CA: San Francisco: Low Impact Design Toolkit for Stormwater

Low Impact Design Toolkit
W h a t w i l l y o u d o w i t h S a n Fra n c i s c o ’s S t o r m wa t e r ?

L O W I M PA C T D E S I G N T O O L K I T i

ii U R B A N S T O R M WAT E R P L A N N I N G C H A R R E T T E ▪ S e p t e m b e r 2 0 0 7

L O W I M PA C T D E S I G N T O O L K I T iii

Urban Watershed Planning Charrette
September 27, 2007
Bayside Conference Rooms, Pier 1
San Francisco, CA

Project Team:

San Francisco Public Utilities Commission
Arleen Navarret: Program Manager, WWPRD
Rosey Jencks: Project Manager
Leslie Webster: Design, Layout, and Illustrations

With research and editorial assistance from:

EDAW
Kerry McWalter
Megan Walker
Mark Winsor

Metcalf & Eddy
Scott Durbin
Kimberly Shorter
David Wood

Cover image:
Map of San Francisco drainage basins and historic hydrology

San Francisco Public Utilities Commission
Stormwater Management and Planning
1145 Market Street, 5th Floor
San Francisco, CA 94103
http://stormwater.sfwater.org

iv U R B A N S T O R M WAT E R P L A N N I N G C H A R R E T T E ▪ S e p t e m b e r 2 0 0 7

Ta b l e o f C o n t e n t s
I ntroduction 2

1. E co-Roofs 6
2. Downspout Disconnection 10
3. Cisterns 14
4. Rain Gardens 18
5. Bioretention Planter s 22
6. Permeable Paving 26
7. Detention Basins 30
8. The Urban Forest 34
9. Stream Daylighting 38
1 0. Constructed Wetlands 42

L O W I M PA C T D E S I G N T O O L K I T 1

Introduction
Summary Thank you for participating in today’s Urban Watershed Planning
Charrette. A charrette is a collaborative session in which a group of
designers, planners or engineers draft a solution to a design problem.
Charrettes often take place in sessions in which larger groups divide
into sub-groups and then present their work to the full group as
material for future dialogue and planning. Charrettes serve as a way
of quickly drafting design solutions while integrating the aptitudes
and interests of a diverse group of people.

While there are many planning challenges facing San Francisco, this
charette will focus on integrating San Francisco’s urban stormwater
into its built environment using green stormwater management
technologies collectively known as “Best Management Practices”
(BMPs), “Low Impact Design” (LID) or “Green Infrastructure.”
The goal is to identify LID techniques that reduce the peak flows
and volumes of runoff entering the combined sewers. LID has the
potential to increase the system’s treatment efficiency by delaying
and/or reducing the volumes of runoff flowing to the combined
sewer, providing stormwater treatment, enhancing environmental
protection of receiving waters, and reducing the volume and
frequency of combined sewer overflows (CSOs). These technologies,
if properly designed can also provide auxiliary benefits that include,
beautification, groundwater recharge, and habitat enhancement.
They can be placed into the existing urban fabric to give streets,
parks, plazas, medians and tree wells multiple functions.

2 U R B A N S T O R M WAT E R P L A N N I N G C H A R R E T T E ▪ S e p t e m b e r 2 0 0 7

Introduction

Pre-development conditions in San
Francisco

Existing conditions in San Francisco

What is Low Impact Design?
Before San Francisco developed into the thriving city it is today, it consisted of a diverse range of habitats
including of oak woodlands, native grasslands, creeks, riparian areas, wetlands, and sand dunes. These
habitats provided food and forage for a wide range of plants, animals and insects. The natural hydrologic
cycle, working its way through each of these ecosystems, kept the air and water clean and recharged the
groundwater.

Today, much of the city is paved or built upon and plumbed with a combined sewer to convey stormwater
and wastewater. Former creeks have been diverted to the sewers and wastewater from homes and runoff
from rain events flow to treatment facilities where it is treated and discharged into the bay and the ocean.
During large storm events, the combined sewer system occasionally discharges partially treated flows
into surrounding water bodies and floods neighborhoods. In areas not served by the combined sewer,
stormwater discharges directly into water bodies untreated. A large quantity of impervious surfaces means
that there are very few places where infiltration can occur and groundwater is depleted.


Introduction

LID is a stormwater management approach that aims to re-create and mimic these pre-development
hydrologic processes by increasing retention, detention, infiltration, and treatment of stormwater runoff at
its source. LID is a distinct management strategy that emphasizes on-site source control and multi-functional
design, rather than conventional pipes and gutters. Whereas BMPs are the individual, discrete water quality
controls, LID is a comprehensive, watershed- or catchment-based approach. These decentralized, small-
scale stormwater controls allow greater adaptability to changing environmental and economic conditions
than centralized systems.

LID has the potential to prevent the volume of combined sewer overflows and localized flooding in San
Francisco by slowing or intercepting stormwater before it reaches the sewer pipes. Roof runoff from buildings
can be intercepted by eco-roofs. The downspouts from roofs can be redirected to landscaped areas or cisterns
where the water can be stored and used during the dry seasons for irrigation or other non-potable uses.

Potential LID additions to urban hydrology

Downspout disconnection
Detention basins
Rain gardens

Eco-roofs

Street tree planting

Constructed wetlands

Bioretention
planters

Cisterns

Stream daylighting

Permeable paving


Introduction

Runoff from streets, parking lots and other paved areas can be directed to detention basins, or bioretention
planters where it is filtered and infiltrated. An expanded urban forest, can also intercept and uptake excess
water. Historic creeks can be returned to the surface and diverted away from the sewer system. Together,
these approaches decrease increase the efficiency of the sewer system and treatment facilities, reduce the
likelihood of flooding and sewer overflows, and recharge our local groundwater reserves.

To d a y ’ s G a m e
For the purposes of the Urban Watershed Planning Charrette each team will be asked to apply appropriate
stormwater BMPs within the boundaries of San Francisco’s four eastern catchments.

Each BMP performs specific functions such as delaying peak flows that reduce flooding and reducing
stormwater volumes that can be quantified based on studies and modeling that has been calibrated for San
Francisco. This booklet introduces and describes the benefits and limitations of each BMP used in today’s
charrette. Each basin has a set of stormwater management goals for peak flow and volume reduction. Your
job is to identify appropriate locations for the BMPs described in this booklet to address surface water
management goals in your basin. Your team then calculates the benefits and costs and determines how
closely you meet your stormwater management goals and stay within your budget. Each turn will consist
of placing your BMP in the landscape and tallying the benefits and costs. Be sure to look for opportunities
for partnerships, multi-purpose projects and synergies between adjacent or nearby developments within the
neighborhood.

The following toolkit describes each BMP and provides specific details on the benefits and limitations,
design details, and the costs of implementation and maintenance.


Eco-Roofs
Summary Green roofs, or eco-roofs, are roofs that are entirely or partially
covered with vegetation and soils. Eco-roofs have been popular in
Europe for decades and have grown in popularity in the U.S. recently
as they provide multiple environmental benefits. Eco-roofs improve
water quality by filtering contaminants as the runoff flows through
the growing medium or through direct plant uptake. Studies have
shown reduced concentrations of suspended solids, copper, zinc,
and PAHs (polycyclic aromatic hydrocarbons) from eco-roof runoff.
The engineered soils absorb rainfall and release it slowly, thereby
reducing the runoff volumes and delaying peak. Rainfall retention
and detention volumes are influenced by the storage capacity of the
engineered soils, antecedent moisture conditions, rainfall intensity,
Intensive eco-roof in Zurich,
Switzerland and duration. A typical eco-roof has been found to retain 50 to 65
percent of annual rainfall and reduce peak flows for large rain events
(those exceeding 1.5 inches) by approximately 50 percent.

Eco-roofs fall under two categories: intensive or extensive. Intensive
roofs, or rooftop gardens, are heavier, support larger vegetation
and can usually designed for use by people. Extensive eco-roofs are
lightweight, uninhabitable, and use smaller plants. Eco-roofs can be
PHOTO BY ROSEY JENCKS

installed on most types of commercial, multifamily, and industrial
structures, as well as on single-family homes, garages, and sheds.
Eco-roofs can be used for new construction or to re-roof an existing
building. Candidate roofs for a “green” retrofit must have sufficient
structural support to hold the additional weight of the eco-roof, which
is generally 10 to 25 pounds per square foot saturated for extensive
roofs and more for intensive roofs.


Eco-Roofs

Benefits Extensive eco-roof in Seattle, WA

• Provides insulation and can lower cooling

costs for the building
• Extends the life of the roof – a green roof can
last twice as long as a conventional roof,
saving replacement costs and materials
• Provides noise reduction
• Reduces the urban heat island effect
• Lowers the temperature of stormwater run-
off, which maintains cool stream and lake
temperatures for ﬁsh and other aquatic life
• Creates habitat and increases biodiversity in Limitations
the city • Poor design or installation can lead to po-
• Provides aesthetic and recreational ameni- tential leakage and/or roof failure
ties • Limited to roof slopes less than 20 degrees
(40 percent or a 5 in 12 pitch)
• Requires additional structural support to
bear the added weight
• Potentially increased seismic hazards with
increased roof weight
• Long payback time for installation costs

based on energy savings
• May attract unwanted wildlife
• Inadequate drainage can result in mosqui-
to breeding
• Irrigation may be necessary to establish
plants and maintain them during extended
dry periods
• Vegetation requires maintenance and can
Extensive eco-roof and detention basin in Germany look overgrown or weedy, seasonally it can
appear dead

C a s e S t u d y : To r o n t o , O n t a r i o
The City of Toronto initiated a green roof demonstration project in 2000 to “find solutions to
overcome technical, financial and information barriers to the widespread adoption of green
roof infrastructure in the marketplace.” In February 2006, the Toronto City Council approved
the Green Roof Pilot Program, allocating $200,000 from Toronto water’s budget to encourage
green roof construction. Subsidies of $10 per square meter ($0.93 per square foot) and up to
a maximum of $20,000 will be available to private property owners for new and retrofit green
roof projects. Additionally, the Green Roof Strategy recommended the following actions:
use green roofs for all new and replacement roofs on city-owned buildings; use zoning and
financial incentives to make green roofs more economically desirable; initiate an education
and publicity program for green roofs; provide technical and design assistance to those in-
terested in green roof building; identify a ‘green roofs resource person’ for each city division;
develop a database of green roofs in the city; conduct and support ongoing monitoring and
research on green roofs; add a green roof category to the Green Toronto Awards; and es-
tablish partnerships with other institutions.


Eco-Roofs

Design Details
An intensive eco-roof may consist of shrubs and small trees planted in deep soil (more than 6 inches)
arranged with walking paths and seating areas and often provide access for people. In contrast, an extensive
eco-roof includes shallow layers (less than 6 inches) of low-growing vegetation and is more appropriate
for roofs with structural limitations. Both categories of eco-roofs include engineered soils as a growing
medium, subsurface drainage piping, and a waterproof membrane to protect the roof structure.

Based on findings from the City of Portland (2006) and the Puget Sound Action Team (2005), roofs with
slopes up to 40 degrees are appropriate for extensive eco-roofs, though slopes between 5 and 20 degrees are
most suitable (slope ration of 1:12 and 5:12). All eco-roofs are assembled in layers. The top layer includes the
engineered soils and the plants. The soil is a lightweight mix that includes some organic material. Under the
soil is a drainage layer that includes filter fabric to keep sediment from the soil in place and a core material
that stores water and allows it to drain off the roof surface. Next is the root barrier, which prevents the roots
from puncturing the waterproof membrane that lies below it, and finally there is the roof structure.

Extensive eco-roof
Layers:
Most suitable slope of 5
to 20 degrees Drought tolerant plants Growing medium
(>2”)
Filter fabric
Leaf screen Gravel
Drainage and
storage
Root barrier
and waterproof
membrane
Roof structure

Overflow enters the gutter system

Cost and Maintenance
A typical roof size of a single-family home in San Francisco is estimated at 1,500 square feet, while commercial
developments are closer to 10,000 square feet. The costs of eco-roofs vary widely depending on the size
and the type of roof but average $18 per square foot to install. Each eco-roof installation will have specific
operation and maintenance guidelines provided by the manufacturer or installer. Once an eco-roof is mature,
maintenance is limited to the vegetation. Intensive eco-roofs generally require more continued maintenance
than extensive roofing systems. In the first few years watering, light weeding, and occasional plant feeding
will ensure that the roof becomes established. Routine inspection of the waterproof membrane and the
drainage systems are important to the roof longevity. Annual maintenance costs are estimated at $5.49 per
square foot, which includes aeration, plant and soil inspection, flow monitoring and reporting.


Eco-Roofs
Case Study: Chicago, IL
Chicago’s first green roof was a 20,000 square foot roof on the City Hall that was constructed
in 2000. In 2005, the city launched its Green Roof Grant Program, awarding $5,000 each to 20
selected residential and small commercial buildings green roof projects (each with a footprint
of less than 10,000 square feet). As of October 2006, more than 250 public and private green
roofs were under design and construction in Chicago, totaling more than 1 million square feet
of green roofs. The city also developed policies that encourage green roof development in
Chicago. For example, all new and retrofit roofs in the city must meet a 0.25 solar reflectance,
which green roofs are effective in meeting but traditional roofs are not. Also, the city offers a
density bonus for roofs that have a minimum of 50 percent vegetative cover.

Case Study: Portland, OR
Portland’s Green Roof Initiative began in the mid 1990s, when the Bureau of Environmental
Services (BES) started investigating the use of eco-roofs to control stormwater in their over-
burdened combined sewer system. In 1999, the Housing Authority of Portland, in cooperation
with BES, built a full-scale green roof on the Hamilton Apartments Building. The eco-roof cost
$127,500 (unit cost of $15 per square foot of impervious area managed), with $90,000 of that
granted by BES. Portland currently has about 80 eco-roofs built (roughly 8 acres) and another
40 in design or construction phases (roughly 10 acres).

Case Study: San Francisco, CA
San Francisco has several completed and in-process eco-roof projects including: the new
Academy of Sciences eco-roof which will be 2.5 acres or approximately 100,000 square feet;
the Environmental Living Center in Hunter’s Point; 2 acre intensive eco-roof on North Beach
Place; the Yerba Buena Gardens downtown which is mostly located above a parking ga-
rage; and Portsmouth Square another public open space over a garage in Chinatown.

References
City of Chicago. 2007 [cited 2007 Jun]. Chicago Green Roofs. Chicago, IL: Office of the Environment. Available
from: http://www.artic.edu/webspaces/greeninitiatives/greenroofs/main.htm

City and County of San Francisco. 2006. Low Impact Development Literature Review. Prepared by Carollo En-
gineers [Unpublished Memo].

City of Portland. 2007 [cited 2007 Jun]. Ecoroofs. Portland, OR: Office of Sustainable Development. Available
from: http://www.portlandonline.com/osd/index.cfm?a=bbehci&c=ecbbd, (June 2007).

City of Portland. 2006 [cited 2007 Jul]. Ecoroof Questions and Answers. Portland, OR: Bureau of Environmental
Services. Available from: http://www.portlandonline.com/shared/cfm/image.cfm?id=153098

City of Toronto. 2007 [cited 2007 Jun]. Greenroofs. Toronto, Canada. Available from:
www.toronto.ca/greenroofs/
Hopper LJ (Editor). 2006. Living Green Roofs and Landscapes Over Structure. In Time Saver Standards for Land-
scape Architecture, 2nd Edition, Hoboken. NJ: John Wiley and Sons, p. 367

Low Impact Development Center, Inc. 2007 [cited 2007 Jun]. Maintenance of Greenroofs. Available from:
www.lid-stormwater.net/greenroofs/greenroofs_maintain.htm

Puget Sound Action Team (PSAT). 2005 [cited 2007 Jul]. Low Impact Development: Technical Guidance Manual
for Puget Sound. Olympia, WA: Puget Sound Action Team and Washington State University Pierce County Exten-
sion. Publication No. PSAT 05-03. Available from: www.psat.wa.gov/Publications/LID_tech_manual05/LID_man-
ual2005.pdf


Downspout Disconnection
Summary Downspout disconnection, also called roof drain diversion, involves
diverting rooftop drainage directly into infiltration, detention, or
storage facilities instead of into the sewer. Rainwater can be harvested
from most types of rooftops. In areas where site conditions allow
infiltration, roof drainage can be conveyed to drainless bioretention
planters, dry wells, or can be simply dispersed onto a rain garden,
lawn, or landscaped area. On sites that are not amenable to infiltration,
roof drains can be routed into cisterns which are available in a range
of materials, sizes, and models, or under drained bioretention planters
that discharge to the sewer (see sections on Cisterns, Bioretention
Planters, and Rain Gardens). Roof rainwater harvesting can retain up
to 100 percent of roof runoff on site, discharging water in excess of
storage capacity flowing to the combined sewer.

Downspouts on DaVinci Middle
School in Portland, OR are directed
to cisterns and a water garden



Benefits Limitations
• Reduces runoff volume and attenuates • Pre-filtration (such as a first-flush diverter) is
peak flows required if water is to be stored
• May decrease water usage through low- • Added complexity for buildings with inter-
ered irrigation requirements nally plumbed stormwater drains
• Low installation costs • Secondary system is required to deal with
• Low maintenance requirements water after it leaves the downspout, such as
• Large variety of implementation locations a cistern or a rain garden
and scales

The cost of roof downspout disconnection for existing buildings varies depending on how the roof is plumbed.
Professional installation of new gutters that direct water to another BMP can cost approximately $2,000 per
household. In addition to these plumbing costs, the cost of the paired BMP (rain garden, cistern, etc.) also
needs to be incorporated and is often the most important element in the system.

Maintenance of disconnected downspouts is relatively light. Regular monitoring should check for litter
in the gutter system to prevent clogs to the connected BMP that would reduce efficiency of stormwater
capture. Checking to ensure that all parts of the system are operating properly is important. Additionally,
maintenance should be performed for the associated BMP as required.

Rainwater harvesting in Australia

PHOTO BY LESLIE WEBSTER



Current conditions

Houses can be plumbed internally
or externally

All roof water goes to the sewer

Cisterns can be placed above
ground, below ground, or inside
the house

Roof water can also be
directed to a rain garden
or other landscaped area
where infiltration is feasible

Overflow goes to the sewer
Disconnection options



Design Details
Downspout disconnection consists of diverting roof runoff to a storage or infiltration BMP. In San Francisco,
many residential properties are plumbed directly to the sewer. Disconnecting a downspout either to collect
water requires installing a diverter that directs water from the pipes into the catchment system. Roof runoff
water is diverted to a storage or infiltration system. Some pretreatment is required before the stormwater
can be stored to prevent clogging from leaf litter. The main considerations for designing downspout
disconnection and rainwater harvesting systems are: roof drainage configuration, site conditions for a
storage tank, construction of new laterals, and desired rainwater uses.


The City of Portland included downspout disconnection in its Cornerstone Projects for reduc-
ing the Combined Sewer Overflows (CSOs). The program began in 1995 and should meet its
goals of reducing CSOs by 94% by 2011. Households and small commercial buildings within
targeted neighborhoods voluntarily disconnect their roof drains from the sewer system and
redirect the flow to either a rain garden or a cistern. Some areas of the city are excluded from
the program because of inappropriate slopes and soils.

The city pays participants $53 per disconnection, or pays for a contractor to do the work.
Community groups earn $13 for each downspout they disconnect. The program currently has
49,000 homeowners participating (about 4,400 disconnections per year from 1995 to 2006),
and has removed approximately 1 billion gallons of stormwater per year from the combined
sewer system. Disconnection costs around $0.01 per gallon of stormwater permanently re-
moved from the sewer system. The new Clean River Rewards program offers stormwater dis-
counts for property owners who control stormwater on site. This is expanding roof disconnec-
tion to other parts of the city.

References

City of Portland. 2007 [cited 2007 Jun]. “Downspout Disconnection” Bureau of Environmental Services, Available
from: http://www.portlandonline.com/bes/index.cfm?c=43081

ual2005.pdf

Texas Water Development Board. 2005 [cited 2007 Jun]. “The Texas Manual on Rainwater Harvesting,” Third
Edition. Austin, Texas. Available from: http://www.twdb.state.tx.us/publications/reports/RainwaterHarvesting-
Manual_3rdedition.pdf

TreePeople. 2007 [cited 2007 Jun]. “Open Charter Cistern,” Available from:
http://www.treepeople.org/vfp.dll?OakTree~getPage~&PNPK=150


Cisterns
Summary Cisterns are a traditional technology employed in arid climates
to capture and store rainwater. Cisterns reduce the stormwater
volume by capturing rainwater for non-potable uses, such as
irrigation or ﬂushing toilets. Suitable for a single house or an entire
neighborhood, cisterns range in size and may be placed above ground
or underground. Smaller, above ground cisterns, also called rain
barrels, are appropriate for single homes. Underground cisterns save
valuable space in urban locations and are more aesthetically pleasing
than surface cisterns but require pumps and other infrastructure in
order to reuse the water, making their maintenance and installation
more expensive. Large underground cisterns can be placed below
various types of open spaces such as parks or athletic ﬁelds.

Case Study: Los Angeles, CA
The TreePeople Open Charter Elementary School Project retrofitted a paved schoolyard with
stormwater treatment train to slow the flow of water, decrease local flooding events, and
decrease the pollutant load. The treatment train consists of three components: a water treat-
ment device; a 110,000 gallon cistern that stores rainwater and feeds the irrigation system;
and a system of trees, vegetation and mulched swales that slow, filter and safely channel
rainwater through the campus. The water capture and treatment project cost $500,000.

TreePeople also completed a project that installed a 250,000 gallon underground cistern in
Coldwater Canyon Park, a 2,700 acre watershed retrofit in Sun Valley, in collaboration with
the County Department of Public Works.


Cisterns

• Reduces runoff volume and attenuates • Poor design, sizing, and siting can lead to
peak flows potential leakage and/or failure
• May decrease water usage if retained for • Storage capacity is limited
irrigation purposes or toilet flushing • Provides no water quality improvements
• Low installation costs • Lower aesthetic appeal (for above ground
• Low maintenance requirements (for above cisterns)
ground cisterns) • Water reuse options limited to non-potable
• Low space requirements (for underground uses
cisterns) • Requires infrastructure (pumps or valves) to
• Good for sites where infiltration is not an op- use the stored water
tion • Inadequate maintenance can result in mos-
quito breeding and/or algae production

The cost of cisterns varies depending on the size and type of cistern. According to the Low Impact Development
Center (2007), small residential rain barrels that connect to the existing gutters can be as inexpensive as
$225-$300 for 200-300 gallons of roof storage. A large scaled surface system costs approximately $40,000
for storage of 20,000 gallons of stormwater. Cisterns installed underground tend to have higher installation
and maintenance costs. Twice annual inspection is advisable to confirm that all the parts are operable and
not leaking. Regular use of the water stored in cisterns between rain events is critical to ensure storage
is available for the next storm event. During the rainy season, it can be difficult to use the stored water if
because irrigation is generally not necessary. The stored water can be used during the rainy season for
other non-potable uses such as toilet flushing or fire suppression.

Case Study: Cambria, CA
Cambia Elementary School
captures and stores run-

www.rehbeinsolutions.com/projects/cambria.html
off water from the entire
school site in a cistern lo-
cated underneath ath-
letic fields and uses the
stored water to irrigate the Photo from Rehbein Solutions, Inc

fields year round. All of the
stormwater on the 12 acre
campus is captured and
stored in large pipes that
are located under 130,000
square feet of new ath-
letic fields. Up to 2 million
gallons of water can be
stored.

Cistern at Cambria Elementary School


Cisterns

Design Details
Proper design, siting, and sizing of cisterns are critical to ensure their full peak flow benefits. Stormwater
from roof downspouts is stored in the cistern until it is pumped out for use, or it reaches capacity and
exits through an overflow valve. Cisterns should be designed to outflow away from building foundations.
Above ground cisterns without a pumping mechanism must be elevated to allow proper water flow. Some
pretreatment is required to prevent clogging (e.g. leaf screens and first-flush diverters) before the stormwater
can be stored to prevent clogging from leaf litter. Cisterns need to have access to air and light to avoid the
production of algae. Generally, cisterns have a raised manhole opening on the top that allows access for
maintenance and monitoring, which should be screened to prevent litter and mosquitoes from entering.

Options for rainwater reuse with a small-scale cistern

Leaf screens on
gutters prevent
clogging

Maintenance
opening has
screen to pre-
vent mosquito
and litter ac- Pump
cumulation

Water can be reused
First flush for non-potable uses
diverter

Sewer backflow pre-
vention device
Overflow enters the
combined sewer


Cisterns

Mosaic above ground cistern in Tennessee
PHOTO BY LESLIE WEBSTER

Case Study: Seattle , WA
Seattle Public Utilities (SPU) recently began their RainCatcher Pilot Program, which consists of
three different types of rainwater collection systems. First is tight-line, which directs rainwater
outflow to a pipe that flows under the yard, through weep holes in the sidewalk reducing
volumes deposited in the storm drain via the curb. The second type, the tight-lined cistern,
includes a cistern at the point of initial outflow that collects water during the storm event and
releases it slowly into the underground pipes. Third, orifice cisterns include an operable valve,
which can be opened during the wet season, discharging a small amount of water onto an
adjacent permeable surface such as a lawn or rain garden to slow down flow, or closed to
store up to 500 gallons of roof runoff, which can be used later for irrigation.

Each cistern costs the SPU a total of $1000 with $325 of that sum paying for the wholesale
purchase of the cistern and $675 to installation and the SPU overhead. The SPU also sells rain
barrels to households in the SPU’s direct service areas. The rain barrels cost $59 each for the
SPU customers and $69 for non-customers. SPU is currently analyzing the impact of cisterns on
the combined sewer system as part of a grant. SPU installs the RainCatcher at no cost to the
participant, provides maintenance and support, and evaluates the performance over time.

References
Los Angeles County. 2002 [cited 2007 Jun]. Development Planning for Stormwater Management: A Manual For
the Standard Urban Stormwater Mitigation Plan (SUSMP). Los Angeles, CA: Department of Public Works. Avail-
able from: http://ladpw.org/wmd/NPDES/SUSMP_MANUAL.pdf

Low Impact Development Center, Inc. 2007 [cited 2007 May]. Cost of Rain Barrels and Cisterns. Sizing of Rain
Barrels and Cisterns. Available from: http://www.lid-stormwater.net/raincist/raincist_cost.htm and
http://www.lid-stormwater.net/raincist/raincist_sizing.htm

Rehbein Environmental Solutions, Inc. 2007 [cited 2007 May]. Cambria Elementary School. Available from: http://
www.rehbeinsolutions.com/projects/cambria.html

Tom Richmond and Associates. 1999 [cited 2007 Jun]. Start at the Source: Design Guidance Manual for Storm-
water Protection. San Francisco, CA: Bay Area Stormwater Management Agencies Association. Available from:
http://scvurppp-w2k.com/pdfs/0203/c3_related_info/startatthesource/Start_At_The_Source_Full.pdf

TreeHugger. 2007 [cited 2007 Jun]. Seattle RainCatcher Pilot Program. Available from: http://www.treehugger.
com/files/2005/03/seattle_raincat_1.php


Rain Gardens
Summary Rain gardens are stormwater facilities integrated into depressed
landscape areas. They are designed to capture and infiltrate stormwater
runoff. Rain gardens include water-tolerant plants in permeable soils
with high organic contents that absorb stormwater and transpire it
back into the atmosphere. Rain gardens slow and detain the flow of
stormwater thereby decreasing peak flow volumes. They also filter
stormwater before it either recharges into groundwater reserves
or is returned to the combined sewer system. The are also easily
customizable and provide both habitat and aesthetic benefits. Rain
gardens are a subset of bioretention planters except that they do
not typically include engineered soils or an under-drain connection.
Their form is regionally variable - in the south and mid-west they
are often less formal, whereas in the west they often take a more
formal shape (see photos to right) Therefore, rain gardens are more
appropriate for residential landscaping or low impervious areas with
well draining soils.

Rain gardens are often small and can be implemented by private
landowners in small yards. They function like larger scaled
bioretention projects with many of the same benefits and limitations.
Stormwater from downspouts can be directed through an energy
dissipater to rain gardens to store and treat water before it makes it
to the sewer system or a receiving water body.


Rain Gardens

• Depth to bedrock must be over 10 feet for
• Reduces runoff volume and attenuates
infiltration based systems
peak flows
• Limited to slopes less than 5 percent, slopes
• Improves water quality
greater than 5 percent require check
• Improves air quality
dams
• Improves urban hydrology and facilitates
• Seasonal fluctuation in water quality ben-
groundwater recharge
efits based on the plants’ ability to filter pol-
• Low installation costs
lutants
• Low maintenance requirements
• Low space requirements
look overgrown or weedy, seasonally it may
• Creates habitat and increases biodiversity
appear dead
in the city
• Site conditions must be conducive to partial
• Provides aesthetic amenity
or full infiltration and the growing of vegeta-
• Easily customizable
tion or an underdrain is needed
• 10 foot minimum separation from ground-
PHOTO FROM www.ci.maplewood.mn.us

water is required to allow for infiltration,
unless the Regional Water Quality Control
Board approves otherwise
• Non-underdrained systems must have mini-
mum soil infiltration rates, no contaminated
soils, no risk of land slippage if soils are heav-
ily saturated, and a sufficient distance from
existing foundations, roads, subsurface in-
frastructure

Residential rain garden in Maplewood, MN

Formal rain garden in Portland, OR



Rain Gardens

Design Details
Rain gardens should be placed at least 10 feet from building foundations and typically collect stormwater
from roofs, small paved surfaces, or landscaped surfaces. The shallow depression fills with a few inches
of water during a rain event. Either the soils must be suitable to infiltrate the collected water or a more
intensive bioretention planter is recommended. Dense vegetation assists with the uptake of pollutants and
the absorption of the stormwater. Rain gardens require a minimum of a 5 percent slope and well-drained
soils to function correctly. Rain gardens are more appropriate for drainage areas less than 1 acre in size.

Typical rain garden

Water from 1 acre or
less of roof, paved or
landscaped surfaces Min. 4’ width
Dense vegetation tolerant
of wet and dry conditions

2-6” Ponding depth

Berm

Min. 10’ from downspout 2-3” Mulch
Optional 12”
sand bed

Native soils suitable
for infiltration

Rain gardens are a relatively low cost and low maintenance stormwater management solution. A resident can
build and install their own rain garden in their front or back yard for very little money. The more elaborate
the garden, the more expensive installation becomes. The cost averages $8 per square foot and are typically
about 600 square feet, making the total cost approximately $5,000. Some level of annual maintenance is
required and is most intensive soon after construction until the garden matures. In the early spring and fall
the garden needs to be weeded and the mulch refreshed bi-annually to encourage healthy vegetation and
pollutant uptake. Mulch and compost improve the soil’s ability to capture water. In the first season irrigation
may be necessary to establish the plants.


Rain Gardens

Rain garden at Glencoe Elementary School

Glencoe Elementary School in SE Portland installed a rain garden in their school grounds in
2003 to prevent neighborhood-wide combined sewer overflow problems by reducing runoff
volumes while providing aesthetic and educational amenities to the schoolyard. The com-
pleted rain garden is a 2,000 square foot infiltration and detention system that manages run-
off from 35,000 square feet of impermeable surfaces with a total cost of $98,000.

References
City of Portland. 2007 [cited 2007 Jun]. Design Report: Rain Garden at Glencoe Elementary School. Portland,
OR: Bureau of Environmental Services. Available from: http://www.portlandonline.com/shared/cfm/image.
cfm?id=147510

Rain Garden Network. 2007 [cited 2007 Jun]. Local, On-Site Solutions for your Local Stormwater Issues. Available
from: http://www.raingardennetwork.com/

Rain Gardens of West Michigan. 2007 [cited 2007 Jun]. Raingardens: Qualities and Benefits. Available from:
www.Raingardens.org

City of Maplewood. 2007 [cited 2007 Aug]. Rain Water Gardens. Available from: http://www.ci.maplewood.
mn.us/index.asp?Type=B_BASIC&SEC=%7BF2C03470-D6B5-4572-98F0-F79819643C2A%7D


Bioretention Planters
Summary Bioretention is the use of plants, engineered soils, and a rock sub-
base to slow, store, and remove pollutants from stormwater runoff.
Bioretention planters improve stormwater quality, reduce overall

volumes, and delay and reduce stormwater runoff peak ﬂows.
Bioretention planters can vary in size from small, vegetated swales to
multi-acre parks; however, there are limits to the size of the drainage
area that can be handled. System designs can be adapted to a variety
of physical conditions including parking lots, roadway median strips
and right-of-ways, parks, residential yards, and other landscaped
areas and can also be included in the retroﬁts of existing sites.
Bioretention in Vancouver, BC
Portland’s Green Streets Program has successfully implemented many bioretention projects
since it began in 2003 including bioretention curb-side planters constructed in the parking
zone on either side of a street, just up stream form the storm drain inlets. One such project, NE
Siskiyou Street, captures runoff from approximately 9,300 square feet of paved surfaces. Total
project cost (excluding street and sidewalk repairs) was $17,000, or $1.83 per square foot of
impervious area managed.

Mississippi Commons, a mixed-use development project incorporates an internal “Rain Drain”
system, which collects stormwater from the 20,000 square foot roof area, which was previously
connected to the combined sewer, and directs it to a courtyard planter. The planter removes
an average of 500,000 gallons of stormwater annually from the combined sewer system and
was designed as an architectural feature for the internal courtyard of the development.

New Columbia, an 82 acre redevelopment area, is Portland’s largest Green Streets site, with
101 vegetated pocket swales for biofiltration, 31 flow-through planter boxes and 40 infiltration
dry wells. It used 80 percent less underground stormwater piping than a comparable tradi-
tional development and 98 percent of the stormwater is retained on the site.



Bioretention planter in
Portland, OR
The installation costs for a bioretention planter in
San Francisco that would be capable of managing
stormwater from a half acre of land is $65,000. Such
a system would be approximately 2,200 square feet
making the cost $39 per square foot. Operations and
Benefits maintenance costs are estimated at $1,168 per acre
per year based on data from the City of Seattle and
• Reduces runoff volume and attenuates adjusted for local factors. Like any landscape feature,
peak flows bioretention planters must be pruned, mulched and
watered until the pants are established. Semi-annual
plant maintenance is recommended including
• Improves urban hydrology and facilitates
groundwater recharge the replacement of diseased or dead plants.
• Lowers the temperature of stormwater run- Other regular maintenance requirements include
off, which maintains cool stream tempera- trash removal and weeding. Because some of the
tures for fish and other aquatic life sediment that enters bioretention planters have the
• Reduces the heat island effect propensity to crust on the soil surface, which limits
the porosity of the soils, some raking of the mulch
in the city
and soil surface may also necessary to maintain
high infiltration rates.
Limitations
• Depth to bedrock must be more than 10
feet for infiltration based systems
• Limited to slopes less than 5 percent
• Seasonal fluctuation in water quality ben-
efits based on the plants’ ability to filter pol-
lutants
look overgrown or weedy, seasonally it may
appear dead
• Site conditions must be conducive to partial
or full infiltration and the growing of vegeta-
tion
• 10 foot minimum separation from ground-
water is required to allow for infiltration,

unless the Regional Water Quality Control
Board approves otherwise
• Must have minimum soil infiltration rates, no
contaminated soils, no risk of land slippage
if soils are heavily saturated, and a sufficient
distance from existing foundations, roads,
subsurface infrastructure, drinking water
wells, septic tanks, drain fields, or other ele-
ments. Bioretention planter in Vancouver, BC



Design Details
During a storm event, runoff may temporarily pond in a bioretention depression as it percolates through the
mulch layer and engineered soil mix. Plant material provides water quality benefits as the roots and soils
uptake some pollutants from stormwater. Bioretention areas can either infiltrate a portion of or all of the
stormwater runoff depending on site and soil conditions. A perforated underdrain pipe is recommended,
in areas with poorly drained native soils. In areas where infiltration is facilitated by well-drained soils,
bioretention planters can be designed without the underdrain, much like rain gardens, to infiltrate the
stormwater. The primary considerations in siting a bioretention planter are space availability, suitability of
the soils for infiltration, rates, depth to groundwater, depth to bedrock, and slope.

Bioretention planters should be designed with a maximum of 6 inches of ponding on the top surface, which
includes mulch and wet-tolerant vegetation. A minimum of 4 feet of engineered soils and a gravel drainage
layer beneath the vegetation allow for proper infiltration. To ensure proper functioning, the maximum
drainage area for a single bioretention cell is 5 acres with a minimum of 5 feet of head to ensure drainage.
Installing an energy dissipater (i.e. grass channel, rip rap, etc.) to slow the water velocity at the entrance to
the bioretention area will minimize the potential for erosion or vegetation damage.

Typical bioretention planter

Dense vegetation tolerant of wet
and dry conditions

Max. 6” ponding
depth
Curb cut
2-3” Mulch
1% Min. Slope Building

Stone used to
dissipate energy Gravel curtain drain
protects building foun-
dation
Min. 4’ engineered soils
Optional sand filter layer

Perforated pipe in
gravel jacket
Infiltration where feasible



Dense vegetation tolerant
of wet and dry conditions

‘Parking Max. 6”
egress zone:’ ponding
concrete pav- depth
ers over sand

Curb cut 2-3” Mulch
Optional sand
Min. 4’ engineered soils filter layer
Perforated pipe
Infiltration where feasible in gravel jacket Street-side bioretention planter
based on Portland’s Green Streets

Seattle Public Utilities’ (SPU) Natural Drainage Program was established in 1999. Street Edge
Alternatives (“SEA Streets”) was SPU’s pilot natural drainage systems project. A residential
block was retrofitted with a narrower, meandering street with flat curbs, lined with vegetated
swales and amended soils on both sides. The swales detain stormwater from the street right-
of-way and properties along the east side of the street, totaling 2.3 acres. The project cost
was $850,000, making the cost per square foot of drainage area managed, not including the
replacement of sidewalks or streets, between $3 and $5.

The second project, the High Point Redevelopment Project, used swales, permeable pave-
ment, downspout disconnection, rain gardens, tree preservation, and bioretention to man-
age runoff from 129 acres of mixed income housing. Construction began in 2003 and will be
complete in 2009.

References
Bioretention.com. 2007 [cited 2007 May]. Components. Design Details. Maintenance. Retrieved at www.biore-
tention.com

City of Portland. 2007 [cited 2007 May]. Sustainable Stormwater Management Green Solutions: Stormwater
Swales and Planters. Portland, OR: Bureau of Environmental Services. Available from: http://www.portlandon-
line.com/shared/cfm/image.cfm?id=123781


City of Seattle. 2007 [cited 2007 May]. High Point Development: Healthy Environment. Seattle, WA: Seattle Hous-
ing Authority. Available from: http://www.seattlehousing.org/Development/highpoint/healthyenviro.html

ual2005.pdf


Permeable Paving
Summary Permeable pavement refers to any porous, load-bearing surface that
allows for temporary rainwater storage prior to infiltration or drainage
to a controlled outlet. The stormwater is stored in the underlying
aggregate layer until it infiltrates into the soil below or is routed to the
conventional conveyance system. Research and monitoring projects
have shown that permeable pavement is effective at reducing runoff
volumes, delaying peak flows, and improving water quality. Several
types of paving surfaces are available to match site conditions,
intended use, and aesthetic preferences. Permeable pavement
systems are most appropriate in areas with low-speed travel and light-
to medium-duty loads, such as parking lots, low-traffic streets, street-
side parking areas, driveways, bike paths, patios, and sidewalks.
Infiltration rates of permeable surfaces decline over time to varying
degrees depending on design and installation, sediment loads, and
consistency of maintenance.
PHOTOS BY ROSEY JENCKS

Top: Permeable pavers a county lane in Vancouver, BC
Bottom: Permeable pavers in Germany


Permeable Paving

• Reduces runoff volume and attenuates • Limited to paved areas with slow and low
peak flows traffic volumes
• Improves water quality by reducing fine • Require periodic maintenance to maintain
grain sediments, nutrients, organic matter, efficiency
and trace metals • Easily clogged by sediment if not correctly
• Reduces the heat island effect installed and maintained
• Improves urban hydrology and facilitates • More expensive than traditional paving sur-
groundwater recharge faces (although these costs can be offset
• Provides noise reduction by not needing to install a curb and gutter
drainage system)
• Depth to bedrock must be greater than 10
feet for infiltration based systems
• Difficult to use where soil is compacted: infil-
tration rates must be at least 0.5 inches per

hour

The estimated installation costs of permeable paving
average $10 per square foot. One of the biggest
maintenance concerns is sediment clogging the
Load-bearing turf block in Vancouver, BC
pores in the paving. For this reason, sediment should
be diverted from the surface and the surface needs
to be cleaned regularly to ensure proper porosity.
Once a year, the paving needs to be inspected and
tested to determine if it is clogged, which can be

done in 5 minutes with a stopwatch and a sprinkler.
Also, broken or damaged pavers need to be removed
and replaced. Maintenance consisting of vacuum
sweeping and pressure washing (as long as water
supply is not limited) has an estimated cost, based
on local labor costs, of $6,985 per acre per year.

Porous asphalt in Portland, OR

The Seattle High Point Project showcases the first “porous pavement” street in Washington
and serves as a testing ground for its use elsewhere. Porous concrete pavement was used
on two city street sections, half of the public sidewalks, and for parking and access on many
of the private properties. Porous pavement sidewalks and gravel-paved driveways are em-
ployed at key sites to help reduce paved or impervious surfaces and infiltrate stormwater.


Permeable Paving

Design Details
Permeable paving consists of a series of layered elements that allows stormwater to penetrate through the
paved surface, be stored, and then either infiltrate into the soils or be slowed and conducted to the sewer
system. The top layer is the permeable paving material, below which is a gravel or sand bedding that filters
large particulates. If a storm event exceeds the capacity of the storage layer, a perforated overflow pipe
directs excess water to the storm sewer.

Common permeable paving systems include the following:

• Permeable hot-mix asphalt: Similar to standard hot-mix asphalt but with reduced aggregate fines
• Open-graded concrete: Similar to standard pavement, but without the fine aggregate (sand and finer) and with
special admixtures incorporated (optional)
• Concrete or plastic block pavers: Either cast-in-place or pre-cast blocks have small joints or openings that can
be filled with soil and grass or gravel
• Plastic grid systems: Grid of plastic rings that interlock and are covered with soil and grass or gravel

Permeable pavements are best suited for runoff from impervious areas. If non-paved areas will drain to
pervious pavements, it is important to provide a filtering mechanism to prevent soil from clogging the
pervious pavement. Soil infiltration rates must also be at least 0.5 inches per hour to function properly.
Site conditions (including soil type, depth to bedrock, slope, and adjacent land uses) should be assessed to
determine whether infiltration is appropriate, and to ensure that excessive sediment and pollutants are not
directed onto the permeable surfaces.

Permeable pavers

Pavers with open
spaces filled with
gravel or sand

Filter layer: fine
gravel or sand

Storage layer:
coarse gravel

Perforated pipe
flows to sewer

Optional geotextile
fabric

Subgrade Infiltration where feasible


Permeable Paving

In 2004, the Bureau of Environmental Services paved three blocks of streets in the Westmore-
land neighborhood with permeable pavement that allows rainwater to infiltrate. They paved
about 1,000 feet of street surface with interlocking concrete blocks. One block of SE Knapp
Street was paved curb-to-curb with permeable blocks. The other streets – SE Rex Street and
SE 21st Avenue – were paved with a center strip of standard asphalt and permeable pave-
ment in both curb lanes. A fourth block was paved curb-to-curb with standard asphalt. New
methods and equipment – like vacuum sweepers – will be used to clean the streets and keep
them free of weeds and debris. The construction cost was $412,000.

In summer 2005, the City of Portland completed paving four blocks of North Gay Avenue. This
is a pilot project to learn how well different pavement materials handle stormwater and hold
up as a street surface. For this reason, the city installed four different pavement combinations
on Gay including porous concrete curb-to-curb; porous concrete in both curb lanes, stan-
dard concrete in the middle travel lanes; porous asphalt curb-to-curb; and porous asphalt in
the curb lanes only. The results of this test are not yet available.

Permeable pavement

Porous asphalt

Filter layer: fine
gravel or sand

Storage layer:
coarse gravel

Perforated pipe
flows to sewer

Optional geotextile
fabric

Subgrade Infiltration where feasible

References
Los Angeles County. 2002 [cited 2007 Jun]. Development Planning For Stormwater Management: A Manual For
The Standard Urban Stormwater Mitigation Plan (SUSMP). Los Angeles, CA: Department of Public Works. Avail-

New York City. 2005 [cited 2007 Jun]. High Performance Infrastructure Guidelines: Best Practice for the Public
Right-of-Way. New York, NY: Department of Design and Construction. Available from: http://www.designtrust.
org/pubs/05_HPIG.pdf

ual2005.pdf

Tom Richmond and Associates. 1999 [cited 2007 Jun]. Start at the Source: Design Guidance Manual for Storm-
water Protection San Francisco, CA: Bay Area Stormwater Management Agencies Association. Available from:
http://scvurppp-w2k.com/pdfs/0203/c3_related_info/startatthesource/Start_At_The_Source_Full.pdf


Detention Basins
Summary Detention basins are temporary holding areas for stormwater that
store peak flows and slowly release them, lessening the demand on
treatment facilities during storm events and preventing flooding.
Generally, detention basins are designed to fill and empty within 24
to 48 hours of a storm event and therefore could reduce peak flows
and combined sewer overflows. If designed with vegetation, basins
can also create habitat and clean the air whereas underground basins
do not. Surface detention basins require relatively flat slopes. Four
types of detention basins are detailed below.

1. Traditional dry detention basins simply store water and gradually
release it into the system. Dry detention basins do not provide water
quality benefits, as they only detain stormwater for a short period of
time. Maintenance requirements are limited to periodic removal of
sediment and maintenance of vegetation. Dry detention basins are
good solutions for areas with poorly draining soils, high liquefaction
rates during earthquakes, or a high groundwater table, which limit
infiltration.

2. Extended dry detention basins are designed to hold the first flush

of stormwater for a minimum of 24 hours. Extended dry detention
basins have a greater water quality benefit than traditional detention
basins because the extended hold time allows the sediment particles
to settle to the bottom of the pond. Collected sediments must be
periodically removed from the basin to avoid re-suspension.

3. Underground detention basins are well suited to dense urban
Stormwater wet pond in Berlin,
Germany (cont.)


Detention Basins

locations where land costs make surface options unfeasible. Underground detention basins work best if
partnered with an ‘upstream’ BMP that provides water quality benefits, like bioretention planters, if water
is not returned to the combined sewer overflow. Underground detention basins need to be on a slight slope
to facilitate drainage but should not be placed on steep slopes because of the threat of erosion. They can
be placed under a roadway, parking lot, or open space and are easy to incorporate into other right-of-way
retrofits.

4. Multi-purpose detention basins are detention basins that have been paired with additional uses such as
large play areas, dog parks, athletic fields or other public spaces. Generally detention basins are only filled
with water during storm events and can act as open spaces during dry weather.

PHOTO BY KIMBERLY SHORTER

Detention basin in Seattle, WA Big Creek multi-purpose detention
basin in Roswell, GA

• Reduces runoff volume and attenuates • Limited pollution removal potential
peak flows • Inadequate drainage can result in mosqui-
• Improves water quality by removing some to breeding
particulate matter, sediment and buoyant • Low aesthetic value (unless designed for
materials (extended dry detention only) multi-purpose)
• Reduces flooding • Site limited by depth to bedrock and slope
• Low maintenance costs • Must have no risk of land slippage if soils are
• Low space requirements (underground heavily saturated, and a sufficient distance
only) from existing foundations, roads, and sub-
• Good for sites where infiltration is not an op- surface infrastructure
tion
• May create habitat and increases biodi-
versity in the city (multi-purpose detention
only)
• May provide open space and aesthetic
amenity (multi-purpose detention only)


Detention Basins

Design Details
Surface detention basins generally consist of a depressed area of land, or an area that is surrounded by
built up berms, where stormwater is directed and stored during storm events. There is a spillway to allow
ﬂows that exceed the designed capacity of the system to reenter the sewer system. Detentions basins
should not be constructed within 25 feet of existing structures and new structures cannot be built on top of
them. Detention basin sizing is important because if runoff exceeds the holding capacity, excess water is
discharged back into the normal conveyance system. Underground detention ﬁlls up during rain events and
stores the water until it can drain back into the combined system.

Traditional dry detention basin

Min. 25’ from structures
Overflow spillway

Berm Designed storm elevation
Erosion Protection

Outflow Max. 4:1 slope
Sediment

Rip-rap, fabric sock, or
Erosion protection trash rack filter sediment
form outflow

Low-flow orifice

Extended Dry Detention Basin

Underground detention basin

Maintenance hatch
Parking lot

Overflow
drains to
sewer Trash racks
Designed storm elevation

Outflow
Sediment


Detention Basins

Typical construction, design and permitting costs for an above ground,
extended detention basin are estimated at $41,000 for a one-tenth of
an acre basin, which can manage stormwater from 2 acres of land.

Detention basins require periodic maintenance and monitoring of
conditions to make sure that sediment accumulation is not a problem.
Underground detention basins must have a maintenance access hatch
that allows system monitoring. Periodically, sediment may need to be
removed to maintain the continued efﬁciency of the system.

Detention basin in Seattle, WA
Case Study: Roswell, GA

The Big Creek Park Demonstration Project includes a multi-purpose detention area to store
and treat runoff from a suburban neighborhood to protect a downstream wetland. The
multi-purpose pond is used for soccer and recreation during dry periods and fills with water
during rain events. Under the sod surface, there is a layer of engineered soil and mixed rock
to improve drainage. The feature includes an outlet structure that ensures drainage within
24 hours to prevent damage to the sod. The total storage volume provided in the 1.7 acre
multi-purpose detention pond is 4.76 acre-feet of stormwater.

Case Study: Kent, WA
Mill Creek Canyon Stormwater Detention Dam is a multi-purpose detention basin located in
Kent, a suburb of Seattle. Built in 1982, this 2.5 acre portion of a larger park can store up to
18 acre-feet of stormwater from 2.2 square miles of pervious and impervious urban surfaces
uphill of the site. A land artist, Herbert Bayer, participated in the design of the park and sculp-
tural earthworks were used to capture the water in spirals and between mounds that park
visitors can traverse using paths and bridges. The project was developed in collaboration
between the King County Arts Council and the Kent Parks Department

References
California Stormwater Quality Association. 2003 [cited 2007 Jun]. Extended Detention Basin. In Stormwater Best
Management Practice Handbook. Available from: http://www.cabmphandbooks.com/Documents/Develop-
ment/TC-22.pdf

City of Seattle. 2000 [cited 2007 Jun]. Flow Controls Technical Requirements Manual. Seattle, WA: Department
of Planning and Development. Available from: http://www.seattle.gov/dclu/codes/Dr/DR2000-26.pdf

Frost-Kumpf, HA. 1995 [cited 2007 Jun]. Reclamation Art: Restoring and Commemorating Blighted Landscapes.
Available from: http://slaggarden.cfa.cmu.edu/weblinks/frost/FrostTop.html

Los Angeles County. 2002 [cited 2007 Jun]. Development Planning For Stormwater Management: A Manual For
The Standard Urban Stormwater Mitigation Plan (SUSMP). Los Angeles, CA: Department of Public Works. Avail-

New York City. 2005 [cited 2007 Jun]. High Performance Infrastructure Guidelines: Best Practice for the Public
Right-of-Way. New York, NY: Department of Design and Construction. Available from: http://www.designtrust.
org/pubs/05_HPIG.pdf


The Urban Forest
Summary
Urban forests made up of publicly and privately maintained street
and park trees offer a myriad of benefits to the urban environment,
including stormwater mitigation. Trees intercept rainfall before it
reaches the ground and uptake the water that does reach the ground,
thereby reducing runoff volume and peak flows. Also, their roots
and organic leaf litter help to increase soil permeability. In addition
to stormwater benefits, trees remove particulates, cool the air and
beautify the city.

In 2003, the City of San Francisco Street Tree Resource Analysis
completed by the Center for Urban Forest Research, reported that
approximately 56 percent of all street-tree planting sites (sidewalk
pavement cuts designated for street tree planting) in the city are
unplanted, ranging from 28 percent in affluent districts to 74 percent
in under served districts (e.g., Bayview-Hunters Point). These
unplanted areas present an opportunity not only for significant
stormwater reductions, but also for addressing environmental justice
issues. The analysis found that San Francisco’s street trees reduce
stormwater runoff by an estimated 13,270,050 cubic feet (99 million
gallons) annually, for a total value to the city of $467,000 per year. On
average, street trees in San Francisco intercept 1,006 gallons per tree
annually. Certain tree species were better at reducing stormwater
runoff than others. Those demonstrating the highest stormwater
reduction benefits were blackwood acacia, Monterey pine, Monterey
cypress, and Chinese elm.


The Urban Forest

Benefits
• Reduces runoff volume and attenuates
peak ﬂows
• Provides shade and therefore may lower
energy costs for buildings
• Decreases soil erosion in parks and open
spaces
• Reduces the heat island effect
in the city
• Can contribute to carbon sequestration

Limitations
• Requires adequate space for planting
• Moderate installation and maintenance
costs
• In some San Francisco neighborhoods, cul-
tural preferences have lead to disagree-
ment about aesthetic value of street trees
• Potential conﬂicts with overhead wires Trees lining a grassy swale in Germany
• Potential to damage underground infra-
structure with roots
• Non-ideal growing conditions can cause
stunting, disease or premature death

Case Study: Los Angeles, CA

Million Trees LA is a plan to plant 1 million trees in Los
Angeles over the next several years. In the first year of
the program, approximately 44,378 trees have been
planted. The Los Angeles Department of Water and
Power (LADWP) “Trees for a Green LA” program, in con-
junction with Million Trees LA, offers free shade trees to

residential electric customers who attend an online or
neighborhood workshop on how to plant and care for
their tree. Non-residential (Home Owners Associations
or apartment owners) can receive free shade trees by
completing the workshop or certifying that a profes-
sional landscape contractor will plant and maintain the
trees.

Trees lining a street and bioreten-
tion planter in Portland, OR


CA: San Francisco: Low Impact Design Toolkit for Stormwater

CA: San Francisco: Low Impact Design Toolkit for Stormwater

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