of the
proposed stormwater facility.
Table 5. Stormwater Facility Hierarchy
Tier Approved Stormwater Facilities
Tier 1
(Onsite Retention)
Infiltration Planter
Infiltration Raingarden
Infiltration Swale
Permeable Pavers
Porous Pavement
Infiltration Trench or Gallery
Drywell
Tier 2
(Extended Filtration)1
Filtration Planter
Filtration Raingarden
Filtration Swale
Water-Quality Vault (non-SFR projects only)2
Tier 3
All other technologies approved for use in the SWMM
and meeting the restrictions of Chapter 6 (Site
Assessment) and Section 7.7 (Stormwater Facility
Selection – Summary)
1 – Tier 2 Facilities must be used to treat stormwater before discharge offsite; 2 – Projects with middle housing excepted
Projects using proprietary technology must incorporate the constraints of the Washington DOE
TAPE program into their design and submit the constraints as part of the stormwater report.
- 48 -
If it is infeasible to treat stormwater created by the new or redeveloped impervious area triggering the
stormwater requirements, the applicant may apply for a variance to treat stormwater from an
equivalent amount of pre-existing untreated impervious area on the property. The required submittals
and criteria for approval of a stormwater variance are provided in the City’s municipal code (LOC
38.25.145). The written approval of the City Engineer for the variance must be included with the
stormwater report.
7.3 ONSITE RETENTION
All projects must provide onsite retention of the 10-yr 24-hr design storm when design infiltration rates
are at least 0.25 inches/hr and when the site assessment completed as part of Chapter 6 (Site
Assessment) supports infiltration.
A facility used to satisfy onsite retention must discharge to the public ROW or an alternative discharge
point that has been pre-approved, in writing, by the Engineering Development Review staff during the
land-use process or prior to construction plan submittal. To ensure proper maintenance and function,
onsite retention systems cannot be directly connected to the underground public stormwater system.
7.4 EXTENDED FILTRATION
Sites that cannot infiltrate the 10-yr 24-hr design storm must provide treatment
of the water-quality design storm, use extended filtration prior to discharging
stormwater offsite, and meet the requirements of Section 7.5 (Stormwater
Facility Selection – Water-Quality Limited Waterways). Large Projects must also
meet the requirements of Section 8.3 (Stormwater Modeling – Flow Control)
and Section 8.4 (Stormwater Modeling – Downstream Capacity Analysis).
Facilities providing extended filtration may be directly connected to the
underground public stormwater system. Facilities not directly connected to the
underground public stormwater system must follow the requirements for offsite discharges in Section
10.3 (Conveyance Design - Outfalls and Offsite Stormwater Discharges).
7.5 WATER-QUALITY LIMITED WATERWAYS
The City is required to reduce concentrations of specific constituents, including phosphorous and
bacteria, by DEQ. Additional stormwater treatment is required for projects that discharge to waterways
that are designated as water-quality limited, have a TMDL, or which are included in the watershed of the
waterway with a TMDL.
Suspended Material
The City’s MS4 permit requires that all projects must reduce suspended solids in stormwater by 80%
prior to discharge offsite. Bacterial reductions are required for all projects that discharge to the
Willamette River and Tualatin River watersheds, and mercury reduction is required for all projects that
discharge to the Willamette River watershed. These requirements are satisfied for impervious areas that
All flows not
retained onsite
must be treated
using extended
filtration.
- 49 -
are treated using facilities approved for onsite retention and extended filtration (see Table 5 in Section
7.2 Stormwater Facility Selection – Facility Hierarchy).
For non-SFR projects that do not include middle housing, proprietary treatments that have been
approved by the Washington DOE TAPE program and which have a General Use Level Designation
(GULD) for Basic Treatment can be used to satisfy these requirements as long as they adhere to the
design and installation requirements specified by the Washington DOE TAPE program and include the
constraints in the stormwater report.
Phosphorous
The City is required by DEQ to reduce phosphorous concentrations in the Tualatin River and Oswego
Lake watersheds. All projects except those that discharge to the Tryon Creek watershed or directly to
the Willamette River are required to reduce phosphorous concentrations by 65% (as per equation in
OAR 340-041-0345(4)(e)). This requirement is satisfied for impervious areas that are treated using
facilities approved for onsite retention and extended filtration.
For non-SFR projects that do not include middle housing, proprietary treatments that have been
approved by the Washington DOE TAPE program and which have GULD approval for Phosphorous
Treatment can be used to satisfy this requirement as long as they adhere to the design and installation
requirements specified by the Washington DOE TAPE program and include the constraints in the
stormwater report.
Copper and Zinc
Prior to discharge offsite, stormwater from non-SFR projects must be treated to reduce copper by 30%
and zinc by 60%. This requirement is satisfied for impervious areas that are treated using facilities
approved for onsite retention and extended filtration.
Proprietary treatments that have been approved by the Washington DOE TAPE program and which have
GULD approval for Metals Treatment can be used to satisfy this requirement for non-SFR projects, that
do not include middle housing, as long as they adhere to the design and installation requirements
specified by the Washington DOE TAPE program and include the constraints in the stormwater report.
7.6 CAPITAL IMPROVEMENTS AND PUBLIC IMPROVEMENTS
Capital Improvement and Public Improvement projects must further reduce suspended solids and
phosphorous, whenever technically feasible, by providing a:
• Water-quality snout and a 2-ft sump for all new and replaced catch basins except where the
previous and subsequent catch basins have snouts and sumps.
• 100% vegetative cover, a 3H:1V slope, and 2-ft (minimum) flat bottom, for all restored or
renovated ditches.
• Culvert orientations that match the stream orientation for all replaced culverts.
- 50 -
7.7 SUMMARY
Table 6 provides a list of the stormwater facilities and their applicability to the setbacks, infiltration
rates, project type, and stormwater facility hierarchy required by the SWMM.
Porous vs Permeable vs Pervious
Many jurisdictions and design engineers use these terms interchangeably. The City uses the
following terms throughout this document (see also Definitions):
Porous Pavement Permeable Pavers
Porous asphalt pavement Permeable Interlocking Concrete Pavers (PICP)
Pervious concrete pavement Permeable Paving Stones
Porous Grid Systems
- 51 -
Table 6. Approved Stormwater Facility Types and Applicability
Facility Type
Tie
r
On
s
i
t
e
Re
t
e
n
t
i
o
n
Fl
o
w
Co
n
t
r
o
l
Wa
t
e
r
Qu
a
l
i
t
y
Tr
e
a
t
m
e
n
t
Ex
t
e
n
d
e
d
F
i
l
t
r
a
t
i
o
n
Ca
p
i
t
a
l
I
m
p
r
o
v
e
m
e
n
t
s
Pu
b
l
i
c
I
m
p
r
o
v
e
m
e
n
t
s
SFR
Pr
o
j
e
c
t
s
1
Pr
e
s
c
r
i
p
t
i
v
e
Si
z
i
n
g
Co
n
t
a
m
i
n
a
t
i
o
n
P
r
e
s
e
n
t
or
H
i
g
h
L
a
n
d
s
l
i
d
e
R
i
s
k
Mi
n
im
u
m
De
s
i
g
n
In
f
i
l
t
r
a
t
i
o
n
Ra
t
e
(i
n
/
h
r
)
Mi
n
im
u
m
S
e
p
a
r
a
t
i
o
n
t
o
Re
s
t
r
i
c
t
i
v
e
L
a
y
e
r
(
f
t
)
2
Pr
o
p
e
r
t
y
L
i
n
e
Se
t
b
a
c
k
(f
t
)
3
St
r
u
c
t
u
r
e
Se
t
b
a
c
k
(
f
t
)
3
Drywell 1 ● ●4 ● 0.25 5 ≥5 ≥10
Infiltration trench or gallery 1 ● ●4 ● 0.25 5 ≥5 ≥10
Porous pavement 1 ● ● ● ● 0.25 3 ≥5 ≥10
Permeable pavers 1 ● ● ● ● 0.25 3 ≥5 ≥10
Planter, infiltration 1 ● ● ● ● ● ● 0.25 3 ≥5 ≥10
Raingarden, infiltration 1 ● ● ● ● ● ● 0.25 3 ≥5 ≥10
Swale, infiltration 1 ● ● ● ● ● 0.25 3 ≥5 ≥10
Planter, filtration 2 ● ● ● ● ● ● ● ● NA NA ≥5 NA
Raingarden, filtration 2 ● ● ● ● ● ● ● ● NA NA ≥5 NA
Swale, filtration 2 ● ● ● ● ● ● ● NA NA ≥5 ≥10
Water Quality Vault8 2 ● ● ●6 NA NA
Detention pond 3 ● ● ● ● NA NA ≥20 ≥20
Detention tanks & vaults5,8 3 ● ● NA NA NA NA
Infiltration pond9 3 ● ● ● ● ● ● 0.25 5 ≥20 ≥20
Proprietary treatment 8,9 3 ● ● ●6 ●6 ●7 —7 —7 —7 —7
Retention pond9 3 ● ● ● ● ● ● NA NA ≥20 ≥20
Sand filter8 3 ● ●6 ●6 0.25 5 ≥5 ≥10
Vegetated Filter strip9 3 ● ● ● 0.25 3 ≥5 ≥10
Wetland, Constructed 3 ● ● ● ● ● NA NA ≥5 ≥10
1 – Does not include middle housing projects on property zoned as low density residential; 2 – Restrictive layers include
bedrock, expansive clays, or groundwater; 3 –Measured from outside edge of facility; 4 – Needs written approval of UIC permit
manager; 5 – Engineering Design Standards contains requirements; 6 – Needs prior approval from City Engineer and Public
Works Director; 7 – Dependent on the Washington TAPE GULD requirements; 8 – Prohibited for middle housing; 9 – Cannot be
used to meet hierarchy requirements of Section 7.2
NA = not applicable; SFR=Single Family Residential
- 52 -
8 STORMWATER MODELING
Modeling is used by stormwater professionals to determine the required facility size and to show, for
projects required to provide Flow Control, that the post-development discharge will not exceed the pre-
development discharge.
Stormwater facility design must be completed by a civil engineer (PE) or a professional hydrologist with
a minimum of 4 years of hydrologic and hydraulic experience. Facilities sized with prescriptive sizing
factors do not require a licensed professional but must show the calculation on the stormwater plan
sheet and in a basic stormwater report. Revised stormwater reports using approved models are required
if deviations from the prescriptive sizing requirements occur during construction. The design storms
used in modeling a project are listed in the table below.
Table 7. Design Storms and Precipitation Depths
Design Storm (24-hr Recurrence Interval) Precipitation Depth1, inches
Water-Quality 1.2
2-year 2.1
5-year 2.8
10-year 3.3
1 – Precipitation depths based on data from Portland HYDRA network’s rain gage at PCC-Sylvania station (1976-2019)
8.1 APPROVED MODELS
The City allows models that use continuous simulation or single event models using the Santa Barbara
Urban Hydrograph (SBUH). The design professional must use a model version that was released within
the last 3 years.
Models using hydrologic soil types, rather than infiltration rates, are not allowed.
Prescriptive sizing uses a simple equation and a conservative sizing factor. It was originally created to
help homeowners and small contractors provide stormwater facilities for relatively simple projects such
as a backyard patio. While available for SFR projects treating up to 6,000 sq ft, it may result in a much
larger stormwater facility than if the project used a model.
Single-Event Models Santa Barbara Urban Hydrograph
Single-event stormwater models based on the SBUH method are allowed for modeling proposed
stormwater systems. Single-event models using SBUH create a hydrograph for a single 24-hr storm
event using the precipitation depth, amount of impervious and pervious areas, and a curve number that
Non-SFR and public stormwater facility design must use a single-event or continuous simulation
model for stormwater design.
- 53 -
represents a land-use. Applicable to urban areas, it calculates the stormwater volume and shows peak
flows and their duration (Debo and Reese 2003). It assumes that the system it is modeling is empty and
that soil conditions have average moisture conditions. An example of a single-event model is HydroCAD.
A Type 1-A 24-hr rainfall distribution is required when using a single-event
model.
The Presumptive Approach Calculator (PAC) cannot be used for designing
stormwater facilities in in the city because of differences between the
design storms, assumptions, and requirements in the City of Lake Oswego
and City of Portland’s stormwater manuals.
Continuous Simulation Models
Continuous simulation models based on the HSPF method are allowed for the design of stormwater
systems. More complex than the single-event method, the continuous-simulation method models
precipitation from multi-day or back-to-back storms which are more typical of storm events in the
Pacific Northwest. Stormwater peak flows and peak flow durations are more accurately predicted
because information from surface runoff, shallow interflow, and infiltration are integrated into the
models. Continuous simulation models take into consideration that a recent storm event may still be
impacting the stormwater system and that the system does not have its full capacity available for the
storm event. Examples of continuous models include EPA-SWMM, HEC-HMS, and others.
Continuous simulation models must use the last 20 years of precipitation recorded at the Portland
HYDRA network’s rain gage at PCC-Sylvania station.
Prescriptive Sizing
Prescriptive sizing may be used for rain gardens or stormwater planters on SFR Projects when design
infiltration rates that are less than 0.5 inches/hour. The prescriptive sizing calculation must be included
in a basic stormwater report (See Appendix A) submitted to the Engineering Development Review staff
for review and approval.
Multiplying the sizing factor by the amount of impervious area to be treated results in the required
surface area of the facility (see Table 8). A total of 6,000 sq ft of impervious area on a SFR property can
be sized using this method.
Table 8. Prescriptive Sizing Factors
Design Infiltration Rate Raingarden Planter
0.00 in/hr to <0.25 in/hr1 9% 6%
0.25 in/hr to <0.50 in/hr2 8% 5%
1 – Flow-through (filtration) facilities; 2 – Infiltration facilities
Stormwater planters and raingardens designed with prescriptive sizing must adhere to the following
design specifications during construction:
• Ponding depth of 1 foot plus a 3-inch freeboard
The City of Portland’s
PAC is not allowed for
designing stormwater
facilities in the City of
Lake Oswego.
- 54 -
• Treatment soil depth of 1.5 ft
• Storage course of 1 ft plus a 3-inch choker course between the storage course and the soil
Raingardens must be constructed with a 3H:1V side slope or flatter and a 2-ft minimum bottom width
and length. Planters must have a minimum interior width of 3 ft.
In addition, facilities that are prescriptively sized must adhere to the following requirements:
• An orifice diameter of 0.5 inches is required for filtration facilities
• An underdrain is required for flow-through (filtration) facilities
• An open bottom must be constructed for infiltration facilities
8.2 MODEL PARAMETERS
Model parameters include the amount of impervious and pervious areas, curve numbers, and infiltration
rates. The total impervious area used for the model must equal the impervious area in the stormwater
report and on the construction plan set.
The basins used in the pre-development model must provide a true representation of the stormwater
flow path. Basins used for the post-development scenario must replicate the proposed stormwater
system.
Void ratios must be 40% for drain rock and 25% for soil mixes unless documentation is provided to
Engineering Development Review staff that soil or rock gradations are different. A material ticket must
be submitted to staff before the final ESC inspection for all stormwater designs confirming that the
different void ratios are representative of the actual soil and rock used in the facility.
Curve Numbers
Curve numbers (CNs) represent the land cover and infiltration capacity present at the site. When
modeling the pre-development scenario, the stormwater professional must use a curve number of 70
for the site. For impervious areas, the CN is 98.
Weighted curve numbers are not allowed due to their significant under-estimation of stormwater
volumes (Cahill, 2012). Curve numbers and time of concentrations must be provided in the narrative of
the stormwater report.
A curve number of 70 must be used when calculating pre-development flow rates.
A revised stormwater report must be submitted to the Engineering Development Review staff for
review and approval if the constructed stormwater facilities do not comply with the prescriptive
sizing specifications. The report must use a continuous or single-event model to show that the
facility, as constructed, meets the SWMM requirements.
- 55 -
Time of Concentration
Time of concentration is the time it takes for runoff to reach the point of discharge from the point in the
basin of interest that is most hydraulically distant. It is affected by the slope, land use, and length. It is
composed of sheetflow, shallow concentrated flow, and channel flow. Equations to calculate the time of
concentration are available in many basic hydrology manuals. The TR20 or TR55 equations can be used
to estimate time of concentration when using the SBUH method.
The time of concentration used in the pre-development model must be a true representation of the site;
for example, a 100-ft flow path on a flat site is approximately 9 minutes.
Infiltration rates
The measured infiltration rate must have a correction factor of 0.5 applied to obtain the design
infiltration rate. The infiltration rate used in the model must be the design infiltration rate. The
maximum design infiltration rate is 10 inches/hour.
For flow-through facilities, the measured infiltration rate is the infiltration rate of the soil. The soil used
must be documented in the stormwater report. A revised stormwater report must be submitted to the
Engineering Development Review staff for review and approval if the infiltration rate of the facility’s soil
differs from the design infiltration rate by 10% or more.
Orifices
Flow control is achieved using an orifice sized to match the pre-development flowrates. The City allows a
minimum orifice diameter of 0.5 inches for vegetated facilities. The minimum orifice diameter for other
stormwater facilities is 1 inch for private facilities and 2 inches for public facilities.
Orifices less than 3 inches must be made of stainless steel, HDPE, or PVC. The orifice diameter must be
greater than or equal to the thickness of the orifice plate.
8.3 FLOW CONTROL
All Large Projects must provide Flow Control unless they can provide onsite retention of the 10-yr 24-hr
storm event. Flow Control requirements are met when the post-development flowrates are less than or
equal to the pre-development flowrates for the 2, 5, and 10-yr 24-hour design storm events.
Detention tanks and vaults cannot be used for flow control on SFR projects, middle housing projects, nor
to fulfill public improvements required as a condition of private development.
Exemptions
Flow Control is not required if the project meets all the following requirements:
Design Infiltration Rate1 = Median Measured Infiltration Rate X 0.5
1 – Maximum is 10 inches/hour
- 56 -
• Discharge is directly to the Tualatin River, Willamette River, or Oswego Lake,
• Discharge does not divert water that would normally go to wetlands,
• Discharge flows only through underground conveyance structures (pipes, catch basins, or
manholes) which extend to the high-water mark of the Tualatin River, Willamette River, or
Oswego Lake,
• The capacity of the downstream public stormwater system plus the Project’s offsite discharge is
sufficient to receive the discharge (maximum capacity of 80%), and
• Stormwater has been treated to the extent required (Section 7.5 – Stormwater Facility Selection
Water-Quality Limited Waterways).
Flow Control Procedure
Small and Large projects must provide onsite retention to the extent possible or treat the water-quality
24-hr storm event (see Figure 11). All projects with an offsite discharge are required to provide
extended filtration and reduce concentrations of pollutants to water-quality limited waterways (see
Section 7.5 Stormwater Facility Selection – Water-Quality Limited Waterways).
Large projects must provide Flow Control unless they discharge directly to an exempted water through
an underground stormwater system (manholes, pipes, catch basins, and inlets) and the offsite discharge
will not cause the downstream public stormwater system to exceed 80% capacity. If the offsite
discharge will cause the downstream stormwater system capacity to be ≥ 80% then a downstream
analysis must be completed. The capacity analysis must follow the steps in Section 8.4 (Stormwater
Modeling – Downstream Capacity Analysis).
Large projects not discharging to an exempt waterway must compare post-development peak flowrates
from the 2,5, and 10-yr 24-hr design storms with their respective pre-development flowrates as follows:
A. If the post-development peak flowrates are less than or equal to the pre-development peak
flowrates, then the design is complete if the offsite discharge meets the requirements of
Chapter 7.
B. If the post-development peak flowrates are more than the pre-development peak flowrates,
then the stormwater facility must be sized to treat the 10-yr 24-hr design storm and meet the
requirements of Chapter 7. The pre-development flowrates from the new design must be
compared with their respective post-development flowrates. If the post-development peak
flowrates are less than or equal to 110% of their respective pre-development flowrates, then
the design is complete if the offsite discharge meets the requirements of Chapter 7.
C. If the post-development flowrates are more than 110% of their respective pre-development
flowrates then the stormwater facility must be sized to treat the 25-yr design storm, provide
maximum flow control (e.g. use a 0.5” orifice), meet the requirements of Chapter 7, and
complete a downstream analysis.
- 57 -
Figure 11. Stormwater Design
Notes: 1 – Offsite discharges to water-quality limited streams must be treated to reduce pollutants as required in Section 7.5 (Water-Quality Limited Waterways).
2 – Post-Development Flowrate must be less than 110% of the Pre-Development Flowrate for each design storm event.
3 – See Section 8.4 Downstream Capacity Analysis
Post = Post-development off-site discharge; Pre = Pre-development off-site discharge; WQ= Water-Quality
- 58 -
8.4 DOWNSTREAM CAPACITY ANALYSIS
A downstream capacity analysis must be completed for Large Projects:
• That are discharging to an exempt water through the public stormwater system OR
• When the post-development flows are greater than the pre-development flows (+10%), flow
control is being implemented to the maximum extent possible, e.g. using a 0.5” orifice, and the
25-yr 24-hour storm is being treated (see Figure 11).
The downstream analysis must use the following parameters to determine the capacity of the public
system:
• Built-out conditions using the City’s most recent Comprehensive Plan or current zoning,
whichever contains the greater amount of lot coverage.
• Maximum lot coverage for properties where middle housing is allowed (see LOC 50.04 for
allowed types and lot coverages for middle housing).
• Type 1A rainfall distribution when using single-event models.
• The last 20 years of precipitation recorded at the Portland HYDRA network’s rain gage at PCC-
Sylvania station when using continuous simulation models.
• Design infiltration rate determined during the site assessment phase of the project.
• Offsite infiltration rates using the following hierarchy:
o Infiltration results listed in existing geotechnical reports.
o Soil types determined by existing geotechnical reports.
o Median infiltration rate of soil types as determined by the NRCS.
For Large Projects that discharge to a non-exempt water, if the offsite discharge causes the downstream
system to be 80% or more of its capacity then the applicant must provide a public improvement (see
Figure 12). If the capacity of the existing stormwater system already exceeds 80% then the applicant
must provide a public improvement.
The use of weighted curve numbers in a capacity analysis is prohibited.
NON-EXEMPT WATERWAYS
A public improvement is required if the existing public stormwater system’s capacity ≥ 80% of
the existing public stormwater system’s capacity OR the offsite discharge + the existing public
stormwater system’s discharge is ≥80% of the existing public stormwater system’s capacity.
- 59 -
Figure 12. Downstream Capacity Analysis
Existing = Pre-development flow in existing public system; Post = Post-development off-site discharge; Pre = Pre-
development off-site discharge.
- 60 -
Large Projects that discharge to an exempt waterway must ensure that the downstream stormwater
system has sufficient capacity to manage the additional flow created by the project. If the post-
development public stormwater system’s capacity is 80% or more then the applicant must provide a
public improvement. The applicant must provide Flow Control (see Section 8.3) if the pre-development
public stormwater system’s capacity is already at 80% or greater.
The downstream analysis must be completed for each of the 24-hr design storms in Table 9. The
analysis must continue downstream to the following location, whichever occurs first:
• The public stormwater system discharges to a waterway OR
• The project’s offsite discharge is less than 10% of the post-development flow in the public
stormwater system.
Table 9. Design Storms for Downstream Analyses
Recurrence Interval Precipitation, inches
25-yr 3.84
50-yr 4.27
100-yr 4.68
Public improvements must extend to a point at which the offsite stormwater discharge is less than 10%
of the post-development flow in the public stormwater system or until the public stormwater system
discharges to a waterway.
EXEMPT WATERWAYS
If the existing public stormwater system’s capacity is already 80% then flow control is required. If
flow control cannot be achieved OR if the resulting flow creates a capacity ≥80% in the public
system then a public improvement is required.
- 61 -
9 STORMWATER FACILITY DESIGN
The design and construction of stormwater facilities must follow specific standards to achieve the water-
quality requirements of the City’s MS4 permit, UIC permit, and TMDL allocations. The standards can be
grouped into those that are generally the same for a facility type (see Section 9.1 – General Design
Standards) and those that are specific to a facility (see Section 9.2 – Specific Design Standards). Both
types of standards must be incorporated into the stormwater design. If a conflict occurs between the
general and specific design standards, the requirement that is most protective of water quality applies
to the design.
The City’s standard details must be used when the construction plans
are submitted to the Engineering Development Review staff for review
and approval. A non-City detail may be used if one is not available from
the City, however the submitted detail is subject to changes by the City
Engineer. The decision of the City Engineer is final.
Stormwater facilities must be designed to be empty within 72 hours of the end of a storm.
9.1 GENERAL DESIGN STANDARDS
Most of the approved stormwater facilities can be grouped into types such as vegetated facilities, UICs,
regional facilities, and proprietary treatment. Each facility type has a set of design requirements that are
the same regardless of the specific facility within the facility type.
Vegetated Facilities
Vegetated facilities include raingardens, planters, swales, and vegetated filter strips. All these facilities
can be used to meet the landscaping requirements of development (see LOC Subarticle 50.06.004).
9.1.1.1 Vegetation
Vegetation must provide 100% coverage at plant maturity and include at
least 3 plant families to avoid catastrophic die-off within the facility.
Perennial forbs may be used for 10% of the plant palette to assist with
aesthetics. Small trees may be used to provide up to 50% of the facility
coverage if the soil depth is increased to three feet. Turf grass is
prohibited from use in vegetated facilities. Appendix C provides
additional information on plant selection.
Plants must be chosen from the approved plant list (see Appendix C), be
in potted containers (minimum 1-gallon) or be ball and burlapped, and
be installed with the root crown level with the ground surface. Root-
bound plants are prohibited.
Plants must have a staggered spacing to maximize flow dispersal and increase plant viability (see Figure
13).
Failure to use the City’s
standard details, when
available, will result in
an incomplete submittal.
Plants in a facility must
represent at least 3
plant families. It is not
appropriate to plant
three types of caryx
however the criteria is
met when planting a
combination of caryx,
deschampia, and juncus.
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Figure 13. Planting requirements for vegetated facilities
ROOT CROWN LEVEL WITH FINISH GRADE
- 63 -
Potted plants must be installed between October 1st and April 30th. Plants may be installed from May to
September but must have temporary irrigation installed to ensure plant viability. Public stormwater
facilities constructed as part of a public improvement or capital improvement project must have
permanent irrigation.
9.1.1.2 Soil
Soil used in stormwater facilities must be a
loam and can have up to 30% fines (see
Figure 14). It must contain a 10% biochar
additive. Biochar suppliers can be found at
www.pnwbiochar.org.
Soil depths for planters, raingardens, and
swales can range from 18 inches to 24
inches, however if trees are incorporated,
the soil depth must be increased to 36
inches. A 6-inch course of soil must be tilled
into vegetated filter strips prior to planting.
Shredded mulch, up to a depth of 2” may
be used to increase plant viability in the
summer but cannot decrease the ponding
depth. If mulch is used, a 3-inch area
around the overflow must be rocked to
prevent mulch from entering the overflow
pipe. Bark nuggets and non-shredded mulch
is prohibited in vegetated facilities because
of their propensity to be transported into
the overflow pipe during storm events.
9.1.1.3 Inlets
Inlets for vegetated facilities adjacent to driveways, flatwork, or streets must be either a curb inlet (see
Standard Detail SD9-02) or an inlet catch basin (see Standard Details SD5-03A or SD5-03B). All inlets
must be at an elevation that is above the facility’s overflow elevation to prevent backwater from
occurring.
Permanent irrigation is required for vegetated public stormwater facilities.
To protect plants from heat-island effects, the use of a top layer of rock in
vegetated facilities is prohibited.
Figure 14. Soil classification
From: Soil Health – Soil Texture and Structure, NRCS Factsheet at
nrcs.usda.gov
Sand Separate, %
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Waterproof connections are required for inlets that are cored through the side of a facility.
All vegetated facilities must provide energy dissipation at the inlet. For raingardens, planters, and
swales, the energy dissipation must be clean rock (1-½” to ¾”) or a concrete pad with 1-inch-tall edging.
Vegetated filter strips must use either a 1ft x 0.5ft rock strip or a non-galvanized 6-inch-wide trench
drain that extends to the edges of the facility to evenly disperse flow. Trench drain grates must have a
load designation following AASHTO M306 standards. A load class B is required for residential driveways
and landscapes, C for commercial applications, D (H20) for industrial applications, and E (HS25) for areas
subject to loads up to 50,000 pounds or 620 psi. Galvanized trench drains are not allowed because of
durability and water-quality concerns.
9.1.1.4 Storage Courses
A storage course may be used for raingardens, swales, and planters. It can range in depth from 12 inches
to 24 inches but must be comprised of clean rock (1-½” to ¾”) that complies with Section 00430 in the
current Oregon Standard Specifications for Construction (ODOT/APWA, 2024 or later).
A 3-inch choker course must separate the soil course from the storage course. It must be comprised of
clean rock (¾” to ½”) that complies with Section 00430 in the current Oregon Standard Specifications for
Construction (ODOT/APWA, 2024 or later).
9.1.1.5 Underdrains
Underdrains are required for filtration facilities and must consist of a perforated pipe that extends a
maximum 2/3 of the length from the overflow to the first inlet. If the overflow is in the center of the
facility, the pipe must not extend no longer than 1/3 of the length of the facility on either side. For
multiple-celled facilities, the underdrain must be installed only in the lowest cell.
A minimum 4-inch diameter perforated pipe must be used for private facilities and a minimum 6-inch
diameter pipe must be used for public facilities.
All pipe connections exiting through a vegetated facility must be waterproof.
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9.1.1.6 Liners
A 30-mil PVC or HDPE liner must be installed in filtration
facilities and extend to a height equal to the ponding
depth. A HDPE liner with equivalent or better
specifications may be used to replace a PVC liner. Spray-
on liners are prohibited from use in stormwater facilities.
In planters, the liner must cover the bottom and the
sides. It must be attached using stainless steel concrete
anchors and a non-rusting aluminum flat bar (2” x 1/8”).
Caulk must be used to prevent water from infiltrating
behind the bar. In filtration raingardens and swales, the
liner must extend to the ponding depth of the facility and
be staked into a 1 ft wide dirt shelf. A 1-ft deep layer of
dirt must be placed on top of the liner to secure it in
place.
9.1.1.7 Outlets
The outlet from a swale, planter, or raingarden must be a beehive structure (see Standard Details SD8-
01 and SD8-02) that is located as far as possible from the inlets. The overflow elevation must be below
the elevation of the lowest inlet. For multiple-celled facilities, the overflow must be in the lowest cell. In
vegetated filter strips, the outlet can be a non-galvanized non-plastic trench drain that discharges to an
approved point of discharge.
9.1.1.8 Construction of Vegetated Facilities
The following notes are required with the construction plan submittal and must be followed during
construction:
• Construction fence must protect the facility footprint plus a 10-ft buffer around it.
• Stormwater must be diverted away from the facility footprint during construction.
• Prior to constructing the facility, the native soil must be scarified to a depth of 6 inches.
Underground Injection Controls
UICs approved for use in the city include infiltration trenches, infiltration galleries, and drywells. All
property owners of UICs are required to notify and obtain DEQ approval of the system. The applicant
must provide a copy of the approval to the City with the post-construction documents (see Section 4.3
Submittals – Post-Construction Submittals).
Owners of UICs that receive stormwater only from SFR roofs are not required to notify DEQ but must
comply with all setback requirements and other UIC regulations.
Spray-on liners are prohibited from use in stormwater facilities.
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UICs may not be used to satisfy public improvement requirements. CIP projects may incorporate UICs
only if no other stormwater facility is feasible. A feasibility analysis must be completed and provided in
the stormwater report. The UIC permit manager must agree in writing to the construction of the UIC.
9.1.2.1 Pretreatment
Pretreatment is required by DEQ prior to discharging stormwater to a UIC. Pre-approved treatment
includes the use of vegetated facilities or sumped catch basins with a water-quality snout. SFR projects
can use a sumped catch basin with a pollution control box. If using a catch basin for pretreatment, the
minimum sump is 18 inches for SFR projects and 36 inches for non-SFR projects. The water-quality snout
or pollution control box must be attached to the outlet pipe.
Cleanouts, vertical standpipes, and PVC pipes cannot be used for pretreatment. Catch basin grates larger
than 2.5 ft X 2 ft cannot be used for SFR projects because the weight of the grate makes maintenance
difficult to complete.
Catch basins used for pretreatment of UICs but placed in landscaping must have a solid top to protect
water quality and reduce the volume of mulch and yard debris entering the system.
9.1.2.2 Maintenance Access
Maintenance access is required for all UICs. Drywells must be installed with a cover located at-grade to
provide maintenance access. Infiltration trenches must include a minimum 6-inch diameter inspection
port. Infiltration galleries must provide a minimum 6-inch diameter inspection port for Small Projects
and a minimum 10-inch diameter inspection port for Large Projects.
9.1.2.3 Construction of UICs
The following notes are required with the construction plan submittal and must be followed during
construction:
• Construction fence must protect the facility footprint plus a 10-ft buffer around it.
• Stormwater must be diverted away from the facility footprint during construction.
• Prior to constructing the facility, the native soil shall be scarified to a depth of 6 inches.
Regional Facilities and Shared Facilities
Ponds and wetlands have traditionally been used to serve more than one property and are allowed
when maintenance responsibility is delegated to a Homeowners Association. Other types of shared
facilities are not allowed due to the complexities of shared maintenance among unaligned homeowners.
Facilities that discharge underground, without first allowing infiltration from the ground
surface, are UICs even if the facility is not normally considered a UIC.
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9.1.3.1 Existing Facilities
Existing regional facilities can be used to fulfill flow control requirements for a project if stormwater
modeling shows that the existing facility has capacity for the proposed stormwater volume and all other
stormwater currently being discharged to it. New projects must still comply with the onsite retention
and water-quality treatment requirements of the SWMM before discharging to the existing facility.
When using an existing regional facility, the applicant must provide a copy of a recorded O&M Plan as
part of the post-construction submittals. It must include all property owners discharging to the facility as
the responsible parties and their signatures.
9.1.3.2 Vegetation
A minimum of nine plant species are required for the treatment area. The facility must be designed so
that it does not require mowing.
Use woody vegetation to provide shade over standing water and to provide structural diversity
surrounding the pond. Maintain a 20-foot minimum distance between hydrophilic trees and shrubs (e.g.,
Oregon ash, alder, willows, and dogwoods) and inlets or outlets to prevent roots from blocking
structures or obstructing maintenance efforts. Trees and shrubs cannot be planted on berms that are
four feet or taller. Trees and shrubs with a fibrous root system (no tap roots) can be planted on berms
shorter than 4 feet. A fibrous root system reduces chances of blowdown, piping, and the creation of
preferential flow paths.
Trees and shrubs must be planted on the south and west sides of a facility, when it is feasible, to lower
water temperatures.
9.1.3.3 Spillways
All ponds and constructed wetlands must provide an emergency overflow route (spillway) that will
convey the 100-yr storm event over the facility’s embankment. A downstream analysis is required for all
spillways to ensure that they do not cause or exacerbate flooding downstream. Additional information is
provided in Section 8.4 (Stormwater Modeling – Downstream Capacity Analysis).
The approved discharge point must be the public stormwater system.
The minimum width of a spillway is 6 ft. The invert elevation of the spillway must be at least 1 foot
above the primary overflow outlet elevation but 1 foot below the top of the embankment.
Spillways must be constructed of concrete or riprap with an apron at the bottom that can contain and
disperse hydraulic jumps that may occur during use. The stormwater report must provide information
on the size and location of hydraulic jumps.
Spillways must be designed by a licensed civil engineer (PE) with at least 4 years of
hydrologic and hydraulic experience.
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9.1.3.4 Other Requirements
The Flow Control structure, or orifice, in a regional facility must be at an elevation that detains the
design storm. A ditch inlet with a box frame and grate (Standard Details SD6-01A and SD6-01C) must be
used as the outlet structure. Overflows must connect directly to the public stormwater system.
Ponds (private and public) must have access roads capable of carrying maintenance vehicles (such as a
track hoe and truck). A 4-ft fence and access gate around the pond is required if the ponding depth is
over 2 feet. The Engineering Design Standards provide additional requirements for access roads.
Staff gauges are required at the inlet and outlet.
Vector control is an important design consideration for any facility that has standing water for extended
periods of time. Bat boxes, diverse planting and other design strategies to encourage biological controls
can help to keep vector populations in balance.
Proprietary Stormwater Treatment
Proprietary stormwater treatment devices may be used for non-SFR projects, except for middle housing
projects, if all the following requirements are met:
• The device is approved by the Washington Department of Ecology (DOE) TAPE (Technology
Assessment Protocol) program and has received a general use level designation (GULD) for the
water quality treatment required for the project.
• The facility design adheres to the Washington DOE TAPE constraints placed on the facility type.
• The Washington DOE TAPE constraints are provided in the stormwater report and construction
plans.
• The applicant has received written approval from the City Engineer during the land-use process
or prior to the submittal of the stormwater report, whichever occurs first.
The City Engineer may require additional conditions for the projects using proprietary treatment. Their
decision is final.
9.2 SPECIFIC DESIGN STANDARDS
The following design standards are required in addition to the general design standards specified in the
previous section.
Section Stormwater Facility Section Stormwater Facility
9.2.1 Drywells 9.2.7 Sand Filters
9.2.2 Permeable Pavers 9.2.8 Swales
9.2.3 Porous Pavement 9.2.9 Trenches and Galleries
9.2.4 Planters 9.2.10 Vegetated Filter Strips
9.2.5 Ponds 9.2.11 Water Quality Vaults
9.2.6 Raingardens 9.2.12 Wetlands
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Drywells (Standard Detail SD1-02)
A drywell is an underground perforated concrete cylinder or
vault that discharges stormwater through its perforations and
into the surrounding soil.
9.2.1.1 Dimensions
Maximum Depth: 16 feet
Maximum Width: 4 feet
9.2.1.2 Materials
Drywells must be precast concrete that conforms with ASTM
C478 (ASTM, 2022).
A minimum 12-inch-wide ring of clean rock (1-½” to ¾”) must be
placed between the perforated section of the drywell and the
surrounding soil and extend the length of the structure. The
rock must comply with Section 00430 in the current Oregon
Standard Specifications for Construction (ODOT/APWA, 2024 or
later).
The bottom of the drywell must have a minimum of 12 inches of densely graded base aggregate (¾” to
0” ) placed under the bottom of the drywell to create a level base. It must comply with Section 02630 in
the current Oregon Standard Specifications for Construction (ODOT/APWA, 2024 or later).
9.2.1.3 Other Requirements
Pretreatment must comply with the UIC
requirements in Section 9.1.2 (General Design
Standards – Underground Injection Controls).
Soil adjacent to the drywell’s drain rock must be
scarified and uncompacted.
To provide access for maintenance, the top of the
drywell must be at ground surface. A lockable lid
is required for private facilities. A manhole cover
is required for public facilities and optional for
private facilities.
Applicability Table
Tier 1
Onsite Retention ✓
Flow Control
Water Quality Treatment
Extended Filtration
Capital Improvement1 ✓
Public Improvement
Single-Family Residential ✓
Prescriptive Sizing
1 – With prior approval of City’s
UIC permit manager
Additional requirements are listed in the general design requirements (Section 9.1.2 General
Design Standards – UICs).
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Porous Pavement (Standard Detail SD9-09)
Porous pavement is a surface designed to infiltrate or treat
precipitation that falls on it. It is not designed to treat or
infiltrate stormwater from other impervious areas. An underlying
storage reservoir beneath the pavement can be used for
temporary storage.
Porous asphalt is an open-graded asphalt that allows precipitation
to infiltrate into underlying soil. It uses a hot or warm asphalt mix
where the percentage of fines is reduced from an impervious
asphalt mix to create a pavement with more voids that can
provide infiltration.
Pervious concrete is a concrete with interconnected voids (15 to
33%) that allows water to flow through the material under
gravity (ACI, 2023). It is comprised of open-graded coarse
aggregate, cement, little or no fine aggregate, and water.
9.2.2.1 Location
Porous pavement can be used for pathways, access roads (maintenance or emergency), and driveways.
Porous pavement cannot be installed:
• In the public ROW without written City Engineer approval obtained during the land-use process
or the submittal of construction plans, whichever occurs first
• Over culverts or bridges
• Over areas of fill
• Above the ground floor of a multi-level parking area
• In areas of frequent sharp turns, i.e., parking lots
• In areas of frequent winter maintenance where de-
icing or anti-icing materials are used e.g.
magnesium chloride, sand, gravel, rock salt
• At industrial sites
• At commercial sites that use or store
petrochemicals or hazardous materials
9.2.2.2 Slopes
Slopes must be 6 percent or flatter. Slopes ≥ 3% must use a
storage course with check dams or concrete weirs to
reduce velocity towards the downslope area.
Applicability Table
Tier 1
Onsite Retention ✓
Flow Control
Water Quality Treatment ✓
Extended Filtration
Capital Improvement1 ✓
Public Improvement
Single-Family Residential ✓
Prescriptive Sizing
1 – Requires prior City Engineer approval
- 71 -
9.2.2.3 Materials
Porous pavement located adjacent to a road or building foundation must have a 30-mil PVC or HDPE
liner or cast-in-place concrete curb that extends beyond the depth of the road base or building
foundation.
A 3-inch leveling course must separate the wearing course from the storage course. It must be
comprised of clean rock (¾” to ½”) that complies with Section 00430 in the current Oregon Standard
Specifications for Construction (ODOT/APWA, 2024 or later).
A storage course is optional for porous pavement and can range from 0.5 ft to 2 ft in depth. If used, it
must be a clean rock (1-½” to ¾”) that complies with Section 00430 in the current Oregon Standard
Specifications for Construction (ODOT/APWA, 2024 or later).
Check dams, if required, must be half the height of the storage course and be comprised of clean rock
(¾” to ½”) that complies with Section 00430 of the current Oregon Standard Specifications for
Construction (ODOT/APWA, 2024 or later).
9.2.2.4 Other Requirements
Porous asphalt design must follow Section 00743 (Porous Asphalt Concrete) in the current Oregon
Standard Specifications for Construction (ODOT/APWA, 2024 or later).
Pervious concrete projects must be designed by a licensed civil engineer (PE) with a minimum of 4 years
of experience designing pervious concrete systems. The specifications and their source must be
submitted with the stormwater report.
Underdrains are not permitted in porous pavement systems.
While important for all stormwater facilities, proper installation and regular maintenance is critical for
these facilities.
9.2.2.5 Construction Notes
During construction, the location must be protected from sediment and runoff.
The subgrade must be protected from compaction, truck traffic, material storage, and construction
equipment.
Permeable Pavers (Standard Detail SD9-10)
Permeable paving stones are set into a high-infiltration material such as sand.
Permeable interlocking concrete pavement (PICP) are solid, precast, modular units that are made of
high-strength Portland Cement concrete and placed on an open-graded bedding course. Joints are filled
with clean aggregate.
- 72 -
Porous grid systems are used for pedestrian and low traffic areas such as patios, driveways, emergency
access lanes, and parking areas used for temporary (overflow) event parking. A high-strength concrete
(5,000 psi or greater) or plastic grid is filled with pea gravel or sand.
Permeable paver designs treat and infiltrate only the precipitation that falls on them. Stormwater from
other impervious areas is not allowed to flow onto them.
Permeable pavers are not suitable:
• In the public ROW unless pre-approved by the City Engineer in
writing;
• Over culverts or bridges;
• Over areas of fill;
• Above the ground floor of a multi-level parking area;
• In areas of frequent winter maintenance where de-icing or
anti-icing materials are used e.g. magnesium chloride, sand,
gravel, rock salt
• At industrial sites; or
• At commercial sites using or storing petrochemicals or
hazardous materials.
9.2.3.1 Slopes
Slopes must be 6 percent or flatter. Slopes ≥ 3% must use a storage course with check dams or concrete
weirs to reduce velocity towards the downslope area.
9.2.3.2 Materials
Permeable pavers located adjacent to a street,
road, or building foundation must have a 30-mil
PVC or HDPE liner or cast-in-place concrete curb
that extends beyond the depth of the road base
or building foundation.
Edge Restraints
• To prevent lateral migration of paving
stones and PICPs, an edge restraint is
required.
• The edge restraint for SFR applications must
be either a spiked metal edge restraint or a
cast-in-place curb. The edge restraint for
non-SFR applications must be a cast-in-
place curb that is at least 6 inches wide and
12 inches deep.
Applicability Table
Tier 1
Onsite Retention ✓
Flow Control
Water Quality Treatment ✓
Extended Filtration
Capital Improvement1 ✓
Public Improvement
Single-Family Residential ✓
Prescriptive Sizing
1 – Requires prior City Engineer approval
- 73 -
Leveling Course
• The leveling course must be 1 to 3-inches thick and consist of clean sand.
Choker Course
• A 3-inch course of clean rock (¾” to ½”) that complies with Section 00430 in the current Oregon
Standard Specifications for Construction (ODOT/APWA, 2024 or later) must be placed between
the leveling course and the storage course.
Storage Course (optional)
• The storage course must be a clean rock (1-½” to ¾”) that complies with Section 00430 in the
current Oregon Standard Specifications for Construction (ODOT/APWA, 2024 or later).
• The depth for paving stones must range between 6 inches to 36 inches. It must provide sufficient
support for vehicles that will use the area.
• The depth of the storage course for PICPs can vary from 6 to 12 inches. Porous grid systems must
have a storage course of 6 inches.
• Check dams, if required, must be half the height of the storage course and be comprised of clean
rock (¾” to ½”) that complies with Section 00430 of the current Oregon Standard Specifications
for Construction (ODOT/APWA, 2024 or later).
9.2.3.3 Other Requirements
Underdrains are not allowed for permeable pavers.
While important for all stormwater facilities, proper installation and regular maintenance is critical for
these facilities.
9.2.3.4 Construction Notes
During construction, the location must be protected from sediment and runoff.
The subgrade must be protected from compaction, material storage, and construction equipment.
Planters (Standard Details SD9-12 and SD9-13)
Planters are vegetated facilities with four structural
walls. They require minimal space because of their
vertical walls. Filtration (flow-through) planters can be
constructed adjacent to building foundations.
9.2.4.1 Dimensions
Treatment areas must be at least 3 feet wide.
The minimum freeboard must be 6 inches for facilities
treating impervious areas that are 3,000 sq ft or
greater. The minimum freeboard for facilities treating
less than 3,000 sq ft is 3 inches. The maximum
freeboard is 12 inches.
The ponding depth must be between 0.5 ft and 1 ft.
Applicability Table
Tier
1(Infiltration
2 (Filtration)
Onsite Retention Infiltration Only
Flow Control Filtration only
Water Quality Treatment ✓
Extended Filtration Filtration only
Capital Improvement ✓
Public Improvement ✓
Single-Family Residential ✓
Prescriptive Sizing ✓
- 74 -
9.2.4.2 Materials
Infiltration planters located next to roads or streets must include a 30-mil PVC or HDPE liner on the side
that is closest to the street or road. Public facilities can use a 6-inch-thick concrete wall extending to the
bottom of the roadbase but it must be monopoured with the weirs or curb. The City’s inspector or
project engineer must be present at the pour.
A storage course is optional for an infiltration facility but required for a filtration facility.
An underdrain is prohibited for infiltration facilities but required for a filtration facility. Requirements
are provided in the general design criteria in Section 9.1.1 (General Design Standards – Vegetated
Facilities).
Planter walls and bottoms must consist of 6-inch-wide concrete with a 28-day compressive strength of
3,000 psi.
9.2.4.3 Other Requirements
Multi-celled planters (see Standard Detail SD9-01) can be
used on sloping properties. They are two or more flat areas
(cells) separated by weirs that allow water to drop with the
slope. All cells must have a dissipation pad at the upstream
side to reduce soil erosion when the stormwater cascades
from the upstream to the downstream cell. The beehive
overflow must be in the lowest cell of the facility.
Pedestrian safety and access must be incorporated into the
planter design. Ensure that planters located next to parking
areas have clearance for pedestrians exiting vehicles.
Water quality planters shall not be located downstream of
detention.
The building foundation must NOT be
used as a wall for a stormwater planter.
Additional requirements, such as storage courses and liners, are listed in the general design
requirements (Section 9.1.1 General Design Standards – Vegetated Facilities).
- 75 -
Ponds
Stormwater ponds are a good choice where there is
a large contributing area and where they can be
easily accessed for maintenance. They differ from
raingardens in size and ownership. Raingardens are
generally owned by one property owner whereas
ponds are generally owned and maintained by a
business or homeowner’s association (HOA).
Ponds approved for use include detention ponds,
infiltration ponds, and retention ponds.
Detention ponds temporarily store water for 24
hours and use an orifice to control the discharge rate
from the pond. There may be incidental infiltration,
but the soil is not adequate for infiltration.
Infiltration ponds temporarily store stormwater while it infiltrates into the soil. Infiltration ponds must
be sized for onsite retention.
Retention ponds have a permanent pool of standing water and must be designed with aeration to avoid
stagnation and minimize disease vectors during the dry summer months. A retention pond may be used
to provide Flow Control if they are designed with additional storage to provide detention of the design
storm. The orifice must be located at an elevation that is above the permanent pool.
9.2.5.1 Dimensions
Include two cells, with the first cell (sediment forebay)
containing approximately 10% of the design surface area. A
pollution control manhole can substitute as a sediment forebay.
Sides slopes must be 3H:1V or gentler.
The length to width ratio must be 3:1 or greater.
The length from the inlet to the outlet must be maximized to
provide the greatest residence time.
For retention ponds, the maximum permanent pool depth is 2
feet. The depth must not exceed 6 feet during the design storm.
Berm embankments must have a minimum top width of 6 feet.
Applicability Table
Tier 3
Onsite Retention
Infiltration and
Retention Only
Flow Control ✓
Water Quality Treatment
Infiltration and
Retention Only
Extended Filtration
Capital Improvement ✓
Public Improvement ✓
Single-Family Residential ✓
Prescriptive Sizing
- 76 -
9.2.5.2 Other Requirements
Ponds cannot be constructed in floodplains or in the area inundated by the 25-year storm for other
waterways.
Ponds must be designed by a licensed civil engineer (PE) with at least 4 years of hydrologic and hydraulic
experience.
Raingardens (Standard Details SD9-15 and SD9-16)
Rain gardens are vegetated facilities with gently sloping
sides. While generally circular in shape, they can adapt
to any shape. Linear rain gardens may look like swales;
however, swales gently slope longitudinally as water is
conveyed through the swale.
They are ideal for residential and commercial sites,
within parking lots, and along roadways.
Sculpture is allowed in a rain garden if the treatment
area is increased to compensate for the sculpture’s
footprint.
9.2.6.1 Dimensions
Raingardens must have a flat bottom of 2 ft x 2 ft with a
maximum cross-slope of 1%.
Sides slopes must be 3H:1V or flatter.
The ponding depth can range from 0.5 ft to 1 ft.
The maximum freeboard is 12 inches. The minimum freeboard must be 6 inches for facilities treating
impervious areas that are 3,000 sq ft or greater. The minimum freeboard for facilities treating less than
3,000 sq ft is 3 inches.
9.2.6.2 Other Requirements
The elevation of the overflow must be lower than the invert elevation of the inlets to the facility. The
design must maximize the distance between the overflow structure and the inlet structures.
Applicability Table
Tier
1 (Infiltration)
2 (Filtration)
Onsite Retention
Infiltration
Only
Flow Control Filtration Only
Water Quality Treatment ✓
Extended Filtration Filtration Only
Capital Improvement ✓
Public Improvement ✓
Single-Family Residential ✓
Prescriptive Sizing ✓
Additional requirements are listed in the general design standards (Section 9.1.3 General Design
Standards – Regional Facilities and Shared Facilities).
- 77 -
Multi-celled raingardens (see Standard Detail SD9-01) can be
used on sloping properties. They are two or more flat areas
(cells) separated by weirs that allow water to drop with the
slope. All cells must have a dissipation pad at the upstream
side to reduce soil erosion when the stormwater cascades
from the upstream to the downstream cell. The beehive
overflow must be in the lowest cell of the facility
Infiltration raingardens located next to streets and roads
must include a 30-mil PVC or HDPE liner on the side that is
closest to the road or street. Public facilities can use a 6-inch-
wide concrete wall extending to the bottom of the roadbase
but it must be monopoured with the weirs or curb. The
City’s inspector or project engineer must be present at the pour.
Sand Filter
Sand filters may be horizontal or vertical. Both require a flow spreader and a sand bed. Vegetation is
optional.
A horizontal sand filter is a shallow rectangular facility that allows stormwater to be filtered through a
sand and gravel bed. A vertical sand filter is a tall
rectangular facility that uses hydraulic head for
filtration through the sand bed
Public sand filters require written approval from the
City Engineer and the Public Works Director obtained
during the land-use process or prior to the submittal of
construction plans, whichever occurs first.
9.2.7.1 Dimensions
The width must be 2 feet or greater.
The length must range from 3 ft to 15 ft.
The depth of the sand course must be 1 foot or
greater.
The length to width ratio for horizontal facilities, or height to width ratio for vertical facilities, must be
2:1.
The minimum ponding depth is 1 foot.
Applicability Table
Tier 3
Onsite Retention
Flow Control
Water Quality Treatment ✓
Extended Filtration
Capital Improvement1 ✓
Public Improvement1 ✓
Single-Family Residential
Prescriptive Sizing
1 – With prior City Engineer approval
Additional requirements, such as storage courses and liners, are listed in the general design
standards (Section 9.1.1 General Design Standards – Vegetated Facilities).
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The minimum freeboard must be 6 inches for facilities treating impervious areas that are 3,000 sq ft or
greater. The minimum freeboard for facilities treating less than 3,000 sq ft is 3 inches. The maximum
freeboard is 12 inches.
The slope must range from 0 to 0.5% towards the overflow for horizontal facilities and towards the
center for vertical facilities.
9.2.7.2 Materials
The walls and bottom of a sand filter must be made of 6-inch-wide concrete with a 28-day compressive
strength of 3,000 psi.
A 3-inch choker course must separate the sand course from the storage course, with clean rock (¾” to
½”) that complies with Section 00430 in the current Oregon Standard Specifications for Construction
(ODOT/APWA, 2024 or later).
An optional storage course can range from 1 foot to 2 feet, with clean rock (1-½” to ¾”) that complies
with Section 00430 in the current Oregon Standard Specifications for Construction (ODOT/APWA, 2024
or later).
A 30-mil PVC or HDPE liner is required along the bottom and sides of the facility. It must extend to the top
of the ponding depth.
An underdrain must be used to collect treated
stormwater from horizontal sand filters. It must be
located at the bottom of the facility for the last 2/3 of
the length of the facility. The perforations must be
located on the top side of the pipe. The pipe must be
a minimum of 4 inches in diameter for private
facilities and 6 inches for public facilities.
The sand bed must consist of a medium sand meeting
the size gradation in Table 10. The contractor shall
obtain a grain size analysis from the supplier to certify
that the No. 100 and No. 200 sieve requirements are
met. The analysis must be provided as part of the
post-construction submittals.
9.2.7.3 Other Requirements
Pretreatment must be provided using a separate compartment, pollution control manhole, or sumped
and snouted catch basin.
For vertical facilities, the hydraulic head must be 4 ft or greater.
U.S. Sieve Number Percent Passing
4 95–100
8 70–100
16 40–90
30 25–75
50 2–25
100 <4
200 <2
Table 10. Sand Media Specifications
From: King County (1998) and Washington DOE (2024)
Using a building’s foundation as a 4th wall for a sand filter is prohibited.
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The overflow for a horizontal facility must be a beehive (see Standard Details SD8-01 and SD8-02).
9.2.7.4 Construction Notes
After construction, flood the facility with 10-15 gallons of clean water per cubic foot of sand to
consolidate the sand course.
Swale (Standard Details SD9-19 and SD9-20)
A swale is a vegetated channel with a gentle
longitudinal slope. They differ from ditches in that they
treat stormwater and are designed for a low velocity
whereas ditches convey stormwater and are designed
for high stormwater velocities.
Swales differ from linear raingardens in that they have
flatter side slopes and lower flow depths. They work
well in parking lots or along roadways and driveways.
9.2.8.1 Dimensions
The minimum length is 100 feet. Sites which cannot
accommodate a minimum 100-foot length should be
designed as rain gardens or planters.
The minimum bottom width is 3 feet.
Side slopes must be 4H:1V or flatter
Longitudinal slopes can range from 0.5 to 4 percent for filtration swales. The longitudinal slope of
infiltration swales must not exceed 0.5%.
The maximum flow depth is 4 inches.
The minimum freeboard must be 6 inches for facilities treating impervious areas that are 3,000 sq ft or
greater. The minimum freeboard for facilities treating less than 3,000 sq ft is 3 inches. The maximum
freeboard is 1 foot.
9.2.8.2 Other Requirements
Velocity must be less than 3 ft per second and provide a residence time of 9 minutes or longer. Rock
check dams spaced a maximum of 10 ft apart can be used to decrease velocity through the facility.
Swales that are constructed with a storage course must provide weirs instead of check dams to minimize
the possibility of creating preferential flow path.
Applicability Table
Tier
1 (Infiltration)
2 (Filtration)
Onsite Retention Infiltration only
Flow Control Filtration Only
Water Quality Treatment ✓
Extended Filtration Filtration Only
Capital Improvement ✓
Public Improvement ✓
Single-Family Residential ✓
Prescriptive Sizing
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Inlets be either a curb inlet (Detail Drawing SD9-02) or inlet catch
basin (Detail Drawing SD5-03A or SD5-03B) for swales receiving
stormwater from flatwork or streets.
Swales located next to roads must include a 30-mil PVC or HDPE
liner on the street side of the swale to minimize degradation of
the road base. Public facilities can use a 6-inch-wideconcrete
wall extending to the bottom of the roadbase but it must be
monopoured with the weirs or curb. The City’s inspector or
project engineer must be present at the pour.
Multi-celled swales must be used for roads with slopes greater
than 4 percent (see Standard Detail Drawing SD9-07). They must
include concrete weirs instead of check dams to avoid
preferential flows through the facility.
Swales must not be located downstream of detention.
Trenches and Galleries (Standard Detail SD9-06)
A trench is a rectangular rocked facility that infiltrates stormwater discharging from an underground
perforated pipe.
An infiltration gallery infiltrates stormwater through an underground perforated pipe. It is structurally
supported to provide more storage than a trench and can consist of multiple rows of storage.
9.2.9.1 Dimensions
The storage course depth for infiltration trenches ranges from 1
to 3 feet. It ranges from 2 feet to 7 feet for galleries and must
comply with the manufacturer’s specifications.
The facility width is a minimum of 2 feet for infiltration trenches
and 4 feet for infiltration galleries.
The depth of the overlying soil for infiltration trenches ranges
from 1 to 3 feet. The depth for galleries will depend on the
manufacturer’s requirements.
9.2.9.2 Materials
A 3-inch choker course must separate the soil overlying the
trench or gallery from the storage course, with clean rock (¾” to
½”) that complies with Section 00430 in the current Oregon
Applicability Table
Tier 1
Onsite Retention ✓
Flow Control
Water Quality Treatment
Extended Filtration
Capital Improvement1 ✓
Public Improvement
Single-Family Residential ✓
Prescriptive Sizing
1 – With prior written approval of City UIC
permit manager
Additional requirements, such as storage courses and liners, are listed in the general design
standards (Section 9.1.1 General Design Standards – Vegetated Facilities).
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Standard Specifications for Construction (ODOT/APWA, 2024 or later). If placed below a driveway, a
choker course is not required.
The perforated pipe used to discharge stormwater into trenches must be placed within the top third of
the storage course. The crown of the inlet for galleries must be in the upper section of the chamber.
The invert elevation of the inlet for a gallery is dependent on the manufacturer specification.
Perforated pipes for trenches must be a minimum of 4 inches for private facilities and 6 inches for public
facilities. Inlet pipes for galleries is dependent on the manufacturer specifications.
The storage course must consist of clean rock (1-½” to ¾”) that complies with Section 00430 in the
current Oregon Standard Specifications for Construction (ODOT/APWA, 2024 or later). If placed below a
driveway, the storage course cannot use the driveway’s base course.
9.2.9.3 Other Requirements
Trenches cannot be located where they will be subject to vehicular traffic. Galleries, however, may be
placed under residential driveways and parking lots because of their structural support.
Because of the large amount of storage rock used for galleries, they must not be placed in a location or
elevation that would cause a violation of Oregon Drainage Law.
9.2.9.4 Construction Notes
Rock for the storage course in a gallery must be placed with equipment located next to, but not in, the
facility in order to protect the infiltration capacity and structural integrity of the structural support.
Dump trucks cannot be used to push rock into the storage course.
To protect the inlet pipe and structural support, rock should be leveled parallel to the pipe or structures.
Vegetated Filter Strips (Standard Detail SD9-05)
A vegetated filter strip is a vegetated area adjacent to an
uncurbed impervious area such as a sidewalk or driveway.
Stormwater is dispersed evenly across the entire width at a low
velocity and depth.
Sidewalks and other walkways may be reverse-sloped and use
vegetated filter strips for stormwater management. Patios less
than 500 sq ft may provide a 0.5% slope to use a vegetated filter
strip to manage stormwater.
Vegetated filter strips are limited to use for walkways, pathways,
driveways, and patios that are less than 500 sq ft. Vegetated
filter strips cannot be used to treat stormwater from motor
courts. Vegetated filter strips cannot be used on driveways
longer than 100 ft or beyond the point at which sheetflow
Applicability Table
Tier 3
Onsite Retention 1 ✓
Flow Control
Water Quality Treatment
Extended Filtration
Capital Improvement ✓
Public Improvement
Single-Family Residential ✓
Prescriptive Sizing
1 – Stormwater report must provide
documentation that onsite retention will be
achieved and can be maintained.
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becomes shallow concentrated flow unless multiple vegetated filter strips and trench drains are used
along the length of the driveway (see Standard Detail SD9-18).
9.2.10.1 Dimensions
The length of a vegetated filter strip can range between
5 to 100 feet as measured in the direction of flow (see
Figure 15).
The width of the facility must extend 6 inches beyond
the edges of the impervious area discharging to the
facility. Alternatively, the impervious area may be
constrained with an edging that is at least 1 inch above
adjacent surfaces to ensure that flows do not bypass the
facility.
9.2.10.2 Other Requirements
Vegetated filter strips must be a minimum of 50 feet
from waterways.
Flow depths cannot exceed 1 inch and flow velocities
cannot exceed 0.5 ft/sec.
The slope of the facility can range between 0.5 and 10 percent if the flow is distributed evenly along the
width of the vegetated filter strip.
• If the slope is between 3 and 5 percent or the width of the impervious area discharging to the
facility is greater than 100 ft, a rock strip (1 ft X 1 ft X 0.5 ft) or a non-galvanized trench drain is
required across the length of the facility to disperse the flow.
• If the slope is over 5%, non-galvanized interior trench drains, or 1-ft X 1-ft gravel check dams
must be constructed along the interior width of the facility to reduce erosion and velocity and to
re-disperse the flow. Check dams must extend 0.5 feet underground to prevent short-circuiting
of the flow.
Additional requirements, such as soil and plants, are listed in the general design standards
(Section 9.1.1 General Design Standards – Vegetated Facilities).
Figure 15. Vegetated Filter Strip
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Water-Quality Vaults
Water-quality vaults can be used for non-SFR and capital
improvement projects, however they cannot be used for middle
housing projects nor to satisfy public improvements required as
a condition of private development.
Water-quality vaults are used to reduce solids. Depending on the
type of filter used, they can also reduce metals, oil, and
phosphorous. They must be constructed of reinforced concrete
with the cartridge filters designed according to the constraints of
the Washington DOE TAPE program for the targeted pollutant
(see Section 7.5 Stormwater Facility Selection – Water-Quality
Limited Waterways). The Washington DOE TAPE constraints for
the targeted pollutant(s) must be included in the stormwater
report.
Wetlands, Constructed
Constructed wetlands are designed to emulate natural wetlands. They have irregular shapes,
variable water depths, and gentle side slopes. They have standing water for part of the year, are
shallower than ponds, and have mottled soil due to long periods of saturated conditions.
Constructed wetlands present an opportunity to integrate wildlife habitats and a public amenity into the
landscape.
Vector (mosquito) control is an important design consideration. Bat boxes, diverse plants, and other
design strategies to encourage biological controls can help to keep mosquito populations in balance.
Applicability Table
Tier 2
Onsite Retention
Flow Control
Water Quality Treatment ✓
Extended Filtration ✓
Capital Improvement ✓
Public Improvement
Single-Family Residential
Prescriptive Sizing
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9.2.12.1 Dimensions
The minimum bottom width is 3 feet.
The minimum length to width ratio is 2:1.
The maximum ponding depth is 3 feet with an average ponding
depth of 2.5 feet.
Side slopes below the design ponding depth must be 5H:1V or
flatter. They must be 3H:1V or flatter above the design ponding
depth.
9.2.12.2 Other Requirements
The outlet and spillway must be at the opposite end of the inlets
to maximize treatment.
Wetlands must either have aerators installed or be oriented with
the prevailing summer winds to minimize anoxic conditions.
Permanent water depth in a wetland should vary in the different cells. For design purposes, the pond
should be designed to have standing water for at least 10 months of the year.
Pretreatment is required and can take the form of a pollution control manhole or a sediment forebay. If
using a sediment forebay, it must contain 10 percent of the design volume. A compacted earthen berm
or concrete weir must extend the width of the wetland to separate the sediment forebay from the
treatment area. The berm height must be 50% to 67% of the ponding height reached during the design
storm.
Applicability Table
Tier 3
Onsite Retention
Flow Control ✓
Water Quality Treatment ✓
Extended Filtration
Capital Improvement ✓
Public Improvement ✓
Single-Family Residential ✓
Prescriptive Sizing
Additional requirements, such as vegetation and spillways, are listed in the general design
standards (Section 9.1.3 General Design Standards – Regional Facilities).
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10 CONVEYANCE DESIGN
Conveyance systems carry stormwater from one location to another through a series of catch basins,
pipes, and manholes. Detention systems control the rate at which stormwater is released to the public
system but do not provide treatment. The requirements for conveyance and detention system design
are primarily provided in the City’s Engineering Design Standards. The information contained in this
Chapter are additional requirements.
Discrepancies in requirements between the SWMM and the Engineering Design Standards or within
either document will default to the requirement that is most protective of water quality.
10.1 DETENTION
Detention tanks and detention vaults can be used to meet flow control requirements for non-SFR
projects, except for middle housing projects. They must follow the requirements of the Engineering
Design Standards with the following changes:
• The minimum diameter for private detention tanks is 36 inches.
• The CN used for pre-development flows calculated during design must be 70.
• A pollution control manhole or other sediment-reduction technique is required at the inlet to
reduce sedimentation in the tank or vault. Public facilities must use a pollution control manhole.
• A flow control manhole is required at the outlet to the detention tank or detention vault to
maintain pre-development flowrates.
• O&M Plans for detention tanks and detention vaults must state that a current confined space
certification is required for all people completing maintenance on the facilities.
10.2 CONVEYANCE STRUCTURES
The Engineering Design Standards contain many of the design requirements and material specifications
for conveyance systems. Public stormwater structures must be designed in conformance with the
Engineering Design Standards and be able to convey stormwater generated from the design storm in the
Engineering Design Standards for the entire upstream catchment. Full build-out, as defined in Section
8.4 (Stormwater Modeling – Downstream Capacity Analyses), must be assumed for the design.
Alignment and Location
Storm lines must run in straight lines, with a constant slope, material, and diameter from manhole to
manhole. The minimum cover for stormline is 3 feet or at a depth that is sufficient to protect it against
damage from construction loads or final traffic loads, whichever is greater.
In public and private streets, new or replaced stormwater mainline locations shall follow these
requirements to the extent possible:
• On the opposite side of the water system,
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• On the south or west side of the street, and
• In the pavement for uncurbed streets.
If the water system is, or can be, located in the center of the street then stormwater mainlines must be
located on the opposite side of the wastewater system.
The Engineering Design Standards provide additional information for utility separations in Chapter 4
(Wastewater).
Private and Public Stormwater System Connections
Private stormwater systems are not allowed in the public ROW and must connect to the public
stormwater system at a 90° angle. A right-of-way permit is required for all private stormwater system
connections to the public stormwater system.
Public stormwater systems must be in the ROW with the exception of treatment or detention facilities
which may be located on City-owned properties. Private stormwater laterals must connect to the public
stormwater system at a 90° angle.
The public stormwater system must not discharge to a private stormwater system.
Stormline and Manholes
The Engineering Design Standards provide most of the standards to be used when designing a pipeshed.
The following standards are in addition to the requirements of the Engineering Design Standards. If a
conflict arises, the standard that is most protective of water quality shall take precedence.
10.2.3.1 Stormline Size and Slope
Public stormwater pipes must have a minimum diameter of 10-inches from inlets and catch basins to
mainlines. Mainlines must have a minimum diameter of 12 inches but they must also be sized to convey
flows from the upstream pipeshed at full buildout, as defined in Section 8.4 (Stormwater Modeling –
Downstream Capacity Analyses), during the design storm event listed in the Engineering Design
Standards with the exception that the minimum design storm event is a 25-yr 24-hr storm event.
Stormline cannot be smaller than that used in the upstream pipeshed. The velocity and slope must not
induce hydraulic jumps within the line.
Private stormline shall be a minimum of 4 inches in diameter and be comprised of material meeting the
requirements for conveyance pipe in the Engineering Design Standards.
At increases in pipe diameter, the crown of the upstream pipe must not be lower than the crown of the
downstream pipe. The invert of the upstream pipe must provide a minimum drop of 0.2 ft to the invert
of the outlet pipe.
The public stormwater system must be in the public ROW and cannot discharge to a private
stormwater system.
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10.2.3.2 Video
All new and replaced public stormline must be videoed to ensure quality construction. The video must
comply with Section 00415 in the current Oregon Standard Specifications for Construction
(ODOT/APWA, 2024 or later) and be submitted to the Engineering Development Review staff or City
Project Engineer as part of the post-construction submittals.
Catch Basins and Inlets
In addition to the spacing required in the Engineering Design Standards, inlets and catch basins must be
provided at the following intermediate locations:
• Where the design flow at the curb line (or berm) exceeds 3 inches in depth or 3 feet in width (as
measured from the curb face or berm) whichever is less and
• At the low point for all cul-de-sacs and dead-end streets with a descending grade.
Water-quality snouts are required for all new and replaced catch basins unless the previous and
subsequent catch basins have a snout and the catch basin in question is not the last catch basin prior to
an outfall. The snout must be attached to the outlet pipe.
Public catch basins must have a minimum 2-ft sump.
Ditches
Ditches provide stormwater conveyance only and are not designed to provide water-quality treatment.
To prevent erosion, ditches must:
• Have sides slopes of 3H:1V or flatter.
• Be sized to carry the design storm at non-erosive velocities.
• Maximum design depth must be 1 ft below adjacent roads.
• If the slope is 4% or greater, the design must provide:
o 6-inch concrete weirs, gabion baskets, or gabion mattresses that do not exceed a 1-ft
elevation drop and,
o energy dissipation at the elevation drop (see Detail SD9-07 Multiple-Celled Facilities).
Quarry spalls may be used for the bottom of the ditch however the side slopes above bank-full
conditions (see Definitions) must be seeded with native grasses and shrubs. If planted, trees must be
located at top of bank and on the south and west sides to assist with temperature TMDL allocations.
Culverts and Bridges
Designs for culverts and bridges must meet these design standards, the Engineering Design Standards,
Oregon Department of Fish and Wildlife (ODFW) criteria, and DSL requirements. Where the
requirements conflict, the criteria most protective of water-quality applies.
ODFW criteria for fish passage (physical and velocity barriers) shall be met for all perennial waterways
and wetlands. Riprap placed to protect abutments shall be placed only below bankfull depth and shall
not constrict channel flow.
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Designs must be submitted to DSL and the ODFW for their review. A copy of their decisions and
requirements must be submitted to the Engineering Development Review staff or the City’s Project
Engineer with other land-use and construction plan submittals.
10.2.6.1 Bridges
Design storms for bridges are listed in the Engineering Design Standards but must comply with the
following:
• For streams without a FEMA-designated 100-year floodplain, bridges must convey the 100-yr
24-hr storm event.
• The vertical clearance between the design water surface and the bottom of the bridge deck shall
be a minimum of 2 feet.
10.2.6.2 Culverts
Design storms for culverts are listed in the Engineering Design Standards. To prevent under-sizing and to
protect riparian corridors and property within the City, new culverts and culvert replacements must
adhere to these requirements:
• Culverts must match the existing slope and orientation of the ditch or waterway.
• Culverts shall be designed such that the headwater a) does not exceed 0.8 times the culvert
diameter OR b) remains 1 foot below the subgrade of the road, whichever is less.
• The culvert diameter must equal or be larger than the diameter of upstream culverts. The
minimum culvert diameter is 10 inches for private culverts and 12 inches for public culverts of
normal length and depth. A 24-inch minimum diameter is required for culverts that are 150 ft
long or that are under 15 ft or more of fill.
• Culverts must be made of materials with a 75-year design life. Due to low service life (ODOT,
2014), galvanized steel culverts are prohibited. Concrete culverts are preferred over metal or
plastic due to their low maintenance and fewer adverse effects to water quality.
• Bottomless culverts shall be used for crossing wetlands and perennial streams. Bottomless
culverts shall provide clearances that meet the requirements of regular culverts.
10.3 OUTFALLS AND OFFSITE STORMWATER DISCHARGES
All stormwater must be treated to comply with Section 7.5 (Stormwater Facility Selection – Water-
Quality Limited Waterways) before discharging offsite. Sheetflow offsite cannot be at erosive velocities
or exceed 100 ft before entering the public stormwater system or a waterway.
Discharges to ditches and waterways must comply with the City’s outfall detail drawing (SD9-08).
Discharges to wetlands cannot increase the pre-development water surface elevation by more than 1
inch.
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10.3.1.1 Location
The point of discharge offsite and outfall locations must be approved by the City Engineer during the
land-use process or the submittal of construction plans, whichever occurs first. In general, the following
criteria apply:
• Discharge cannot be to another property’s stormwater facility.
• For maintenance reasons, outfalls and offsite discharge locations cannot be under a bridge or in a
culvert.
• Offsite discharges cannot connect to the public system as a blind tee.
• To prevent slope failure, outfalls cannot be located within 200 ft of steep slopes unless a downslope
pipe or other means of velocity reduction are installed to the toe of the slope to prevent erosion and
hydromodification (see Section 10.3.1.2 – Velocity).
• Flow spreaders cannot be used to discharge stormwater on slopes greater than 5%.
10.3.1.2 Velocity
Stormwater discharged offsite must have energy dissipation adequately designed to prevent erosion at
the point of discharge. Design calculations must include the pre- and post-development peak discharge
and the peak flow duration. The City Engineer may require additional energy dissipation based on these
factors and the soil type of the receiving area.
10.4 PUBLIC CONVEYANCE EXTENSION
Extensions of the public stormwater system required as a condition of a project must continue to the far
side of the property, i.e. "to and through", to allow connections for upstream properties. Except as
otherwise provided, the extension of the public conveyance systems to serve any parcel or tract of land
shall be done by and at the expense of the property owner or applicant.
All extensions must be constructed in the public ROW and adhere to the Engineering Design Standards
and the SWMM.
Extensions must be sized for a built-out scenario using the assumptions and parameters detailed in
Section 8.4 (Stormwater Modeling – Downstream Capacity Analyses).
10.5 EASEMENTS
The requirements for easements, access roads, and access pathways are primarily provided in the
Engineering Design Standards. Access roads are required for all stormwater facilities that require
mechanized equipment for maintenance or that are located more than 150 feet from the public ROW.
Access pathways must obtain prior written approval from the City Engineer and the Public Works
Director during the land-use process or the submittal of construction plans, whichever occurs first.
Easements for railroad crossings shall be obtained by the applicant and all terms for the easements shall
be met by the applicant and contractor.
- 90 -
Utility construction within easements shall minimize land disturbance, especially to trees and other
vegetation. Any disturbed areas within easements shall be stabilized and restored to the condition prior
to development unless requirements more protective of water-quality are required through the
municipal code.
All stormwater systems in easements shall be explicitly labeled as “Private” or “Public” on the plat.
10.6 ENCROACHMENTS
An encroachment permit is required for private structures, pavement, retaining walls, vegetation, or
landscaping located in the public ROW or in a public easement.
Encroachments are not permitted in areas 1) with a history of flooding, 2) that require frequent
maintenance, or 3) used as an access road or pathway to a stormwater system.
The City Engineer may include additional requirements as a condition of approval.
The approval or denial of an encroachment by the City Engineer is final.
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11 MAINTENANCE OF STORMWATER FACILITIES
Stormwater facilities, private and public, must be maintained to preserve the treatment capacity and
comply with Flow Control requirements. The responsibility for maintenance is determined during the
land-use process or the submittal of construction plans, whichever occurs first. Entities responsible for
O&M of a stormwater facility must be listed on the recorded O&M Plan. Public facilities constructed as
part of the capital improvement program or as a condition of private development become the City’s
responsibility after the warranty is completed and when the City has formally accepted the facility.
11.1 OPERATIONS AND MAINTENANCE PLANS
An O&M Plan is required for all projects required to provide stormwater
management. The O&M Plan must describe how to properly maintain
each stormwater facility, the frequency of maintenance, and the party
responsible for maintenance. The maintenance of access roads and
pathways must be included in an O&M Plan.
Developers must record O&M Plans for private stormwater facilities in
the County of Record and provide a copy of the recorded O&M Plan
(with the County stamp) to the Engineering Development Review staff prior to receiving approval of the
final ESC inspection. Public facility O&M Plans for water-quality vaults and proprietary treatment
facilities must be provided to the Engineering Development Review staff or the City’s Project Engineer
prior to the City’s acceptance of the facility.
The cover sheet required for private O&M Plans must be obtained from the Engineering Development
Review staff. Appendix D contains a maintenance template that may be used with the cover sheet.
11.2 FACILITY ACCESS
An access road or access pathway must be constructed when public stormwater facilities cannot be
accessed from the public ROW with industry-standard maintenance equipment. An access road is
required for mechanized maintenance, and an access pathway is required for facilities that use hand
tools, do not require mechanized maintenance, and which are no farther than 150 ft away. The
Engineering Design Standards contain the design standards for access pathways and roads.
11.3 INSPECTIONS
Inspection frequency depends on the type of stormwater facility but must be completed annually or
when:
• The system does not drain within 48 hours or is otherwise not operating as designed.
• Nuisance conditions are present. Nuisance conditions include, but are not limited to,
accumulated sediment, overflowing during smaller storms, stagnant water with algae, insect
breeding, odors, discarded debris, or safety hazards.
O&M Plans must state
that unmaintained
facilities can result in a
required replacement
of the facility.
- 92 -
Inspection Records
The owner or responsible party identified in the O&M Plan is required to keep all records of
maintenance. The party responsible for maintenance must document repairs and keep the records for a
minimum of five years. The records may be kept electronically but must be available to the City Engineer
upon request.
City Inspections
The City reserves the right to inspect private facilities to confirm that they are functioning as designed.
Examples of inspections may include but are not limited to:
• Routine inspections
• Random inspections
• Inspections based upon complaints or other notice of
possible violations
• Joint inspections with other agencies inspecting under
environmental or safety laws
• Inspection of facilities operating under a Partial Letter of
Acceptance
Property Sale and Facility Ownership Transfer
When ownership of public stormwater management facilities is transferred to and accepted by the City,
the transfer shall include all maintenance and access easements.
If a property is transferred to another owner, the owner of the stormwater facility shall:
• Inform the new owner(s) or responsible party of the existence of private stormwater facilities on
the site, their restrictions (including design capacity and setbacks), and the requirement to
inspect and maintain them
• Provide the new owner with a copy of the maintenance records that document the facility
inspections and any completed repairs.
11.4 MAINTENANCE
While the maintenance requirements and frequency are specific to each facility type, size, and location,
there are some general requirements. The applicant can provide additional maintenance steps however
the following procedures must be incorporated into O&M Plans.
Catch Basins
Routine cleaning of catch basins is one of the most important methods for protecting the quality of
waterways and for protecting the infiltration capacity of stormwater facilities. Catch basins must be
cleaned annually or when the sump is reduced by one-third whichever occurs first.
All stormwater management
facilities, whether located on
private or public property,
shall be accessible for City
inspection.
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Detention Tanks and Vaults
Remove oil, sediment, and debris from the facility when it
exceeds 1/3 of the sump.
Annually inspect the vault or tank for structural integrity and
make necessary repairs.
Drywells, Infiltration Trenches, and Infiltration Galleries
If overflows occur during small storms or if water remains for more than 48 hours, check for clogging of
the pipes or perforations. Remove any clogs through vacuum suction rather than jetting to avoid
clogging of the infiltration area and reducing the lifespan of the facility.
UICs and their pretreatment systems must be inspected annually for structural integrity. As a rule,
repairs should be made during the summer when they are relatively empty.
Porous Pavements and Permeable Pavers
Debris and sediment must be removed from porous pavement and
permeable pavers each year. The party responsible for maintenance
can either dry sweep the area or hire a company that does
regenerative air vacuuming. After vacuuming permeable pavers,
restore the sand or crushed rock between them.
If moss prevents infiltration or creates a slip hazard, use physical removal measures. Herbicides and
other chemical applications cannot be used for moss removal.
Annually inspect the edge restraints in areas with permeable pavers for structural integrity and repair as
necessary. Over time, settling may occur in areas using permeable pavers. Replace aggregate base,
washed sand, and broken pavers as needed. Refer to the manufacturer’s recommendations for detailed
maintenance procedures.
Planters, Raingardens, and Swales
Annually inspect pipe inlets and outlets for water tightness and caulk as
necessary. For planters check the structure for concrete spalls or cracks
and repair as necessary.
Inspect liners for structural integrity in filtration facilities. Repair holes and
tears and close gaps. For planters, check the metal bar attaching the liner
to the concrete and caulk as necessary to maintain a water-tight
connection. Replace attachment bars that have rusted.
For raingardens, ensure that the liner retains at least 6 inches of cover
(soil) at the top of the facility.
Sealing porous pavement
or permeable pavers is
prohibited and a violation
of the municipal code.
The use of
herbicides and
pesticides to remove
weeds in vegetated
facilities isa
violation of the
municipal code.
Maintenance of below-ground
facilities, such as tanks and
vaults, must be completed by
people who have current
confined space certifications.
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Remove sediment at the inlets if it exceeds 1 inch or is interfering with plant growth. In areas of erosion,
rake the area level and replace rock and soil as needed to maintain a level surface.
Vegetation must cover 100 percent of the surface area of the facility. Replace dead or diseased plants.
Remove weeds manually and replant unvegetated areas. Do not use pesticides, herbicides, or fertilizer.
Check for clogging if ponding occurs longer than 48 hours after a storm event. Clogging can occur in the
outlet pipe of a facility or by creating a layer of sediment on top of the soil. Remove clogs in pipes and
rake the soil to a depth of 4 to 6 inches to regain the infiltration capacity. If no clogs are present in the
pipes and raking is ineffective then replace the soil.
Ponds and Wetlands
Ponds and wetlands must be maintained and inspected annually. Remove sediment that has
accumulated at the inlet, rake to remove rills, and replace rock as necessary. Remove weeds and
replant, as necessary, to maintain 100% vegetative cover of the surface area of the facility. Prune trees
and shrubs as necessary to maintain protect traffic sightlines and pedestrians from plant overhang.
Inspect the interior overflow areas and orifice for structural integrity and repair or replace as necessary.
Remove accumulated sediment. Sediment at the bottom of the facility, even thin layers, can create
areas of dead vegetation and reduce infiltration.
Inspect the emergency spillway. Remove all trees and shrubs within 20 feet of the spillway. Check for
spalling and cracks and repair as necessary. If repaired areas deteriorate more than one year, consult a
civil engineer (PE) with geotechnical experience for solutions.
Inspect the outlet for clogging if ponding occurs longer than 48 hours for detention and infiltration
ponds. Remove clogs and sediment as necessary.
Sand Filters
The filter medium, including sands and gravels, must allow adequate infiltration. Annually remove
sediment that has accumulated on the surface and check the inlets and outlets for structural integrity.
Remove sediment and debris from structural components. Repair cracks and replace damaged
components as necessary. Rake the sand bed to create a level surface.
Inspect the liner if present. Repair holes and tears and close gaps. Re-attach liner as necessary. Caulk the
liner at the attachment bar and at pipe inlets and outlets to maintain waterproof connections. Replace
the liner attachment bar if it is rusted.
If the facility is vegetated, remove weeds and replant as necessary to retain 100% cover at plant
maturity. Do not use pesticides, herbicides, or fertilizer.
If ponding occurs longer than 48 hours, rake to a depth of 6 inches to restore the infiltration capacity or
replace the soil. Contact a stormwater engineer if ponding continues to occur beyond 48 hours.
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Vegetated Filter Strips
Filter strips must be inspected annually. Remove accumulated sediment at the inlet and outlet. Evaluate
the vegetation, remove weeds, and replace dead plants to ensure 100 percent coverage. Do not use
herbicide, pesticides, or fertilizer.
Check the inlet and treatment area for erosion. Rake areas where rills have occurred and add rock or
soil, as necessary, to create a level surface. Where flow spreaders or trench drains are being used,
remove sediment and check for structural integrity. Replace or repair as necessary.
Wetlands
Constructed wetlands must be inspected annually. Remove weeds and replant as necessary. Do not use
herbicides, pesticides, or fertilizer. Check depths and remove accumulated sediment from the interior of
the wetland as necessary to retain the design ponding depth.
Check the overflow for structural integrity and remove accumulated sediment. Ensure that the orifice is
not clogged and is functional; repair or replace as necessary.
Inspect the emergency spillway. Remove all trees and shrubs within 20 feet of the spillway. If it is a dirt
or vegetated spillway, rake and replace soil in areas of erosion and repair areas of seepage. If it is a
concrete spillway, check for spalling and cracks and repair as necessary. If repaired areas deteriorate
more than one year, consult a civil engineer (PE) with geotechnical experience for solutions.
If ponding occurs longer than 48 hours, check the outlet for clogging and remove clogs as necessary.
Sediment in the bottom of the pond, even thin layers, can create areas of dead vegetation and reduce
infiltration. Rake to a depth of 6 inches and replant to achieve 100% vegetative cover.
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12 EROSION AND SEDIMENT CONTROL
Erosion is the movement of soil from land to water or from one area of land to another area. It can be
natural such as beach dunes shifting or the appearance of oxbows in a stream system. It can also be
anthropogenic such as streambank slumping or rills caused by stormwater from development or excess
stream sediment caused by a discharge from a construction site. Because the erosion rate on
construction sites has been estimated to be 500 times greater than on natural sites and because
mercury is naturally present in soil, DEQ has determined that minimizing erosion and stormwater
discharges from construction sites are effective methods for reducing mercury in streams (DEQ, 2019).
Sediment is soil that has been moved by water from one area to another. It can be natural or
anthropogenic such as 1) downcutting in a stream system caused by excessive flows (natural or
anthropogenic) or 2) solids moved by stormwater from impervious or unvegetated areas to waterways.
12.1 REGULATORY OVERVIEW
ESCs are implemented at a construction site to ensure that an illicit
discharge does not occur at the site. In addition to the SWMM, the
City’s municipal code, MS4 permit, and TMDL Implementation Plan
affect which ESCs are required at a construction site.
Municipal Code
Covered under LOC Chapter 52 of the municipal code, the ESC program requires a permit for any project
that disturbs at least 500 square feet or is within 50 feet of waterways. Landscapers and homeowners
with projects disturbing between 500 and 1,000 sq ft are required to submit a simple ESC plan for
review and approval by the City (see Appendix E for template).
In conjunction with LOC Chapter 52, LOC Article 38.25 prohibits the “discharge, directly or indirectly, of
any pollutant into the surface water management system, private storm drainage system connected
to the surface water management system, or receiving water within city limits. This includes
discharges as a result of an unintentional spill or deliberate dumping .”
MS4 Permit
The City’s MS4 permit requires that sediment discharges do not occur from construction sites. It
requires staff review of plans, inspections of construction sites, and enforcement. Discharges offsite that
reach the public stormwater system are illicit discharges and a violation of the applicant’s ESC permit as
well as the City’s municipal code.
TMDL Implementation Plan
Mercury in sediment accumulates in fish tissue during their life cycle and can become toxic for people
who consume fish. In 2019, the EPA and DEQ revised the mercury TMDL allocation for the City sharply
downwards to combat these issues.
The ESC requirements are
mandated by the Clean
Water Act and enforced
by DEQ through the
City’s NPDES MS4
permit.
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The City’s TMDL Implementation Plan requires source control and ESCs to control sediment movement
and erosion. Steep slopes, riparian areas, and riparian-adjacent projects are required to use coconut coir
blankets to stabilize soil during construction. Non-SFR projects are required to retain a certified ESC
professional to ensure that ESCs remain functional.
12.2 EROSION AND SEDIMENT MOVEMENT
Factors that influence erosion and sediment movement include 1) soil type, 2) slope gradient, 3) slope
length, and 3) project size and timing.
Soil Type
Soil type determines if the ESCs at a construction site should focus on erosion control or sediment
containment. Erosion control is necessary with coarse-grained soil such as loams and sands. Fine-
grained soil such as silt and clays require more focus on sediment controls. Most of the soil types in the
city are fine-grained, however the west side of the city contains coarse-grained soil because of the
Missoula Floods (see Figure 8 in Section 6.2.1 Geotechnical Conditions – Soil Classification).
Slope Gradient
Soil particle size affects the stability of slopes. The angle
of repose is a soil property that affects slope stability and
is based on particle size. Beyond the angle of repose, a
slope becomes unstable resulting in slumping. It is very
important when grading a construction site, excavating a
trench, or during stream restoration.
As a rule, doubling the slope increases the potential for
erosion by five times. Unprotected steep slopes can
become unstable during relatively small storms.
While not intuitive, sites with large excavations or flat
grades can have difficulty with sediment control because
rainfall ponds on the site and, for fine-grained soil, do not
infiltrate easily. A flat site can have difficulty keeping
sediment from being tracked offsite by construction
vehicles. Construction entrances may need refreshing
more frequently on a flat site and a project manager may
need to install a tire wash facility to reduce tracking or
temporary sediment settling tanks (e.g. Baker tanks) to reduce sediment in construction stormwater.
The same conditions and solutions may be necessary for a site with a large below-ground excavation as
seen on commercial projects with underground facilities.
Slope Length
The slope gradient combined with slope length determines the amount and method of soil movement.
Long steep slopes can result in a landslide however even long gentle slopes can result in rills and gullies.
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As the length of a slope increases, stormwater velocity increases and erodes unvegetated areas. As a
rule, doubling the slope length increases erosion by four times.
Project Size and Timing
Vegetation is the most effective form of erosion control, especially for large projects where the work
tends to move from one area to the next and rarely has work occurring everywhere. Breaking a large
construction site into smaller areas of work (phasing) is very effective in reducing disturbed areas and
the likelihood of erosion and sediment movement. Even small projects can benefit from leaving
vegetation in place until clearing is necessary for construction work.
Construction of a project in the summer requires different ESCs than if the project is constructed in the
winter. The dry summers of the Willamette Valley require attention to dust control whereas projects
started in the winter require more sediment control to reduce tracking offsite and the likelihood of a
sediment fence failure. ESC professionals managing sites with large excavations, such as commercial
developments, need to determine if temporary sediment settling tanks or dewatering facilities will be
needed to reduce the volume of ponded water onsite.
12.3 ESC PERMITS
Projects that disturb at least one (1) acre of soil or are part of a common development must apply to
DEQ for a construction stormwater (1200-C) permit. Once approved by DEQ, a copy of the permit, with
the site plan, must be provided to the City’s ESC Inspector for review. Unless there are special
circumstances such as riparian areas, land disturbance within 50 ft of a waterway, or steep slopes on a
project, the DEQ-issued 1200-C permit will be accepted for the project.
Applicants must apply for a City ESC permit for all projects that disturb less than an acre and which are
not part of a common plan of development. An online application on the City’s website is available for
applicants.
ESC Plans
ESC plans must be submitted as part of the application; they include a narrative as well as a site plan.
The narrative must describe the proposed project and location as well as the ESCs to be used and any
riparian areas or slopes greater than 15%. The site plan sheet(s) must provide the requirements outlined
in Section 4.1.1 (Pre-Construction Submittals – Erosion and Sediment Control Sheet).
Certified Professionals
Certified ESC professionals are required for non-SFR projects. The City strongly encourages applicants of
SFR or large landscaping projects to engage the services of a certified professional to expedite permit
issuance, decrease time for inspection approvals, and reduce lost time due to enforcement.
Applicants that use a certified professional can see faster permit approvals and fewer enforcement
actions. Certified ESC professionals know how to create and execute an appropriate ESC plan as well as
how to install, inspect, and maintain the measures.
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12.4 EROSION AND SEDIMENT CONTROL SELECTION
The City has adopted Water Environment Services’ Erosion
Prevention and Sediment Control Manual for its ESC program.
Because the manual is used for urban and rural projects, some
of the ESCs in the manual are not applicable to the urban
environment of the city and could cause a violation of the
City’s MS4 permit. Therefore, the decision of certified ESC staff
in the Engineering Department is final for plan review and site
inspections.
Required Erosion and Sediment Controls
Every project must provide a construction entrance, catch
basin inserts, proper waste management including
covered/closed dumpsters, sediment fences around the project perimeter, and wattles near tree
protection areas. A leak-proof concrete wash-out is required to be onsite before the foundation pour
and during all activities that involve the use of cementitious materials such as grout, mortar, concrete,
or stucco. All projects adjacent to Oswego Lake or its canals must provide silt curtains.
Construction Access and Parking
As the primary access point for construction, the construction entrance must be installed before any
work is done including clearing and grubbing (see Standard Detail E1-06). Using rock with fines or not
properly refreshing the rock during construction can result in sediment leaving the construction site with
subsequent citations and other enforcement actions.
Construction site parking is a common source of land
disturbance and tracking. Keep parking limited to hard
surfaces or compacted graveled areas. Do not block the
construction entrance with crew parking, job trailers, or
materials.
Load and unload materials within the construction site
perimeter and not from the street. Even small equipment
can cause serious land disturbance or tracking from the
site. Material spills or waste discarded outside the
construction site perimeter can result in a stop work order,
citations, fines or a combination of these actions.
The City has adopted Water
Environment Service’s Erosion
Prevention and Sediment Control
Manual (available on the City’s
website.
Questions and inquiries about the
Erosion Control Program can be
answered by contacting the City’s
Erosion Control Inspector.
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12.4.2.1 Maintenance
The function of a construction entrance is to minimize the amount
of sediment leaving a site. It also provides focused access for
vehicles to enter and leave a site.
•Keep construction entrances of dirt and debris. Refresh rock as it
becomes filled with soil from the site.
•Sweep streets adjacent to the construction site at the end of
each day.
12.4.2.2 Optional Controls
For smaller projects with limited access, a construction pathway can be used instead of a construction
entrance. The pathway must use plywood or steel sheeting to reduce land disturbance and limit damage
to existing root systems. For larger projects or larger equipment near water bodies, interlocking
pathway boards capable of withstanding 600 psi can be used to minimize sediment tracking.
A tire wash facility may be necessary for large sites or projects where the construction entrance has
been shown to be ineffective in eliminating tracking offsite.
Sediment Control
The function of a sediment control is to prevent sediment from leaving a site. It detains stormwater and
allows fine-grained particles to settle out.
Required sediment controls include sediment fences, catch basin inserts, wattles near tree protection
areas, and silt curtains for projects adjacent to Oswego Lake.
12.4.3.1 Sediment Fences (Detail E-03)
Sediment fences are required at the limits of clearing for all
projects. If construction fencing is not used at the site,
sediment fences must be used to protect and delineate
sensitive lands, placed around the perimeter of stormwater
facility locations, and installed at the project limits.
On slopes, the fence must be at a set elevation (across the
slope), rather than sloping (following the descent of the
property), to avoid channelized runoff and fence failure.
Multiple lines of sediment fence may be necessary on long or
steep slopes to avoid failure of the fence during storm events.
Sediment fence must be trenched in to a depth of six inches (usually denoted by a line on the sediment
fence) and staked. The fence must be straight and not sag. Lengths of sediment fence are joined
together by wrapping two end stakes together at least 2 times.
Washing dirt or debris into
the street, curb inlets, or
catch basins is a violation of
the ESC permit and municipal
code.
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12.4.3.2 Catch Basin Inserts (E1-01)
Catch basin inserts are required to protect all catch basins within 200 feet of the project perimeter. They
are a final effort to avoid a sediment discharge into the stormwater system. They are NOT a primary line
of defense.
12.4.3.3 Wattles
Wattles are required at tree protection areas and can be used as secondary protection on the site. They
must be installed between the tree protection fence and the disturbed area and staked down. Wattles
cannot be used as a primary ESC anywhere other than tree protection areas.
12.4.3.4 Maintenance
Sediment must be removed from sediment fences when it reaches 1/3 of the height of the fence. The
weight of water ponding next to a sediment fence will break the wooden stakes of the fence, allow an
uncontrolled discharge from the site, and result in a violation of the ESC permit. If sediment hasn’t been
removed from a fence, it can exacerbate the situation because the water’s weight will be centered
higher on the fence line.
Stakes in a fence must be replaced when they are broken and fence sections must be replaced when a
hole develops in the fence material.
Catch basin inserts must be replaced when they become 1/3 filled with sediment.
Wattles must be replaced when they become flattened or filled with sediment. Driving over them will
increase replacement frequency.
Sediment removed from sediment control measures can be distributed back on flat areas of the site
after dewatering. A 50-ft vegetated buffer must separate riparian areas and stormwater facility locations
from areas where sediment is re-distributed.
12.4.3.5 Optional Controls
Buffer zones may be used to prevent run-on and to protect riparian areas. Preserving natural vegetation
reduces the amount of soil disturbed during at one time on a construction site. It can be used to phase
construction of large projects or as a sediment control for small projects.
Biobags or dewatering bags may be used to filter stormwater. They can be used to provide additional
protection for the required sediment fences and catch basin inserts but cannot be used to replace them.
Biobags must be replaced when flattened or filled with sediment. Dewatering bags must be replaced
when they become clogged or when turbid water is leaving the bag.
If additional sediment removal or additional time for removal is needed then temporary sediment
settling tanks can be used to provide the time necessary to remove sediment. If chemicals are needed to
enhance sediment removal, pH monitoring is required to keep the pH between 6.5 and 8.5 before
stormwater is discharged offsite.
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Run-on Control
While sediment control is the primary ESC used in the city, project managers should minimize the
amount of water entering the site from adjacent properties. Channelized stormwater or groundwater
should be diverted from exposed soil or steep slopes to reduce erosion and sediment transport.
Water from adjacent properties should be diverted from unvegetated areas on a site and routed to the
downslope side of the property for discharge off the site. If it is allowed to reach unvegetated areas, the
project will require additional sediment controls to filter the water before it is discharged offsite.
Swales and pipe slope drains can be used to divert water aware from disturbed areas. Both will require
an outfall at the perimeter of the site to reduce velocity and prevent erosion at the discharge point.
Swales on slopes will require check dams to reduce water velocity and trap sediment.
12.4.4.1 Maintenance
Remove sediment and regrade below outfalls to repair erosion. Placing a geotextile or coconut coir
blanket below the rock used for the outfall will reduce scour and maintenance time.
Dust Control
Dust from a construction site must be minimized during dry weather. Using water trucks to wet
disturbed soil is the required method for dust control in the city. Other approved methods for
minimizing dust include seeding, mulching, or using coconut coir blankets.
Stabilize Slopes and Disturbed Areas
Unvegetated areas and slopes are a source of sediment during storm events. Areas that have been
cleared, but are not under active construction, must be stabilized until active construction. If the soil will
be exposed during winter, it must be covered at the end of each day.
Cover unvegetated areas with 3 inches of straw, compost mulch, wood chips, gravel, or other ground
cover to minimize land disturbance and reduce the potential for tracking. Coconut coir blankets are
required for all projects with slopes 15% or greater (see Detail E1-04) and for all areas that are 50 feet
(measured horizontally) or closer to a waterway (See Detail E1-05).
12.4.6.1 Maintenance
Replace or add more ground cover when patches of bare soil can be seen. Repair holes in coconut coir
blankets or replace them if the holes are too large to adequately repair.
12.4.6.2 Optional Controls
Surface roughening, or tracking, can be used to prevent erosion of a slope. Ensure that equipment tracks
up and down the slope, instead of cross-slope, to prevent forming channels on the slope.
Pipe slope drains can be used to divert water from disturbed sloped areas but are not a substitute for
stabilizing the soil.
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Permanent or temporary seeding may be used to stabilize soil. Hydroseeding or using a bonded-fiber
matrix is the easiest method for large, sloped areas or stockpiles that need to be stabilized.
Plastic sheets can be used but must be surrounded by sediment fence when used on a slope or to cover
a stockpile. Rain falling on a plastic sheet will create runoff on the construction site. It should be
diverted before reaching unvegetated areas to reduce erosion and sediment movement on a site.
Stockpile Areas
Material storage areas must be identified on the ESC plan. Materials, other than soil, must be placed on
a hard surface such as a gravel base, or plywood. Soil stockpiles must be encircled by sediment control
such as a sediment fence. When plastic sheeting is used, tie-downs or anchors are required to keep the
plastic from becoming airborne (see Detail E1-07). A dual barrier is recommended for large stockpiles.
Soil stockpiles must be covered daily during active construction. They must be covered or hydroseeded if
unworked for 14 days or longer.
12.4.7.1 Maintenance
Repair holes in sheeting and maintain equidistant spacing of tie-downs/weights. Replace sheeting if
holes cannot be adequately repaired. Remove material from behind barriers when it reaches 1/3 of the
height of the barrier.
12.4.7.2 Optional Controls
Stockpiles of compost, bark dust, topsoil, or other amendments may be placed on tarps if the storage
time onsite is minimized, the volume is minimal, and methods are installed to divert water or control
discharge from the stockpile.
Source Control
Project managers must minimize, store, control, and dispose of construction waste properly. Closed
trash containers, spill response kits, and concrete washout containers are required for every project.
Construction waste must be stored away from stormwater facilities and waterways.
All concrete wash-out, mortar, grout, wet saw
slurry, and other liquid wastes must be
contained in leak-proof prefabricated pans.
Onsite construction of washout containers is
prohibited.
Ground or open-pit dumping is prohibited.
Additional requirements may be required by
the City’s certified ESC professionals.
12.4.8.1 Maintenance
Leak-proof concrete washout containers must be removed after a concrete pour is completed or when
they become 2/3 full including washwater and rainfall.
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Trash leaving a site is a violation of the ESC permit. Construction
sites must be cleaned of outdoor trash at the end of each day or
immediately when strong wind gusts are forecast. Trash must be
placed in leak-proof covered containers that are marked as
“Trash”. Small (up to 55-gallon) containers must be emptied
weekly or when ¾ full. They can be emptied into larger
containers. Larger containers such as drop-off bins must be
emptied monthly or more frequently if needed.
Discarded building materials should be sorted by recyclability
such as steel and wood. They must be stored in closed recycling
containers.
Spill response kits must be in easily-accessible areas and have
prominent signage. Kit materials must be replenished as soon as
possible after they are used so that the kit has its maximum spill
response capability. Staff must be properly trained in spill
response with documentation of the training available to City
staff upon request.
Wet Weather Requirements
ESC requirements for the wet weather season (October 1st to May 31st) are in addition to standard ESC
requirements and must be included, or incorporated by reference, into the ESC plan.
• All stockpiled material must be fully covered with secured plastic sheeting and isolated with silt
fencing or check dams/wattles at the toe of the slope unless being actively accessed.
• All unvegetated areas must be covered at the end of each workday with a 3-inch minimum
depth of straw, compost mulch, or wood chips.
• Inspections are increased to weekly and within 24 hours of storm events exceeding 0.5 inches
of precipitation within 24 hours.
12.5 PRE-CONSTRUCTION
ESC effectiveness is increased with good project management. These practices include efficient
scheduling, project phasing, awareness of weather conditions, and training.
Schedules and Phasing
Scheduling can be a very effective means of reducing construction impacts. Reducing the number of
contractors onsite reduces the amount of parking needed, the likelihood of tracking sediment offsite,
the likelihood of accidental spills, and the amount of waste to manage at the site.
Phasing, when used, must be communicated to Engineering Development Review staff during plan
review with each phase and its ESCs documented on a separate plan sheet.
Source control includes:
•Storage of solid waste
including concrete waste.
•Spill prevention and
response.
•Disposal of fluids and
wastes.
•Control of emissions from
painting, finishing, and
coating of buildings and
equipment.
•Storage or transfer of solid
materials, by-products, or
finished products.
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Weather Forecasts
Weather forecasts including expected precipitation amounts are available
from the National Weather Service (www.weather.gov) or local weather
forecasting websites. Hourly precipitation forecasts are available at these
websites in addition to radar. Some local websites include forecasts by
the minute for 2-hr windows. Using the City’s two primary zip codes of
97034 (east side) and 97035 (west side) can result in more precise
weather forecasts.
Activities should be scheduled, as much as possible, based on the season
and daily weather forecast. For example, grading activities should occur
during dry periods and disturbed areas must be stabilized if rain is
forecast within 24 hours.
Soil stockpiles must be covered and sediment fences must be inspected to ensure that they are in good
condition and will be able to withstand the amount of runoff expected from a storm. Catch basin inserts
must be inspected and replaced if sediment is near the 1/3 depth required for replacement to reduce
flooding and failure of the insert during a storm.
Training
Staff and subcontractor training on ESC requirements is required for all projects. If construction
practices result in an ESC violation, the permittee will pay any fines associated with the violation even if
it was caused by a subcontractor. Stop work orders apply to the entire site.
12.6 CONSTRUCTION
After the ESC application is approved and the permit is issued, the applicant must install the
construction entrance before installing the other approved ESCs because of the likelihood of equipment
tracking soil offsite after unloading materials. After the site has been protected by the ESCs, the
applicant must schedule the initial inspection and obtain approval from the City ESC inspector before
clearing and grubbing.
Establishing protective buffers and minimizing grading at the beginning of a project and around sensitive
lands, such as riparian areas and stormwater facilities, will reduce the chance of an ESC or sensitive
lands violation.
Minimizing the extent and duration of exposed soils and maximizing the preservation and protection of
site features, especially areas to be used for stormwater management, will reduce potential erosion and
Clearing and grubbing before approval of the 1st ESC inspection is a
violation of the ESC permit.
Project managers
must be aware of
changing weather
conditions at all
times. Not being
aware of a wind or
precipitation event is
not a defense against
an ESC violation.
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sedimentation. Compaction of areas used for infiltrating stormwater can lead to failure and subsequent
reconstruction of the facility before approval of the final ESC inspection.
Inspections and Inspection Logs
City ESC staff will routinely visit project sites to ensure that the approved
ESCs are properly installed and functional. While three inspections are
mandatory and must be scheduled with the City’s ESC Inspector,
construction sites that are active for more than 6 months may receive
additional inspections.
ESC inspectors hired by the applicant or contractor must be, at a minimum,
knowledgeable in ESC maintenance. Inspectors for subdivisions or on
properties zoned as commercial, industrial, or multi-family must possess
certification as an ESC professional.
Inspections should occur on a routine basis and before large storm events. All ESC inspections at the
construction site must be documented during the project. The inspections must be recorded in an
inspection log documenting 1) the date of the inspection, 2) the amount of rainfall during and 24-hours
prior to the inspection, 3) the type and location of the inspected ESCs, 4) observed issues and any
corrective actions taken, and 5) the inspector’s name and signature. Inspections must be documented
with photographs and any sampling results.
The inspection log must be on site and available to City or DEQ inspectors upon request.
Four weather stations, located in various areas, are maintained by the City and can provide information
on the amount of precipitation required for the inspection logs. The weather station location and the
collected information is available at the water conservation page on the City’s website.
Significant Discharges
The following discharges from a construction site are considered significant.
They must be included on the permittee’s inspection log and documented
with photographs:
•Earth slides or mud flows.
•Evidence of concentrated flows such as the presence of rills, channels, or
gullies.
•Turbid water leaving the construction site.
•Sediment from the construction site tracked onto public or private streets.
•Sediment on adjacent property that is not part of the project.
When a discharge occurs, the ESC permittee is responsible for informing the
City’s ESC Inspector, updating the ESC plan to prevent another occurrence,
and submitting the modified plan to the City’s ESC Inspector for review and
Approval of an ESC
plan by the City does
not relieve the
permit holder from
the responsibility to
ensure that ESCs are
properly installed
and maintained at
the construction site.
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approval. Unreported discharges discovered during a City inspection are a violation of the ESC permit
and subject to citations, fines, stop work orders, and permit revocation.
Modifications to Approved ESC Plans
It is important to modify the ESC Plan to respond to changing conditions at the
construction site. Additional controls must be installed immediately if site
conditions deteriorate because of rain or ineffective controls. A modified ESC
Plan documenting these changes must be sent within 2 business days to the
City ESC Inspector for review and approval.
Changes not needed to respond to a time-sensitive event must be documented
as a modification on the previously-approved ESC plan and sent to the City’s
ESC Inspector for approval prior to making the change in the field.
Enforcement
To avoid a citation, contractors and developers must inspect and maintain all ESCs on a regular basis.
Stop work orders, citations, fines, and permit revocation may be issued for failure to install and maintain
required ESCs, or for allowing sediment or other pollutants to enter waterways or the public stormwater
system.
Post Construction
After a project has been completed, the disturbed area must be stabilized using permanent landscaping
material. When the disturbed area has been stabilized, the ESCs may be removed from the site and from
adjacent catch basins. For developments with multiple buildings or structures, ESCs must remain until
the last building or structure has been completed and all disturbed soil is stabilized.
Stop work orders apply to the entire construction site.
Mulched areas larger than 100 sq ft which have no other stabilization
method will not be considered stabilized.
Ineffective ESC
Plans are subject
to fines and
enforcement.
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13 DEFINITIONS
303(d) list
A list developed in conformance with Section 303(d) of the federal Clean Water Act that identifies
waters that do not meet water quality standards and where a total maximum daily load (TMDL) needs to
be developed.
Access Road
The area on private property that extends from the public ROW and is used to access and maintain a
stormwater facility.
Aggregate
Rock of specified quality and gradation.
Aggregate, Fine
Crushed rock, crushed gravel, or sand that passes a ¼” sieve.
Altered
Changed from the City’s original approved design.
Applicant
For the purposes of the SWMM, an applicant is an individual, group, or legal entity that submits
documents to the City Engineer for review and approval.
Approval
For the purposes of the SWMM, approval is the written notice that Engineering staff have reviewed and
approved the documents submitted in compliance with stormwater requirements.
As-Built
Drawings showing the constructed surface and subsurface utility infrastructure and their surveyed
locations on the vertical and horizontal plane.
Base Course
A layer of material placed under the surface wearing course of a pavement and its bedding course in
order to support them. For porous pavements, the base course is open-graded.
Bankfull
Elevation at the lower edge of perennial vegetation in the riparian corridor. It is the depth reached in a
waterway during the 1.2-yr 24-hr storm event.
Base Flood
The flood having a one percent chance of being equaled or exceeded in any given year (100-yr flood).
For areas influenced by the Willamette River, it is the elevation reached during the 1996 flood.
Reference LO Maps and LOC Figure 50.05.011-A, Maps A to D.
Base Flood Elevation
The elevation to which floodwater is anticipated to rise during the base flood.
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Bedrock
The native, contiguous, consolidated rock underlying the surface of the Earth. Above bedrock is usually
an area of broken and weathered unconsolidated rock or soil. Bedrock is sometimes exposed on the
surface, indicating that soil has been eroded.
Beneficial Use
The purpose or benefit to be derived from a waterway as designated by the Oregon Water Resources
Department or the Oregon Water Resources Commission.
Bridge
A single or multiple span structure, including supports, that carries motorized and non-motorized
vehicles, pedestrians, or utilities on a roadway, walk, or track over a watercourse, highway, railroad or
other feature.
Built-out
Land developed to the maximum extent allowed by the zoning designations in the City’s Comprehensive
Plan.
Capacity
The flow volume that a conveyance structure is designed to transport. See also Design Capacity and Full
Capacity.
Catch Basin
A structure, usually with a sump to provide sediment storage, that collects and conveys stormwater to a
stormline or a stormwater facility.
Channel
The land features (bed and banks) that confine a stream.
Channelized Flow
Stormwater flow that generally occurs 300 feet or more from the point of origin along the flow path but
which is not in an open channel. See also Shallow Concentrated Flow.
Check Dam
A rock structure embedded in an open channel or stormwater facility to control water depth or velocity.
Choker Course
A layer of aggregate placed into a facility to provide stability and reduce finer material above it from
migrating to the coarser layer below it.
Civil Engineer
For the purposes of the SWMM, a civil engineer is an engineer recognized by OSBEELS as a licensed
professional who has at least 4 years of experience in hydraulics and/or hydrology.
Cleanout
An access port, less than 6 inches in diameter, used to clean or inspect underground pipes or structures.
Clearing
Activity that removes vegetative cover while leaving the root system intact.
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Construction
Includes, but is not limited to, clearing, grading, excavation, and other site preparation or ground-
disturbing work related to the construction of residential buildings and non-residential buildings, and
heavy construction (e.g. highways, streets, bridges, tunnels, pipelines, transmission lines, and industrial
non-building structures).
Contaminated Soil
Soil which does not meet the Oregon Department of Environmental Quality’s Clean Fill Determinations.
Contractor
The person, partnership, firm or corporation that is licensed in Oregon to complete construction. The
term shall also include the Contractor’s agents, employees and subcontractors.
Contour
A vertical location of the ground surface as depicted by a line drawing.
Conveyance System
A system of structures used to detain and convey stormwater in a manner that allows efficient flow to a
discharge point.
Course
A material placed in one or more layers or lifts to a specified thickness.
Cross-Section
The image formed by a plane cutting through an object, at right angles, to a central axis. Generally used
for conveyance structures and stormwater facilities.
Crown
The inside top of a pipe.
Culvert
A conduit, open on both ends, that conveys water under a road, driveway or embankment in order to
connect two stream segments or open channels.
Curve Number (CN)
A unitless number used in SCS hydrologic models that is a function of the soil type, land cover,
infiltration capacity, and land use.
Designated Management Agency (DMA)
A federal, state or local governmental agency that has legal authority of a sector or source contributing
pollutants, and is identified as such by the Department of Environmental Quality in a TMDL (OAR 340-
042-0030(2)).
Design Capacity
The flow volume or rate that a stormwater facility or structure is designed to contain, treat, detain,
convey, or infiltrate to meet a specific performance standard. See also Full Capacity.
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Design Storm
A storm event with a specific recurrence interval, it is the expected amount of precipitation that will fall
within a specified amount of time, e.g., 24 hours.
Detention
The storage and controlled release of stormwater during and after a storm event.
Development
Any manmade change to unimproved real property, including, but not limited to, construction,
installation or alteration of a building or other structure, change of use, land division, establishment or
termination of a right of access, storage on the land, grading, clearing, removal or placement of soil,
paving, dredging, filling, excavation, drilling or removal of trees.
Discharge
The volume of water or stormwater moving from one point to another over a specific time period, e.g.
cubic feet per second (cfs).
Discharge point
The location at which water or stormwater leaves a watershed or structure.
Ditch
A constructed open channel that conveys stormwater and where infiltration is incidental.
Easement
The area of land granted for use by the City for the construction, reconstruction, operation,
maintenance, inspection, and repair of utilities.
Edge Restraint
An edging or curb that surrounds permeable pavers and prevents lateral movement under loading.
Emergency
An imminent event or circumstance causing or threatening environmental damage, including but not
limited to, fires, explosions, floods, tornadoes, earthquakes, volcanic activity, and spills or releases of oil
or hazardous material or wastes.
Engineering Development Review Staff
City staff with responsibilities for stormwater management. For capital improvement projects (CIPs), it
means the CIP Project Manager.
Energy Dissipation
The use of rock or concrete to reduce stormwater velocity.
Erosion
The weathering of soil or detachment and movement of soil particles by water, wind, ice, or gravity.
Erosive Velocity
The velocity at which flowing water exceeds a soil’s critical shear stress and is capable of dislodging and
transporting soil. Calculated using the Shield’s formula or estimated using the Hjulstrὂm diagram.
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Establishment Period
The time to ensure satisfactory establishment and growth of planted materials.
Excavation
Any act by which soil or rock is cut into, dug, quarried, uncovered, removed, displaced, or relocated.
Existing
Buildings, facilities or conditions, which are already in existence, constructed, or officially authorized.
Extended Filtration
The treatment of stormwater using a stormwater facility that contains natural or engineered media and
which results in a reduction of sediment and hydrophobic pollutants.
Facultative
Vegetation that can thrive in both wetlands and uplands.
Fill
Placement of any soil, sand, gravel, clay, mud, debris, refuse, or any other material, organic or inorganic.
Filtration
The treatment of stormwater using vegetation, sand, soil, or other media designed to specifically
remove a pollutant but which does not include infiltration.
Fine Aggregate
See Aggregate, Fine.
Fines
Soil that passes a #200 sieve and commonly referred to as silt and clay.
Flatwork
Flatwork includes, but is not limited to, sport courts, covered decks, outdoor kitchens, spas and pools
with associated decking, hardscaping, driveways, sidewalks, patios, and parking lots.
Floodplain
The low-lying area near a waterway that is subject to inundation during a specific flood, e.g. 100-yr
flood.
Flow Control
The controlled release of stormwater, through orifices, that reduces post-development flowrates to pre-
development flowrates prior to discharge offsite.
Flow-Through
See Filtration.
Freeboard
The vertical distance from the design water surface elevation to the elevation at which water overflows
a structure.
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Full Capacity
For analysis of conveyance and detention structures in the public stormwater system, it is 80% of the
design capacity. See also Design Capacity.
Grading
Altering the contour, topography, or natural cover of the land.
Gravel
Particles of rock, rounded or not, that will pass a 3-inch sieve but are retained on a No. 4 sieve.
Green Infrastructure
Stormwater infrastructure that 1) uses vegetation or other pervious surfaces to treat or infiltrate
stormwater or 2) allows the storage of rainwater for later use. See also Low Impact Development.
Groundwater
Subsurface water that occurs in the saturated zone of a geological formation and which fluctuates
seasonally. Seasonal high groundwater, for the purposes of calculating separation distance to
stormwater facilities, includes perched groundwater.
Hardscaping
Landscaping that provides structure and functionality to outdoor areas and which results in an impervious
surface. It includes, but is not limited to, water features, fire pits, retaining walls, covered arbors and
gazebos, and playgrounds not using woodchips or lawn as a surface.
Hazardous
A material or waste that exhibits one or more of the following characteristics: toxic, corrosive, irritant,
strong sensitizer, flammable, combustible, or pressure-generating through decomposition, heat or other
means.
Hydraulics
The study of the conveyance capacity of a stream or stormwater system given a specified amount of
water at a given time.
Hydraulically restrictive soil layer
A situation where infiltration is restricted because of bedrock or soil that is either glacially consolidated
soil with more than 50% fines or glacially unconsolidated soil with more than 70% fines.
Hydrograph
A graphic display of the discharge of stormwater over time. The area under the hydrograph represents
the total volume of stormwater created during a storm.
Hydrology
The study of water as it occurs in the atmosphere, on the surface, or underground.
Hydromodification
Modification of a stream channel due to development that either directly or indirectly results in
elevated streamflow and peak velocities when compared to undeveloped conditions.
Immediate
See Imminent
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Imminent
Actively occurring or expected to occur within the next 72 hours
Impervious Area
See Impervious Surface.
Impervious Surface
A manmade surface that prevents or retards the entry of water into the underlying soil. Non-traditional
surfaces include but are limited to: artificial turf, building eaves, walkways, compacted gravel areas,
pools, flatwork, hardscaping, and packed earthen materials.
Infiltration
The movement of fluid through soil. See also Surface Infiltration.
Infrastructure
Publicly-owned structures used to transport vehicles or to convey stormwater, water, or wastewater.
Infrastructure, Stormwater
Structures and vegetated areas used to treat, detain, or convey stormwater from impervious areas to
the approved discharge point or outfall.
Inlet
A structure used to convey stormwater into a stormwater facility or into a stormline.
Inspector
City Engineering representative authorized to inspect and report on project performance.
Invasive Plant
A non-native plant that causes economic or environmental harm and is capable of spreading to new
areas of the State (ORS 570.750).
Invert elevation
The elevation of the lowest part of the inside of a pipe, culvert, or ditch.
Land disturbance
Any activity that alters the land surface in a way that modifies its characteristics and that affects its
potential for runoff or erosion.
Landscaping
An activity that modifies the physical features of an area of land.
Large Project
A project that 1) creates, 2) replaces, or 3) creates and replaces 3,000 square feet or more of impervious
area.
Licensed Professional
A licensed civil engineer or a hydrologist with a minimum of 4 years of hydrologic and hydraulic
experience.
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LO Maps
The City’s GIS map of contours, watersheds, waterways, and utilities (wastewater, water, and
stormwater) among other layers.
Low Impact Development
A stormwater management approach that seeks to mitigate the impacts of increased runoff and
pollution using a set of planning, design, and construction approaches that promote the use of natural
systems for infiltration, evapotranspiration, and reuse of precipitation.
Microclimate
A local climate that is different from the overall climate of an area and has, among other characteristics,
1) temperatures affected by large impervious areas or adjacent concrete or brick, 2) shading from tall
buildings or large tree canopies, or 3) wind affected by traffic or tall buildings.
Middle Housing
Duplexes, triplexes, quadplexes, cottage clusters, and townhouses in residential zones.
Monopour
A construction technique where adjacent concrete structures are poured at the same time with no
separation to create a monolithic structure.
Municipal separate storm sewer system (MS4)
As defined by 40 CFR 122.26(b)(8), “a conveyance or system of conveyances (including roads with
drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm
drains): (i) Owned or operated by a state, city, town, borough, county, parish, district, association, or
other public body (created to or pursuant to state law) including special districts under state law such as
a sewer district, flood control district or drainage district, or similar entity, or an Indian tribe or an
authorized Indian tribal organization, or a designated and approved management agency under Section
208 of the Clean Water Act that discharges into waters of the United States. (ii) Designed or used for
collecting or conveying stormwater; (iii) Which is not a combined sewer; and (iv) Which is not part of a
Publicly Owned Treatment Works (POTW) as defined at 40 CFR 122.2.”
National Pollutant Discharge Elimination System (NPDES)
A program initiated by the U.S. Congress in 1972 and amended in 1987 as part of the Clean Water Act
that regulates discharges into navigable or regulated waters.
Naturalized Plant
A non-native plant that co-exists with native plants but does not cause environmental or economic
harm.
Noxious Weed
An invasive plant that is not easily controlled with chemical, biological, or physical methods, and that is a
priority of the Oregon Noxious Weed Control Program for prevention and control due to its adverse
economic or environmental impact.
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Onsite Retention
Infiltration of a designated stormwater volume such that there is no discharge offsite for the specified
storm event.
Open channel
An open linear depression, natural or manmade, that is used to convey channelized water.
Operations and Maintenance
Regularly-scheduled activities required to keep stormwater assets and their components functioning in
accordance with their design objectives. It does not include repairs to a failed or failing structure or
repairs required to avert an emergency situation.
Operations and Maintenance (O&M) Plan
A plan that describes a stormwater system’s operation and the regularly-scheduled maintenance
needed to maintain its treatment capacity.
Orifice
A structure used to control the flowrate out of a stormwater facility or conveyance structure.
Outfall
A structure used to discharge stormwater to a receiving water.
Overflow elevation
The elevation at which water overflows or enters the outlet of a stormwater treatment or detention
facility.
Peak flowrate
The maximum rate of flow during or after a storm event.
Perched Groundwater
Groundwater that is not defined as an aquifer and that accumulates above a layer of soil or rock that has
low permeability.
Permeable
A material that wicks precipitation to the surrounding clean rock for infiltration into the soil, i.e.,
permeable pavers.
Permeable interlocking concrete pavement (PICP)
Concrete pavers placed in an interlocking pattern with clean aggregate (not sand) used to fill in the
joints between them.
Permeable Pavers
A paving stone or grid system surrounded by clean rock that infiltrates the precipitation that falls on it,
i.e. PICPs, permeable paving stones, and porous grid systems.
Pervious
A surface that is able to infiltrate precipitation.
Planter
A vegetated stormwater facility with vertical concrete walls.
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Pollutant
Dredged spoils, solid waste, incinerator residue, filter backwash, sewage, garbage, sewage sludge,
munitions, chemical wastes, biological materials, concrete wash water, paints, radioactive materials
(except those regulated under the Atomic Energy Act of 1954, as amended [42 U.S.C. 2011 et seq.]),
heat, wrecked or discarded equipment, rock, sand, cellar dirt, industrial wastes, municipal wastes, or
agricultural wastes that are discharged into a waterway.
Ponding Depth
The depth of water, minus freeboard, reached in a stormwater facility before it overflows the top of the
facility or enters an overflow structure.
Porous
A material that is able to infiltrate precipitation, i.e., porous asphalt pavement or porous grid systems.
Porous Pavement
Pavement that infiltrates precipitation falling on it. Includes porous asphalt pavement and pervious
concrete pavement.
Pre-development
Represented as a curve number of 70, it is the representation of conditions present prior to
development or land alteration by Anglo-Europeans, i.e., conditions present at the time of the Lewis and
Clark expedition.
Pretreatment
Treatment of stormwater to remove a specific pollutant prior to stormwater entering a stormwater
facility or structure.
Professional Engineer
A civil engineer licensed by OSBEELS. See also Civil Engineer.
Public Improvement
An improvement to the public infrastructure required as a condition of private development and built by
a private entity.
Raingarden
A vegetated stormwater facility with a level bottom, sloped sides, and a length under 100 ft.
Receiving water
The waterway receiving stormwater from a public or private stormwater system. Virtually all receiving
waters are Waters of the State, and include “lakes, bays, ponds, impounding reservoirs, springs, wells,
rivers, streams, creeks, estuaries, marshes, inlets, canals, the Pacific Ocean within the territorial limits of
the State of Oregon, and all other bodies of surface or underground waters, natural or artificial, inland
or coastal, fresh or salt, public or private (except those private waters that do not combine or effect a
junction with natural surface or underground waters) that are located wholly or partially within or
bordering the state or within its jurisdiction.” (ORS 468B.005(10)).
Recurrence Interval
The number of years historically separating storms or floods of a specified size, e.g. 10-yr storm event or
100-yr flood.
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Redevelopment
The removal of impervious area on a property combined with construction of impervious area that may
or may not coincide with the original impervious area footprint. Construction may or may not be
completed at the same time as the impervious area removal.
Regional Facility
A stormwater facility receiving stormwater from multiple properties and which provides detention,
retention, or infiltration. For the purposes of the SWMM, a regional facility includes constructed
wetlands, retention ponds, detention ponds, and infiltration ponds.
Renovation
Structural repair or alteration of a failing or failed stormwater facility that is not regularly-scheduled
maintenance or nonstructural repair or alteration of a failed stormwater facility that is not regularly-
scheduled maintenance. See also Operations and Maintenance.
Replace or replacement
The removal of an impervious surface that exposes soil followed by the placement of an impervious
surface. Replacement also means the construction of a stormwater facility to substitute for an existing
stormwater facility. It does not include repair or maintenance activities on failing or failed structures or
facilities.
Restoration
Nonstructural repair or alteration of a failing stormwater facility that is not regularly-scheduled
maintenance. See also Operations and Maintenance.
Retention
The process of collecting and holding stormwater with no outflow.
Right-of-Way (ROW)
Public land used for public roads and utilities.
Rill
Small channels less than 12 inches deep that are created by flowing water.
Santa Barbara Urban Hydrograph (SBUH)
A hydrologic model that converts the runoff from a design storm into a hydrograph and routes it
through an imaginary reservoir.
Seasonally-High Groundwater
The highest elevation reached by groundwater from October to May. See also Groundwater.
Sedimentation
Deposition of sediment.
Sensitive lands
Areas that the City has designated as sensitive and that have environmental significance such as
wetlands, stream corridors, and tree groves, and that are more sensitive or easily damaged by
development impacts than non-sensitive land.
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Shallow Concentrated Flow
Concentrated flow ranging from 1 inch to 6 inches in depth that has a defined shallow path. See also
Channelized Flow.
Sheetflow
Laminar flow that is less than 1 inch in depth, not channelized, and generally occurs from the point of
origin to 100 feet along a flow path. See also Shallow Concentrated Flow.
Single-Family Residential (SFR)
Projects completed on properties zoned as low or medium density residential and that are not
considered as middle housing.
Small Project
A new or redevelopment project that creates and/or replaces at least 1,000 sq ft of impervious area but
less than 3,000 sq ft.
Source Control
A structure or operation intended to prevent pollutants from coming into contact with stormwater
through physical separation.
Spall, Concrete
The weathering or deterioration of concrete resulting in pieces breaking off from the main structure.
Spillway
An outlet used to pass flows exceeding a pond or wetland’s design capacity.
Standard Details
The set of detail drawings contained in the City’s Standard Construction Specifications and Drawings.
Steep slope
Topography with a slope of 15 percent or more.
Storm Drain
A structure that receives stormwater from a street and conveys it to a stormline or stormwater facility.
Stormwater
Water from precipitation, primarily rainfall and snowmelt, that falls on impervious surfaces and does not
infiltrate into the soil.
Stormwater facility
Structures designed to treat, infiltrate, retain, detain, or convey stormwater.
Stormwater System
A series of connected stormwater structures and treatment facilities that manage stormwater.
Stream
A naturally flowing surface water that produces a definable channel and which can be perennial,
intermittent, or ephemeral. It includes areas where development was allowed to divert the natural
stream channel but does not include ditches, swales, or overflow channels from stormwater facilities.
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Sump
The distance from the bottom of a structure to the bottom (invert) of an outlet pipe.
Surface Infiltration
As defined by OAR 340-044-0005, surface infiltration is fluid movement from a ground surface into the
underlying soil. See also Infiltration.
Surface water management system
The natural and manmade facilities utilized by the surface water management utility to regulate the
quantity and quality of surface water, including drainage easements, stormwater conveyance and
treatment facilities, and waterways.
Surface water management utility
The entity that plans, designs, constructs, maintains, administers, and operates the City’s surface water
management system, and the regulations for facility control. The surface water management utility also
establishes standards for design and construction.
Swale
A linear vegetated facility with sloped sides, check dams, and a length of 100 feet or more. They differ
from ditches in the reduced velocity and gently sloped sides.
Time of Concentration
The time required for stormwater to travel from the most hydraulically distant point in an area of
interest to the discharge point.
Total Maximum Daily Load (TMDL)
The amount of an identified pollutant, as determined by DEQ, that a specified waterway can receive and
still meet water quality requirements.
Underdrain
A prefabricated perforated PVC pipe used to collect treated stormwater in a facility.
Underground Injection Control
A structure that discharges stormwater into the underlying soil without allowing surface infiltration. For
the purposes of the SWMM, UICs approved for treating stormwater include infiltration trenches,
infiltration galleries, and drywells.
Utility
The public wastewater, water, and stormwater systems used to provide City services.
Vegetated Facility
A stormwater facility that uses vegetation and a specific soil mix to treat stormwater. Vegetated
facilities include raingardens, planters, swales, and vegetated filter strips.
Water-Quality Snout
A structure installed over an outlet pipe that minimizes the volume of oil, floatables, solids, and trash
entering the outlet pipe.
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Waters of the State
Lakes, bays, ponds, impounding reservoirs, streams, creeks, estuaries, marshes, inlets, canals, the Pacific
Ocean within the territorial limits of the state of Oregon, and all other bodies of surface or underground
waters, natural or artificial, inland or coastal, fresh or salt, public or private (except those private waters
that do not combine or effect a junction with natural surface or underground waters) that are wholly or
partially within or bordering the state or within its jurisdiction [OAR 340-045-0010].
Waterway
Waters of the State as well as springs, wells, rivers, and all other bodies of surface or underground
waters, that are located wholly or partially within the city or for which the City has jurisdictional
authority.
Wearing Course
The top surface of a pavement system which is directly subject to traffic loads.
Wetland
An area that is inundated or saturated by surface water or groundwater at a frequency and duration
sufficient to support, and that under normal circumstances does support, a prevalence of vegetation
typically adapted for life in saturated soil conditions, commonly known as hydrophytic vegetation.
Wetlands generally include, but are not limited to, swamps, marshes, bogs and similar areas.
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14 REFERENCES
American Concrete Institute (ACI), June 2023, Pervious Concrete – Report, ACI PRC – 522-23.
American Public Works Association (APWA) and Oregon Department of Transportation (ODOT), 2021,
Standard Specifications for Construction.
American Society of Civil Engineers (ASCE), 2018, ASCE Standard 68-18: Permeable Interlocking Concrete
Pavement.
American Society for Testing and Materials (ASTM) International, 2018, D3385, Standard Test Method
for Infiltration Rate of Soils in Field using Double-ring Infiltrometer.
ASTM International, 2019, E1903, Standard Practice for Environmental Site Assessments: Phase II
Environmental Site Assessment Process.
ASTM International, 2022, C478/C478M, Standard Specification for Circular Precast Reinforced Concrete
Manhole Sections.
ASTM International, 2022, D8152-18, Standard Practice for Measuring Field Infiltration Rate and
Calculating Field Hydraulic Conductivity Using the Modified Philip Dunne Infiltrometer Test.
Bumbaco, K.A., C.L. Raymond, L.W. O’Neill, D.J. Hoekema, 2024, 2023 Pacific Northwest Water Year
Impacts Assessment.
Cahill, T.H., 2012, Low Impact Development and Sustainable Stormwater Management, McGraw-Hill.
Dalton, M., Mote, P.W., Snover, A.K. 2013. Climate Change in the Northwest: Implications for our
Landscapes, Waters, and Communities. Washington, D.C.: Island Press.
Debo, Thomas N. and Andrew J. Reese. 2003. Municipal Stormwater Management, 2nd ed. Lewis
Publishers, CRC Press LLC. Boca Raton, Florida.
Fleishman, E., editor, 2023, Sixth Oregon Climate Assessment. Oregon Climate Change Research
Institute, Oregon State University, Corvallis, OR DOI 10.5399/osu/1161.
Greger, M., and Landberg, T, 2023, Removal of PFAS from water by aquatic plants, Journal of
Environmental Management, Vol 351, 29 December 2023.
Gresham, City of, 2025, Stormwater Management Manual.
King County. 1998. King County, Washington Surface Water Design Manual. Department of Human
Resources.
Lane, E.W. 1955. Design of Stable Channels. Transactions of the American Society of Civil Engineers,
Volume 120, pages 1-34.
Leopold, L.B., and T. Maddock. 1953. The Hydraulic Geometry of Stream Channels and Some
Physiographic Implications. US Geological Survey Professional Paper, 252.
- 123 -
National Resource Conservation Service (NRCS), 2007, National Engineering Handbook, Part 630
Hydrology, Hydrologic Soil Groups.
NRCS, Soil Texture Calculator, Multi-Point Texture Triangle at
https://www.nrcs.usda.gov/resources/education-and-teaching-materials/soil-texture-calculator
Oregon Department of Agriculture, 2024, Noxious Weed Policy and Classification System
Oregon Department of Environmental Quality (DEQ), 2019, Clean Fill Determinations.
Oregon Department of Environmental Quality (DEQ), 2019, Final Revised Willamette Basin Mercury Total
Maximum Daily Load.
Oregon Department of Transportation, 2014, Hydraulics Manual.
Oregon Invasive Species Council, 2025, https://www.oregoninvasivespeciescouncil.org/
OTAK. 2009, City of Lake Oswego Clean Streams Plan.
OTAK, 2022, Technical Memorandum: Stormwater Plan Review Procedures.
Portland, City of, 2020, Stormwater Management Manual.
Seattle, City of, 2017, Subsurface Investigation and Infiltration Testing for Infiltrating BMP’s, Stormwater
Manual – Appendix D.
Syranidou, E., Christofilapoulos, S, and Kalogerakis, N., 2017, Juncus spp. – the helophyte for all (phyto)
remediation purposes?, New Biotechnology, Vol 38, Part B, September 2017, pp 43-55.
Washington Department of Ecology, 2024, Stormwater Management Manual for Western Washington.
~ 124 ~
APPENDIX A STORMWATER REPORT TEMPLATE
A basic stormwater report may be completed for projects that use prescriptive sizing. All other projects
must use the regular stormwater report.
~ 125 ~
STORMWATER REPORT
Date: ___________________________
Building Permit # or Land-Use Case: ________________
Tax Lot Number: _______________________________
Nearest Address: _______________________________
Proposed Stormwater Facilities: ____________________
______________________________________________
Engineering Company: _________________________
Address: _____________________________________
___________________________________________
Engineer Name and License Number:
___________________________________________
Stormwater Facility Type of Impervious Area
(roof, driveway, parking lot, etc.) Impervious Area Treated Type Tier
Total Impervious Area
Project
Overview
1. Describe project objective (such as an addition, new building, remodel, or
redevelopment), development type (e.g. Commercial, SFR, Multi-Family Residential,
Middle Housing, or Industrial), and zoning of tax lot (or future zoning of completed
project).
2. Discuss whether the impervious area classifies the project as a Small Project or a
Large Project.
3. Discuss the applicable requirements.
Existing
Conditions
1. Soil Description:
a. Predominant soil class on the property.
b. Presence and extent of hydraulically-restrictive layers onsite and the depth (bgs)
c. Presence and extent of contaminated soil or an onsite wastewater system present
onsite.
2. Geotechnical Hazards:
a. Discuss slopes within 50 ft of the tax lot including any slopes that are 15% or
greater.
b. Discuss the presence or absence of weak soils and faults.
c. Discuss landslide risk (as defined by DOGAMI)
~ 126 ~
3. Hydrological Constraints and Hazards
a. Discuss the groundwater elevation (or estimated depth to groundwater) and the
presence or absence of a wetland, stream, or sensitive lands within 100 ft of the tax
lot
b. Discuss the presence of a 100-yr floodplain designation.
c. Discuss extent of fill and the elevation of neighboring properties relative to the
subject property.
4. Infiltration Rate:
a. Discuss the infiltration test method used, the test locations, and the depth at
which the tests were completed.
b. State the median measured infiltration rate. Provide a table of the test results.
Infiltration
Feasibility
1. Discuss Source Controls to be used on the site (see Chapter 5 – Source Control).
2. Discuss final contours of the property and the final elevation at property limits in
relation to the neighboring properties.
3. Discuss the results of a groundwater mounding analysis if 10,000 sq ft or more of
impervious area will be infiltrated on the property.
4. Discuss the design infiltration rate and whether it and the other factors of this
section support onsite retention.
5. Determine whether the system will provide onsite retention or water-quality
treatment.
Stormwater
Facility
Selection
1. Using the existing conditions previously discussed and the design infiltration rate,
discuss if a Tier 1 stormwater facility is feasible or infeasible. If a Tier 1 infiltration
facility is infeasible, discuss whether a Tier 2 facility is feasible or infeasible.
2. Discuss proposed location(s) of stormwater facilities and separation from
groundwater including whether groundwater elevations require the control of
buoyant forces..
3. Discuss how the stormwater facility will provide the water-quality treatment
required by Section 7.5 (Water-Quality Limited Waterways)
4. Discuss point of offsite discharge or location of the connection to the public
system (if filtration facilities are used)
5. Discuss any design constraints placed on proposed proprietary facilities by the
Washington TAPE GULD designation
Model Results
1. Discuss model used (SBUH vs HSPF) and the inputs to the model e.g. curve
number, time of concentration, design infiltration rate, total impervious area.
2. Discuss design requirements used in model such as longitudinal slope, storage
course depth, soil and ponding depth (for vegetated facilities), void ratios, etc
3. Discuss facility size (length, width, height)
Flow Control
Analysis (for
1. Discuss whether the project is exempt from flow control and why.
~ 127 ~
Large Projects
only)
2. Compare the pre-development flowrates with the post-development flowrates at
the respective design storms.
3. Discuss the number of orifices needed, if any, and their type, diameter, location,
and elevation.
Downstream
Capacity
Analysis
1. Discuss the method for determining the existing flows in the public stormwater
system e.g. zoning, areas where middle housing densities were used, and the
calculations or assumptions for determining impervious area and infiltration rates.
2. Using the results from the stormwater model, discuss the pre- and post-
development flowrates offsite and the existing flowrates of the public stormwater
system above and below the property. Discuss the location at which the offsite
discharge is less than 10% of the existing public system flows and the length from
the offsite discharge location to this point.
3. Discuss the existing capacity of the downstream public stormwater system and
whether stormwater from the project will cause the existing capacity to exceed 80%.
4. Discuss proposed stormwater improvements if required by the downstream
analysis.
Summary Summarize the sections above and discuss how the proposed stormwater
management plan complies with the requirements of the SWMM.
Appendices
1. Existing Conditions Map (including a stormwater basin map)
2. Proposed Conditions Map
3. Composite Utility Map
4. Watershed/Pipeshed Schematic Used in Model
5. Stormwater Model Calculations including time of concentration, curve number,
hydrograph, pre-development/post-development flowrates, and pipe capacity
graphics (if applicable).
5. WOE TAPE GULD design constraints
6. Written approval of variances or exemptions
7. Infiltration Report
8. Geotechnical Report (if applicable)
9. Proposed Operations and Maintenance Plan
Maps
1. Pre-development map showing existing buildings and other structures, infiltration
test locations and the areal extents of contaminated soil, historical fill, leach fields,
hydraulically-restrictive layers, weak soils, historical landslides, steep slopes,
waterways, and floodplains.
2. Post-development map showing the location of required source controls, retained
and proposed stormwater facilities and the conveyance to and from the facilities to
the approved offsite discharge point.
3. Composite utility plan sheet.
4. Stormwater plan sheet.
5. Catchment areas and the impervious area treated for each stormwater facility.
~ 128 ~
BASIC STORMWATER REPORT
Date: ___________________________
Building Permit or Land Use Case #: ________________
Tax Lot Number: _______________________________
Address or Nearest Address:
_______________________________
Proposed Stormwater Facilities: ____________________
______________________________________________
Designer Company: ________________________
Address: ___________________________________
___________________________________________
Designer Name and Contact Information:
___________________________________________
Stormwater Facility Drainage Type
(roof, driveway, parking lot, etc.) Impervious Area Treated Type Tier
Total Impervious Area
Project
1. Describe project objective (such as an addition, new building, remodel, or
redevelopment), development type (e.g. Commercial, SFR, Multi-Family Residential,
Middle Housing, or Industrial), and zoning of tax lot (or future zoning of completed
project).
2. Discuss whether the impervious area classifies the project as a Small Project or a
Large Project.
3. Discuss the applicable SWMM requirements .
Existing
Conditions
1. Soil Description:
a. Predominant soil class on the property.
b. Presence and extent of hydraulically-restrictive layers onsite and the depth (bgs).
c. Presence and extent of contaminated soil or an onsite wastewater system.
d. Discuss extent of fill and the elevation of neighboring properties relative to the
subject property.
2. Geotechnical Hazards:
a. Discuss slopes within 50 ft of the tax lot including any slopes that are 15% or
greater. b. Discuss the presence or absence of weak soils and faults.
c. Discuss landslide risk (as defined by DOGAMI)
~ 129 ~
3. Hydrological Constraints and Hazards:
a. Discuss the groundwater elevation encountered during testing (or the estimated
depth to groundwater if not encountered) and the presence or absence of a
wetland, stream, or sensitive lands within 100 ft of the tax lot.
b. Discuss the presence of waterways and 100-yr floodplain designation.
4. Infiltration Rate:
a. Discuss the infiltration test method used, the test locations, and the depth at
which the tests were completed.
b. State the median measured infiltration rate and the calculated design infiltration
rate.
Infiltration
Feasibility
1. Discuss the final contours of the property, final elevation at property limits in
relation to the neighboring properties.
2. Discuss the design infiltration rate. Determine if onsite retention is supported by
the information obtained during the site assessment.
Stormwater
Facility
Selection
1. Using the facility hierarchy of Section 7.2 (Stormwater Facility Selection – Facility
Hierarchy), discuss why the proposed facilities are appropriate for use at the site.
2. Discuss the proposed location(s) of stormwater facilities and groundwater
separation including whether it is necessary to control buoyant forces.
3. Discuss how the stormwater facilities provide the water-quality treatment
required by Section 7.5 (Water-quality Limited Waterways).
4. Discuss the point of offsite discharge or, for filtration facilities, the location of the
connection to the public system.
Prescriptive
Sizing
1. Discuss the appropriateness of the use of prescriptive sizing for the project and
the total amount of impervious area that has historically used prescriptive sizing on
the applicant’s property.
2. Discuss the number and dimensions of the facilities needed to manage
stormwater on the site and the sizing factor used in the analysis.
3. Provide a table listing each stormwater facility, impervious area treated, and the
prescriptive sizing factor used. Include any facilities on the site that previously used
prescriptive sizing and that will be retained with this project.
Summary Summarize the sections above and, discuss how the proposed stormwater
management plan complies with the requirements of the SWMM
Appendices
1. Existing Conditions Map
2. Proposed Conditions Map (including a stormwater basin map)
3. Composite Utility Map
4. Written Approval of Variances or Exemptions
Maps 1. Pre-development map showing existing buildings and other structures,
infiltration test locations, and the areal extents of contaminated soil, historical fill,
~ 130 ~
leach fields, hydraulically-restrictive layers, weak soils, historical landslides, steep
slopes, waterways, and floodplains.
2. Post-development map showing the location of retained and proposed
stormwater facilities and the conveyance to and from the facilities to the approved
offfsite discharge point.
3.Composite utility plan sheet
4. Stormwater plan sheet
5. Catchment area map delineated with the impervious area treated for each
facility.
~ 131 ~
APPENDIX B INFILTRATION TEST LOG AND REPORT TEMPLATE
~ 132 ~
INFILTRATION TEST LOG
Date: ___________________________
Building Permit or Land-Use Case Number:
______________________________________________
Tax Lot:___________________________________________
Address or Nearest Address:
______________________________________________
Company Name:__________________________
Company Address:________________________
_______________________________________
Tester Name/Contact Information:
_______________________________________
Soil Classification
Depth (ft, bgs) Soil Type
Infiltration Test
Test Method:_____________________________
Testing Depth, ft bgs:_________________
Presaturation Time:________ to ________
Test Hole ID#:_________
Test Hole Diameter or Length/Width, ft:
_____________________________________
Time
Time
Interval (hr)
Measurement
(ft)
Water Level Drop
(ft)
Infiltration Rate
(inches/hr) Notes
Test #1
NA NA NA
Test #2
NA NA NA
Test #3
NA NA NA
Median Measured Infiltration Rate:________________ inches/hr
~ 133 ~
INFILTRATION REPORT
Date:_______________________
Building Permit OR Land-Use Case Number:
____________________________________________
Tax Lot:_____________________________________
Address (or nearest Address:____________________
____________________________________________
Company Name:____________________________
Company Address:___________________________
___________________________________________
Contact Name ________________________________
Project
Description Describe the existing conditions including slope and photos
Infiltration
Discuss the infiltration test method used, number and descriptive location of the tests,
depth at which the tests were completed, the measured infiltration rates, and the
median measured infiltration rate.
Soil
Description
1. Describe the soil class at the infiltration depth for each test.
2. Describe the location and extent of any hydraulically-restrictive layers present on
the site and the elevation (ft, bgs).
Geotechnical
Conditions
1. Discuss geology and geotechnical hazards present onsite.
2. Discuss location and extent of contaminated soil present onsite.
Hydrologic
Conditions
1. Discuss the groundwater elevation (or estimated depth to groundwater) and the
presence or absence of a wetland or sensitive lands within 100 ft of the tax lot.
2. Discuss the location and extent of onsite wastewater systems including the leach
field.
Summary
1. State the median and design infiltration rates.
2. Provide a recommendation on the viability of using infiltration facilities on the site.
Appendices
1. Map showing the location of test sites.
2. Map showing the location of geotechnical and hydrological hazards, if any.
3. Boring Logs with moisture estimation (dry, moist, wet).
4. Infiltration Test Log.
~ 134 ~
APPENDIX C PLANT SELECTION AND APPROVED PLANT LIST
Plants play an important role in the function of stormwater
facilities and, with the right plant palette, can also contribute
aesthetically to the surrounding area. Landscape design must
thrive under local conditions such as soil moisture, light
exposure, nearby infrastructure, setbacks, sight distance
requirements, expected pedestrian traffic, and existing nearby
plant communities. Plants must adapt to local micro-climates at
private facility locations without permanent irrigation. In
addition to the “right plant right place” concept, it is important
to choose plants that do not require fertilizers or pesticides to
thrive.
C.1 Vegetative Diversity
Healthy and densely planted vegetation in a stormwater facility
improves its performance. Each vegetated stormwater facility requires a minimum number of plant
families to avoid catastrophic die-off within the facility due to weather conditions or pest infestations.
Diverse vegetation in stormwater facilities significantly improves stormwater treatment and plant
survivability. Plant diversity helps to:
• Reduce sediment. Plants act as physical filters by
capturing sediment and associated hydrophobic
pollutants. Facultative plant species are particularly
suited to reducing sediment load because of their
ability to thrive in saturated unsaturated soil.
• Reduce stormwater velocity. Vegetation reduces the
velocity through a stormwater facility and allows
additional settling time for pollutants in stormwater.
• Increase infiltration. Plant families have different
types of root systems (fibrous vs tap) and depth. Root
diversity protects a soil’s infiltration capacity.
• Reduce erosion. Fibrous root systems are more effective in preventing erosion in a stormwater
facility especially near inlets and other areas of high velocity or elevation drop.
• Reduce pollutant concentrations. Plants absorb pollutants
through their roots and store them to varying degrees. For
example, carex rostrata (beaked sedge) is excellent for
absorbing PFAS (Greger and Landberg, 2023). Plants in the
Juncus family are suitable for the removal of many heavy
metals (Syranidou, E., et al., 2017).
Right Plant - Right Place
For a vegetative stormwater
facility to function long-term, the
landscape professional must
consider plant placement and
optimum growing conditions.
Vegetative facilities are required
to maintain a minimum plant
coverage at plant maturity. The
width and height of each plant
species must be considered, as
well as the number of plants in
the facility, to provide adequate
coverage.
Stormwater filtration
results in sediment removal
as well as the reduction of
metals, phosphorus, and
other pollutants.
~ 135 ~
C.2 Invasive Species, Native Species, and Naturalized Species
Native species are plants that have adapted to our climate and
are a normal component of our ecosystem. They provide
benefits to our waterways and to our wildlife. Native species are
approved for all stormwater facilities and their use is strongly
encouraged for landscaping throughout the city.
Plants are defined as naturalized, invasive, or noxious by the
Oregon Department of Agriculture (ODA) based on tendencies to
become the dominant plant in a vegetative community, their
ability to spread quickly, and the difficulty involved to eradicate
them from an area. Invasive and noxious plants are prohibited
from use in a stormwater facility (see Table C-1).
Naturalized species are plants that are introduced from other areas of the world but co-exist with our
native species without dominating the ecosystem. They provide some benefit to wildlife but, in general,
do not provide a main food source or habitat for native wildlife.
Invasive species do not provide wildlife habitat nor are a main food source for native wildlife. Noxious
species are a subset of invasive species that are extremely hard to eradicate once they have colonized
an area. Invasive and noxious species are prohibited from use in stormwater facilities.
Clean Water Services
While Table C-1 provides a wide list of plants that are known to be noxious or invasive in Oregon,
the landscape professional must consult the current invasive lists available from ODA and the
Oregon Invasive Species Council to determine whether a plant is prohibited in a stormwater facility.
~ 136 ~
Table C-1. Invasive and Noxious Weeds in Oregon
Plant Latin Name Plant Common Name Plant Latin Name Plant Common Name
Acaena novae-zelandiae Biddy-biddy Hypericum perforatum St. John’s Wort
Aegilops, spp Goatgrass Impatiens glandulifera Policeman’s helmet
Adonis aestivalis Pheasant’s Eye Iris psuedocorus Yellow Flag Iris
Ailanthus altissima Tree of Heaven Isatis tinctoria1 Dyer’s Woad
Alhagi pseudalhagi Camel Thorn Linaria, spp Toadflax
Alliaria petiolate1 Garlic Mustard Lagarosiphon major Curly waterweed
Amorpha fruticose Indigo Bush Lamiastrum galeobdolun Yellow Archarngel
Anchusa officinalis1 Common Bugloss Lathyrus latifolius Perennial Peavine
Arundo donax1 Giant Reed Lepidium, spp2 Whitetop, Pepperweed
Brachypodium sylvaticum False Brome Limnobium laevigatum South American spongeplant
Bryonia alba White Bryonia Ludwigia, spp2 Water primrose
Buddleja davidii Butterfly Bush Lysimachia vulgaris Garden Yellow Loosestrife
Butomus umbellatus Flowering Rush Lythrum salicaria Purple Loosestrife
Carduus, spp Thistle Myriophyllum, spp Watermilfoil, Parrotfeather
Chondrilla juncea1 Rush Skeletonweed Nymphoides peltate Yellow Floatingheart
Centaurea spp. Knapweed, Star Thistle Odontarrhena spp Yellow tuft
Cirsium, spp Thistle Onopordum acanthium Scotch Thistle
Clematis vitalba Old Man’s Beard Orabanche minor Small broomrape
Conium maculatum Poison Hemlock Peganum harmala Wild Rue
Convolvulus arvensis Field Bindweed Phragmities australis Common Reed
Cortaderia jubata Jubata Grass Pilosella, spp2 Hawkweed
Crataegus monogyna English Hawthorn Phalaris arundinacea1 Reed Canary Grass
Crupina vulgaris1 Common crupina Potentilla recta Sulfur Cinquefoil
Cuscuta japonica Japanese Dodder Pueraria lobata Kudzu
Cynoglossum officinale Houndstongue Ranunculus ficaria Lesser Celadine
Cyperus esculentus Yellow Nutsedge Rosa canina Dog Rose
Cytisus, spp2 Broom Rosa rubiginosa Sweet Briar
Daphne laureola Spurge Laurel Rubus ameniacus Himalyan Blackberry
Delairea odorata German Ivy Salvia aethiopis Mediterranean Sage
Dipsacus laciniatus Cutleaf teasel Salvinia molesta Giant Salvinia
Echium spp2 Buglass Senecio jacobaea1 Tansy Ragwort
Egeria densa S American Waterweed Silybum marianum Milk Thistle
Erica lusitanica Spanish Heath Spartina, spp Cordgrass
Euphorbia, spp2 Spurge Sphacrophysa salsula Swainson Pea
Fallopia, spp Knotweed Tamarix ramosissima1 Saltcedar
Galega officinalis Goatsrue Taeniatherum caput Medusahead Rye
Geranium, spp Geraniums Tribulus terrestris Puncture Vine
Hedera, spp Ivy Tussilago farfara Coltsfoot
Heracleum mantegazzianum Giant Hogweed Ulex europaeus1 Gorse
Hydrilla verticillate Waterthyme Ventenata dubia Ventenata Grass
1 – Noxious; 2 – Some species are noxious; Source: ODA, 2024 and Oregon Invasive Species Council, 2025
~ 137 ~
C.3 Plant Selection
Choosing the plant palette for a stormwater facility should take into account the micro-climate at the
facility, plant availability, and maintenance of the facility (see Figure C-1). Information such as the post-
construction landscape and desired aesthetics will also affect the final landscape design.
C.3.1. Post-Construction Landscape and Microclimate
The design professional should become familiar with the post-construction conditions and microclimate.
Some questions to consider include:
• Is the adjacent area to be used for parking or bike lanes (plant overhang)?
• Is the adjacent landscape a street or a backyard?
• What will be the post-construction light conditions (morning sun, afternoon shade, sun all day)?
• Is the facility located near a tall building or high traffic street where plants will be buffeted by
wind?
• Is the entity responsible for maintenance of the facility a homeowner, business, or the City?
C.3.2 Plant Palette
Select plants that will provide for year-round water quality and aesthetic functions. A stormwater facility
can contain up to 10% forbs to improve aesthetics.
Vegetation coverage within the planted facilities must be shown on the site plan, with a diagram and
calculation for a minimum of 100% coverage at maturity. 100% mature coverage must be achieved
within one year following construction and shall never drop below 90% coverage.
C.3.3 Mature Plant Considerations
Consideration should be given to the mature height, as well as width, of the plants installed in a
stormwater facility. Sightlines need to be preserved at intersections for vehicular and pedestrian safety
when the plant is at maturity. If trees or large shrubs are being installed, the stormwater facility is
required to have 2 feet of topsoil to provide adequate room for the tree to mature. This is especially
important for public facilities.
C.4 Plant Availability
The landscape professional must ensure that plants are available for the
expected planting timeframe. Fall and early spring are the most common
times for planting because they allow some root growth prior to the plant
stress that occurs during the dry summers in the city. A temporary irrigation
system must be installed if the project schedule assumes that plants will be
installed from early summer to early fall.
Permanent irrigation
is required in public
stormwater facilities.
~ 138 ~
Figure C-1. Considerations for Plant Selection in a Stormwater Facility
C.5 Plant Maintenance
The landscape professional must choose plants that have maintenance requirements commensurate
with the experience normally associated with the type of responsible party. A stormwater facility for a
SFR project must contain plants that are easy to care for whereas a business that is likely to have a
landscape professional on staff could have a stormwater facility with plants that require more attention.
The following questions should be considered:
• Does the planting design provide easy access for maintenance?
• Will plant roots or overhang negatively affect inlets, outlets, or pedestrian traffic?
• Are the plants prone to diseases that require pesticides, herbicides, or fungicides? Maintenance
~ 139 ~
of plants in stormwater facilities cannot include the use of pesticides, herbicides, or fungicides.
• Do the plants require regular pruning? Plants where pruning will adversely affect the mature
shape of the plant due to sightlines, overhead power, or other utility conflicts must not be
installed in the stormwater facility.
Native Plants
Native species are a priority for stormwater facilities. They:
•Are adapted to local soil, hydrology, and climate conditions.
•Protect the infiltration capacity of soil in the stormwater facility.
•Compete with invasive species to prevent monocultures.
•Are more resilient to pests and diseases.
Native vegetation provides the following benefits for stormwater facilities. They:
•Blend into adjacent natural areas, open spaces, and
neighborhoods.
•Create a park-like visual experience for the community.
•Provides wildlife habitat (food and shelter).
•Encourage the presence of pollinators.
~ 140 ~
C.6 Plant List
Lake Oswego Plant List: Rushes
Junws acuminatus
Taper-Tip Rush
Juncus effusus var
pacificus
Pacific Rush
Juncus ensifalius
Dagger-Leaf Rush
Juncus occidentalis
Western Rush
Juncus patens
Spreading Rush
Juncus eftusus
soft Rush
Scirpus microcarpus
Small-fruited Bulrush
Wet
Moist to Wet
Mo,st to Wet
Moist to Wet
Moist to Wet
Moist to Wet
Moist to Wet --
Sun to Part Sun 3 ft
Sun 2 ft
Sun 2 ft
Sun 1 to 2 ft
Sun 2 ft
Sun 1 to 2.5 ft
Sun to Part Sun 2 ft -
3ft
3 ft
2 ft
2 ft
2 ft
1 ft
2 ft
Purple flowers In spring.
Drought tolerant. Attracts
birds and pollinators.
Evergreen.
Brown flowers in summer.
Grass-like.
Attracts birds.
Iris-like leaves.
Attracts birds.
Evergreen.
Blue-green foliage with clustered
brown seedheads In summer.
Attracts birds.
Evergreen.
Semi-evergreen.
Semi-evergreen.
OFF P Os
Lake Oswego Plant List: Sedges F �,
� � o
Op EGO
my
m
zz w
ek E
�N 3
q.,
Dense spreading sedge with
Corex morrowii Moderate to Wet Part Shade
to Shade lft 2ft foliage effect of dark green
Ice Dance Sedge with white borders.
Evergreen sedge that typically
Corex comae
grows in dense weeping clumps
1 New Zealand Sedge Moist to wet Sun 1 to 2 ft 1 to 2 ft of thin hair-like leaves.
Good for slowing
Corex denso flow and trapping
Dense Sedge Moist to Wet Sun to Part Sun 2 It 2 ft sediment.
Evergreen.
Pleasant colour that
Corex dipsacea changes throughout the
'- Autumn Sedge
,, Moist to Wet Sun to Shade 3 ft 1 to 1.5 ft season.
a '
=� Semi-evergreen.
Corex obnupto Moist to Wet Sun to Part Sun 3 ft 2 ft Upright seed heads.
Slough Sedge
This chocolate colored sedge is
Corex tenuiculmis an extrraordinary complement
gg New Zealand Hair Sedge Moderate to Moist Sun to Shade 1 It 2 ft to any garden.
- J W{•�- Evergreen.
A hlY -
if
Corex testoceo Moderate to Wet Sun to Part Sun 2.5 ft 2 ft Light green leaves that develop red
Orange Sedge or orange highlights.
1,A E
Lake Oswego Plant List: Ferns �
U J O
OPEGO�
wmm
w
mm
zz Ee o E
o!E d
Ed ' L
9
C1 N
Delicate fronds with black
Adiantum pedatum stems. Prefers shade.
Moist to Wet Part Sun to Shade 2 ft 1 to 2 ft
Maidenhair Fern Will go dormant in hot
temperatures.
6. _ i
Athyrium filix femino Large delicate leaves.
Lad Fern Moist to Wet Part Sun to Shade 3 ft 1.5 ft
Y Attracts birds.
Blechnum spicant Moist to Wet Part Sun to Shade 2 ft 1.5 ft Attracts birds.
Deer Fern
Evergreen.
Dryopteris arguta Attracts wildlife.
Moist Part Sun to Shade 3 ft 3 ft
Wood Fern Evergreen.
Gymnocarpium Bright green triangular fronds.
disjunctum Moist Part Sun to Shade 2ft 4ft
Western Oak Fern Attracts wildlife.
n� Polystichum munitum Attracts wildlife and birds.
A Dry to Moist Part Sun to Shade 3k 3ft
•'� 'f•. Sword Fern
Evergreen.
Woodwardiafimbriata Moist to Wet Part Sun to Shade 6ft 3ft Evergreen,
Giant Chain Fern
Lake Oswego Plant List: Grasses and Sedums
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Agrostis exarata
Spike Bentgrass
Bromus vulgaris
Columbia Brome
Deschampsia
caespitosa
Tufted Hairgrass
Sedum oreganum
Oregon stonecrop
Sedum spathulifolium
Broadleaf Stonecrop
Moist to Wet
Dry to Moist
Dry to Moist
Moist to Wet
Dry to Moist
Sun to Part Sun
Sun to Part Sun
Sun to Part Sun
Sun to Part Sun
Sun to Shade
0.3 ft
2 to 3 ft
1 to 3 ft
1 to 3 ft
0.5 ft
1 ft
2 ft
2 ft
1 ft
1 ft
Reddish brown narrow and erect (spike-like) seedheads.
Cool season bunchgrass.
Attracts wildlife, birds, and pollinators.
Drooping seedheads.
Cool season bunchgrass. Prefers shade.
Attracts wildlife, birds, and pollinators.
Nodding seedheads.
Cool season grass.
Attracts wildlife, birds, and pollinators.
Succulent groundcover with yellow flowers from spring to summer.
Attracts pollinators.
Evergreen.
Succulent groundcover with yellow flowers in spring.
Attracts birds and pollinators.
Evergreen.
O4X A E Os
Lake Oswego Plant List: Perennials ,
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w Blue,purple,or white flowers in
Anemone oregano Moist Part Sun to Shade 0.5 to 1 ft 0.25 ft late spring and early summer.
Oregon Anemone Attracts birds and pollinators.
Blue and white flowers in late
Aquilegia coeruleo spring to early summer.
Colorado Blue Moist Shade 1 to 2 ft 1 ft Attracts pollinators.
Columbine Plant in the fall(needs cold to
germinate).
Red to orange flowers in spring.
Aquilegio formosa Moist Sun to Part Sun 3 ft 1 to 2 ft Attracts pollinators.
Red Columbine Prefers well-drained soil.
Aster suspicotus Purple flowers from
Douglas Aster Dry to Moist Sun to Part Sun 1 to 1.5 ft 1 ft summer to fall. Attracts
birds and pollinators.
Chame
Magenta to rose-colored
Fireweed
gustiJglium
Fireweed Dry to Moist Sun to Part Sun 1 to 6 ft 1 ft flowers from summer to fall.
Purple tubular flowers in spring
Menzies'Larkspur Delphiniumkspur sii Dry to Wet Sun to Part Sun 1 to 2 ft 1 ft to early summer.
Attracts birds and pollinators.
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Pink flowers spring to
Dicentro formosa summer.Attracts birds and
Moist Part Sun to Shade 1.5 ft 1.5 ft pollinators.
Pacific Bleeding Heart
Will go dormant during hot
temperatures.
OF tiA E 0`ttL
Lake Oswego Plant List: Perennials (U o�
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Pink flowers in summer
Goulthfollowed by edible berries. Also
Oregon
Wiriantergreen
tergreen Moist to Wet Part Sun 0.25 ft 1 to 3 ft known as Oregon Teaberry.
Oregon Wintergreen Attracts birds and pollinators.
Evergreen.
Gentian parryii Moist to Wet Sun i to 2 ft 1 ft
Mountain Gentian Blue flowers in summer.
Hydrophyllum teniupes Moist Sun to Shade 1 to 3 ft 1 ft White to lavender bell-shaped
Pacific Waterleaf flowers in early summer.
Iris tenax Purple flowers in spring.
Oregon Iris Dry to Moist Sun to Part Sun 1 ft 1.5 ft
6 Attracts birds.
Lupins polyphyllus Purplish blue flowers In summer.
Large-Leaved Lupine Moist to Wet Sun to Part Sun 3 ft 2 ft Attracts birds and pollinators.
Liriopespicata Lavender flowers late
Moderate to Dry Sun to Part Shade I to 2 ft I to 2 ft Creeping Lilyturf summer.Goundcover,
speading.
Fragrant creamy white panicle
Malanthemum of flowers In early summer
racemosa followed by red berries.
Moist Part Sun to Shade I to 3It 1.5 ft
- � 4 Western False Attracts wildlife and birds.
Solomon's Seal
Slow growth.
Lake Oswego Plant List: Perennials F
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' Mimulus guttatus Yellow tubular flowers in
Common Monkeyflower Moist to Wet Sun to Part Sun 0.5 to 2.5 ft 0.5 ft summer.
Attracts birds and pollinators.
N Blue flower with white center in
Baabyby Blue
la menziesii e Eyes Moist Sun to Part Sun 0.5 to l ft 0.25 It spring and early summer.
Attracts pollinators.
yt,
Yellow flowers in summer.
Hypoxis hirsuta Attracts birds and
-- _ Yellow Star Grass Dry to Moist Sun to Part Sun 7ft 1.5 ft
pollinators.Needs welt-
drained soil.
Purple flowers in summer.
Penstemon serrulatus Moist to Wet Sun to Part Sun 1 to 2 ft 1 ft Attracts birds and pollinators.
Cascade Penstemon
Semi-evergreen.
y
Sagi
� aaria latifolio Wet Sun to Part Sun 1 to 3 ft 1 ft White flowers in summer to
Wap' Wapato early fall.
Sidalceo cusickii Rosy pink flowers In summer
Cusick's Checkermallow, Moist to Wet Sun to Part Sun 2 to 5 ft 1 It that deepen In color as they age.
Attracts pollinators.
AL .
' Viola sempe"t.rens Yellow flowers in spring.
Evergreen Violet Moist to Wet Part Sun to Shade 0.35ft 0.25ft
Evergreen.
Lake Oswego Plant List: Medium to Small Shrubs F
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Ceono[hus velutinus
Small pink-white Bowers
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Snow Brush Dry to Moist Sun 2 to 6 ft 3 It and white berries.
Attracts birds.
Evergreen.
Pink flowers in spring followed by
Gaultheriashollon blue-black berries in the fall.
Salal Dry to Moist Part Sun to Shade Ito 3ft 3to Oft Grows taller in shade.
Attracts birds and pollinators.
Evergreen.
yam,
- Yellowflowers in spring followed
Mahonia oquifolium by edible fruit. Bronze fall color
Tall Oregon Grape Dry to Moist Sun to Part Sun 4 to 6 h 4 it Attracts birds and pollinators.
Evergreen.
r^ Yellow flowers in spring followed
G + Mahonia nervosa by blue fruit.
Dry to Moist Sun to Part Sun 2ft 2h Attracts wildlife and birds.
Dull Oregon
Grape
Evergreen
Fragrant white flowers In late
Philodelphus lewisii Orange spring to midsummer.
Mock Oran Dry to Moist Sun to Part Sun B to 7 h 3 to 5 It Attracts birds and pollinators.
Prefers well-drained soil.
Reddish pink flowers in early spring
Ribessanguineum followed by edible bluish black
Dry to Moist Sun to Part Sun 4to8ft 7ft berries in fall.
Red Flowering Current Attracts birds and pollinators.
Prefers well-drained soil.
Pale pink fragrant Flowers In
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Rosa ip Rose arpa 3ft summer with orange-red rose hips
Baldhip Rose Moist to Wet Sun to Shade 3 to 5 k in the fall.
Soft spines(no thorns).
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Lake Oswego Plant List: Medium to Small Shrubs
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Fragrant flowers in late spring to
•,i; t�` Rosa nutkana midsummer with large scarlet rose
Nootka Rose Moist to Wet Sun to Part Sun 5to7ft 4to5 ft hips in the fall.
.ts Attracts birds and pollinators.
Curved thorns.
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A.a ram.
White flowers In spring followed by
Rubus parviJlorus pinkish red fruit in the summer.
Thimbleberry Moist to Wet Part Sun to Shade 5 It5 it Large soft leaves.
Attracts wildlife,birds,and
pollinators.
Magenta flowers in the spring
_ Rubus spectabilis followed by an orange-red fruit in
Salmonberry Moist to Wet Sun to Part Sun 6 to 8 ft 8 ft the summer.
_ Attracts wildlife,birds,and
pollinators.
Upright pyramidal dark pink
Spimea douglosii Flower in summer. Shrub shape is
Douglas Spirea ft Moist to Wet Sun to Part Sun 4to5 It 4to 6 more compact In the sun.
ram,
Attracts birds and pollinators.
Pink bell-shaped flowers In late
spring to summer followed by
Vaccinum ovatum
edible dark blue berries in the fall.
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- Evergreen Huckleberry
Slow growth.
Dry to Moist Sun to Shade 3 to 6ft 3 ft g
Attracts wildlife,birds,and
" s pollinators.
Evergreen.
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Yellow or pink flowers in spring
Vaccinum parvifollum followed by red tart berries In the
Red Huckleberry Dry to Moist Part Sun to Shade 3to 6ft Oft fall.
a Attracts wildlife,birds,and
pollinators.
White flowers in late spring or
early summer followed by red
Vibumam edule qtt edible berries. Red fall color.
Highbush Cranberry Moist to Wet Sun to Part Sun 6ft
Attracts wildlife,birds,and
pollinators.
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Lake Oswego Plant List: Small Trees or Large Shrubs
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_ Multiple-stemmed tree in sunny
areas. Grows as a vining shrub in
Acercircinatum Soft shady areas.
Moist to Wet Part Sun to Shade SS ft
Vine Maple Red-orange fall color.
Attracts wildlife and birds.
White flowers in early spring
Amelanchier olnifolio followed by edible black fruit.
I Mimi Dry to Wet Sun to Part Sun 10 to 18ft loft
Serviceberry
Attracts birds and pollinators.
Pink flowers in early summer.
Reddish flaky bark for winter
Arctostaphylos interest.
columbiana Dry to Moist Sun loft Soft
Hairy Manzanita Attracts birds and pollinators.
Evergreen.
Fragrant blue flowers in spring and
fall.
Ceanothus thyralflorus
Blue Blossom Dry Sun 16 it6 to 8 It Attracts birds and pollinators.
Evergreen.
Pacific nuttallod Moist to Wet Part Sun to Shade 2oft Soft spring. Orange Large white to to nk blooms In
Pacific Dogwood purple fall color.
White flowers in late spring. Blue
Corns sericea Inedible fruit in summer.
Red-Osier Dogwood Moist to Wet Sun to Part Sun 8toloft 6to8ft Red bark in sunny areas.
Attracts birds and pollinators.
Edible nuts. Yellow leaves in fall.
Corylus cornuta
Beaked Hazelnut Dry to Moist Part Sun 12ft 12ft Attracts wildlife.
Prefers well-drained soil.
IIIIIIIIIIIIIIIII,
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Lake Oswego Plant List: Small Trees or Large Shrubs F
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Euonymus occidentalis
Western Wahoo Moist Part Sun to Shade 10 to 15 ft 10 ft Red and yellowfall color.
White to cream flowers in late
spring to late sumer.Flowers
Holodiscus discolor turn light brown and stay on
Oceanspray Dry to Moist Sun to Part Sun loft 6ft plant through winter.
Vase-shaped.
Attracts birds and pollinators.
Yellow flowers in spring and
Lonicera involucrota Moist[o Wet Sun to Part Sun 10 to 12 ft 7 ft summer.Paired inedible black
Black Twinberry berries.
Attracts birds and pollinators.
Fragrant pinkish white flowers in
spring.Edible tart apples and
Malus Fusco Moist to Wet Sun to Part Sun 10 to 30 ft 15 ft orange-red color in fall.
Pacific crabapple
Attracts wildlife,birds,and
pollinators.
White flowers in early spring.
1pi++r� Oemleria cerosiformis Bluish-black plum-shaped fruit in
Indian Plum Dry to Moist Part Sun to Shade loft 5ft fall.
Attracts wildlife and birds
White flowers in late spring. Red
Physocarpus copltatus seed clusters in summer and fall.
Pacific Ninebark Moist to Wet Sun to Part Sun 10 to 12 ft 5 to 8 ft Shredded bark for winter interest.
Attracts birds and pollinators.
Fragrant white flowers in spring
which are favored by cedar
Braes
followed by Inedible red cherries
Bitter Cherry
nata Moist to Wet Sun to Part Sun 20 ft 15 ft waxwings.
Attracts wildlife,birds,and
pollinators.
IIIIIIIIIIIIIIII'
Lake Oswego Plant List: Small Trees or Large Shrubs
Prunus virginiana
Common Chokecherry
Rhamnus purshiono
Cascara
Sombucus coeruleo
Blue Elderberry
Sombucus rocemoso
Red Elderberry
Sorbus sitchensis
Sitka Mountain Ash
Toxus brevifo/io
Pacific Yew
Viburnam dovidii
David Viburnam
Moist to Wet
Moist
Dry to Moist
Dry to Wet
Dry to Moist
Moist to Wet
Dry to Moist --
Sun to Part Sun 15 ft 12 ft
Sun to Part Sun 20 ft 15 ft
Sun to Part Sun 15 ft 15 ft
Sun to Shade 15 ft 15 ft
Sun to Part Sun 10 to 12 ft 5 ft
Sun to Shade 10 ft 10 ft
Sun to Shade 10 ft 8 ft -
White flowers In spring followed by edible tart cherries In summer.
Attracts wildlife, birds, and pollinators.
Shrub or small tree depending on
the conditions. Yellow fall color.
Dome-shaped white flowers
followed by edible blue berries.
Attracts wildlife, birds, and
pollinators.
Fragrant white flowers and red
be rries .
Attracts wildlife, birds, and
pollinators.
White flowers In early summer.
Tart red fruit in fall .
Attracts wildlife and birds.
Slow growth. Cited height is at 10
years. It can reach 40 ft.
Attracts birds.
Evergreen.
Small white flowers in spring
with black plum fruit in fall. Red
fall foliage.
Attracts wildlife, birds, and
Lake Oswego Plant List: Large Trees
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Arbutus menziesii
Pacific Madrone
Betula papyrifera
Paper Birch
Pinus Ponderosa
Ponderosa Pine
Fraxinus latifolia
Oregon Ash
Tsuga heterophylla
Western Hemlock
Dry to Moist
Moist to Wet
Moist
Dry to Moist
Dry to Moist Sun to Shade
Sun
Sun to Part Sun
Sun to Part Sun
Sun to Part Sun
70 ft
30 to 70 ft
60 ft
50 ft
120 to 200 ft
20 to 30 ft
20 ft
20 ft
35 ft
25 ft
Fragrant white flowers in spring followed by red fruit in the fall.
Attracts wildlife and birds. Peeling
bark.
Needs good drainage. Slow growth.
Evergreen.
Not recommended for areas near patios or parking areas.
Peeling bark.
Attracts wildlife and pollinators.
Attracts wildlife, birds, and pollinators.
Fragrant textured bark.
Attracts wildlife and birds.
Evergreen.
Graceful delicate needles.
Attracts wildlife and birds.
Evergreen.
~ 154 ~
APPENDIX D OPERATIONS & MAINTENANCE TEMPLATE
Date:___________________________
Building Permit # or Land-Use Case: ________________
Tax Lot:_______________________________________
Nearest Address:_______________________________
Owner Name:____________________________
Owner Address, if different:
___________________________
Stormwater
Facility
Descriptive
Location
Drainage Type
(roof, driveway, parking lot, etc.) Impervious Area Treated (sq ft)
Total Impervious Area (sq ft)
Stormwater
Facility Season Maintenance
~ 155 ~
APPENDIX E SIMPLE EROSION AND SEDIMENT CONTROL
TEMPLATE
~ 156 ~
Figure E-1. Example Simple Erosion and Sediment Control Plan