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CA: San Francisco: Low Impact Design Toolkit for Stormwater


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Low Impact Design Toolkit for Stormwater

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CA: San Francisco: Low Impact Design Toolkit for Stormwater

  1. 1. Low Impact Design ToolkitW 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
  2. 2. 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
  3. 3. L O W I M PA C T D E S I G N T O O L K I T iii
  4. 4. Urban Watershed Planning CharretteSeptember 27, 2007Bayside Conference Rooms, Pier 1San Francisco, CAProject Team:San Francisco Public Utilities CommissionArleen Navarret: Program Manager, WWPRDRosey Jencks: Project ManagerLeslie Webster: Design, Layout, and IllustrationsWith research and editorial assistance from:EDAWKerry McWalterMegan WalkerMark WinsorMetcalf & EddyScott DurbinKimberly ShorterDavid WoodCover 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.orgiv 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
  5. 5. 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 42L O W I M PA C T D E S I G N T O O L K I T 1
  6. 6. 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
  7. 7. IntroductionPre-development conditions in SanFranciscoExisting conditions in San FranciscoWhat is Low Impact Design?Before San Francisco developed into the thriving city it is today, it consisted of a diverse range of habitatsincluding of oak woodlands, native grasslands, creeks, riparian areas, wetlands, and sand dunes. Thesehabitats provided food and forage for a wide range of plants, animals and insects. The natural hydrologiccycle, working its way through each of these ecosystems, kept the air and water clean and recharged thegroundwater.Today, much of the city is paved or built upon and plumbed with a combined sewer to convey stormwaterand wastewater. Former creeks have been diverted to the sewers and wastewater from homes and runofffrom 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 flowsinto 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 meansthat there are very few places where infiltration can occur and groundwater is depleted.L O W I M PA C T D E S I G N T O O L K I T 3
  8. 8. IntroductionLID is a stormwater management approach that aims to re-create and mimic these pre-developmenthydrologic processes by increasing retention, detention, infiltration, and treatment of stormwater runoff atits source. LID is a distinct management strategy that emphasizes on-site source control and multi-functionaldesign, rather than conventional pipes and gutters. Whereas BMPs are the individual, discrete water qualitycontrols, LID is a comprehensive, watershed- or catchment-based approach. These decentralized, small-scale stormwater controls allow greater adaptability to changing environmental and economic conditionsthan centralized systems.LID has the potential to prevent the volume of combined sewer overflows and localized flooding in SanFrancisco by slowing or intercepting stormwater before it reaches the sewer pipes. Roof runoff from buildingscan be intercepted by eco-roofs. The downspouts from roofs can be redirected to landscaped areas or cisternswhere 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 paving4 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
  9. 9. IntroductionRunoff from streets, parking lots and other paved areas can be directed to detention basins, or bioretentionplanters where it is filtered and infiltrated. An expanded urban forest, can also intercept and uptake excesswater. 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 thelikelihood of flooding and sewer overflows, and recharge our local groundwater reserves.To d a y ’ s G a m eFor the purposes of the Urban Watershed Planning Charrette each team will be asked to apply appropriatestormwater 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 reducingstormwater volumes that can be quantified based on studies and modeling that has been calibrated for SanFrancisco. This booklet introduces and describes the benefits and limitations of each BMP used in today’scharrette. Each basin has a set of stormwater management goals for peak flow and volume reduction. Yourjob is to identify appropriate locations for the BMPs described in this booklet to address surface watermanagement goals in your basin. Your team then calculates the benefits and costs and determines howclosely you meet your stormwater management goals and stay within your budget. Each turn will consistof placing your BMP in the landscape and tallying the benefits and costs. Be sure to look for opportunitiesfor partnerships, multi-purpose projects and synergies between adjacent or nearby developments within theneighborhood.The following toolkit describes each BMP and provides specific details on the benefits and limitations,design details, and the costs of implementation and maintenance.L O W I M PA C T D E S I G N T O O L K I T 5
  10. 10. 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 bePHOTO 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. 6 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
  11. 11. Eco-Roofs Benefits Extensive eco-roof in Seattle, WA • Provides insulation and can lower cooling PHOTO BY ROSEY JENCKS 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 costsPHOTO BY ROSEY JENCKS 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. L O W I M PA C T D E S I G N T O O L K I T 7
  12. 12. Eco-RoofsDesign DetailsAn 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 extensiveeco-roof includes shallow layers (less than 6 inches) of low-growing vegetation and is more appropriatefor roofs with structural limitations. Both categories of eco-roofs include engineered soils as a growingmedium, 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 withslopes up to 40 degrees are appropriate for extensive eco-roofs, though slopes between 5 and 20 degrees aremost suitable (slope ration of 1:12 and 5:12). All eco-roofs are assembled in layers. The top layer includes theengineered soils and the plants. The soil is a lightweight mix that includes some organic material. Under thesoil is a drainage layer that includes filter fabric to keep sediment from the soil in place and a core materialthat stores water and allows it to drain off the roof surface. Next is the root barrier, which prevents the rootsfrom 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 systemCost and MaintenanceA typical roof size of a single-family home in San Francisco is estimated at 1,500 square feet, while commercialdevelopments are closer to 10,000 square feet. The costs of eco-roofs vary widely depending on the sizeand the type of roof but average $18 per square foot to install. Each eco-roof installation will have specificoperation 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 maintenancethan extensive roofing systems. In the first few years watering, light weeding, and occasional plant feedingwill ensure that the roof becomes established. Routine inspection of the waterproof membrane and thedrainage systems are important to the roof longevity. Annual maintenance costs are estimated at $5.49 persquare foot, which includes aeration, plant and soil inspection, flow monitoring and reporting.8 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
  13. 13. Eco-RoofsCase 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.ReferencesCity of Chicago. 2007 [cited 2007 Jun]. Chicago Green Roofs. Chicago, IL: Office of the Environment. Availablefrom: 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. Availablefrom:, (June 2007).City of Portland. 2006 [cited 2007 Jul]. Ecoroof Questions and Answers. Portland, OR: Bureau of EnvironmentalServices. Available from: of Toronto. 2007 [cited 2007 Jun]. Greenroofs. Toronto, Canada. Available 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. 367Low Impact Development Center, Inc. 2007 [cited 2007 Jun]. Maintenance of Greenroofs. Available Sound Action Team (PSAT). 2005 [cited 2007 Jul]. Low Impact Development: Technical Guidance Manualfor Puget Sound. Olympia, WA: Puget Sound Action Team and Washington State University Pierce County Exten-sion. Publication No. PSAT 05-03. Available from: O W I M PA C T D E S I G N T O O L K I T 9
  14. 14. 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. PHOTO BY ROSEY JENCKSDownspouts on DaVinci MiddleSchool in Portland, OR are directedto cisterns and a water garden10 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
  15. 15. Downspout DisconnectionBenefits 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 scalesCost and MaintenanceThe 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 perhousehold. In addition to these plumbing costs, the cost of the paired BMP (rain garden, cistern, etc.) alsoneeds 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 litterin the gutter system to prevent clogs to the connected BMP that would reduce efficiency of stormwatercapture. 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 WEBSTERL O W I M PA C T D E S I G N T O O L K I T 11
  16. 16. Downspout Disconnection 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 options12 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
  17. 17. Downspout DisconnectionDesign DetailsDownspout 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 collectwater requires installing a diverter that directs water from the pipes into the catchment system. Roof runoffwater is diverted to a storage or infiltration system. Some pretreatment is required before the stormwatercan be stored to prevent clogging from leaf litter. The main considerations for designing downspoutdisconnection and rainwater harvesting systems are: roof drainage configuration, site conditions for astorage tank, construction of new laterals, and desired rainwater uses.Case Study: Portland, OR 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.ReferencesCity 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]. “Downspout Disconnection” Bureau of Environmental Services, Availablefrom: Sound Action Team (PSAT). 2005 [cited 2007 Jul]. Low Impact Development: Technical Guidance Manualfor Puget Sound. Olympia, WA: Puget Sound Action Team and Washington State University Pierce County Exten-sion. Publication No. PSAT 05-03. Available from: Water Development Board. 2005 [cited 2007 Jun]. “The Texas Manual on Rainwater Harvesting,” ThirdEdition. Austin, Texas. Available from: 2007 [cited 2007 Jun]. “Open Charter Cistern,” Available from: O W I M PA C T D E S I G N T O O L K I T 13
  18. 18. 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.14 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
  19. 19. CisternsBenefits Limitations • 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 productionCost and MaintenanceThe cost of cisterns varies depending on the size and type of cistern. According to the Low Impact DevelopmentCenter (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 $40,000for storage of 20,000 gallons of stormwater. Cisterns installed underground tend to have higher installationand maintenance costs. Twice annual inspection is advisable to confirm that all the parts are operable andnot leaking. Regular use of the water stored in cisterns between rain events is critical to ensure storageis available for the next storm event. During the rainy season, it can be difficult to use the stored water ifbecause irrigation is generally not necessary. The stored water can be used during the rainy season forother non-potable uses such as toilet flushing or fire suppression.Case Study: Cambria, CA Cambia Elementary School captures and stores run- 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 SchoolL O W I M PA C T D E S I G N T O O L K I T 15
  20. 20. CisternsDesign DetailsProper design, siting, and sizing of cisterns are critical to ensure their full peak flow benefits. Stormwaterfrom roof downspouts is stored in the cistern until it is pumped out for use, or it reaches capacity andexits 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. Somepretreatment is required to prevent clogging (e.g. leaf screens and first-flush diverters) before the stormwatercan be stored to prevent clogging from leaf litter. Cisterns need to have access to air and light to avoid theproduction of algae. Generally, cisterns have a raised manhole opening on the top that allows access formaintenance 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 sewer16 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
  21. 21. Cisterns Mosaic above ground cistern in TennesseePHOTO 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: Low Impact Development Center, Inc. 2007 [cited 2007 May]. Cost of Rain Barrels and Cisterns. Sizing of Rain Barrels and Cisterns. Available from: and Rehbein Environmental Solutions, Inc. 2007 [cited 2007 May]. Cambria Elementary School. Available from: http:// 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: TreeHugger. 2007 [cited 2007 Jun]. Seattle RainCatcher Pilot Program. Available from: http://www.treehugger. com/files/2005/03/seattle_raincat_1.php L O W I M PA C T D E S I G N T O O L K I T 17
  22. 22. 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.18 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
  23. 23. Rain Gardens Benefits Limitations • 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 • Vegetation requires maintenance and can • 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 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 PHOTO BY ROSEY JENCKS L O W I M PA C T D E S I G N T O O L K I T 19
  24. 24. Rain GardensDesign DetailsRain gardens should be placed at least 10 feet from building foundations and typically collect stormwaterfrom roofs, small paved surfaces, or landscaped surfaces. The shallow depression fills with a few inchesof water during a rain event. Either the soils must be suitable to infiltrate the collected water or a moreintensive bioretention planter is recommended. Dense vegetation assists with the uptake of pollutants andthe absorption of the stormwater. Rain gardens require a minimum of a 5 percent slope and well-drainedsoils 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 infiltrationCost and MaintenanceRain gardens are a relatively low cost and low maintenance stormwater management solution. A resident canbuild and install their own rain garden in their front or back yard for very little money. The more elaboratethe garden, the more expensive installation becomes. The cost averages $8 per square foot and are typicallyabout 600 square feet, making the total cost approximately $5,000. Some level of annual maintenance isrequired and is most intensive soon after construction until the garden matures. In the early spring and fallthe garden needs to be weeded and the mulch refreshed bi-annually to encourage healthy vegetation andpollutant uptake. Mulch and compost improve the soil’s ability to capture water. In the first season irrigationmay be necessary to establish the plants.20 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
  25. 25. Rain Gardens PHOTO BY ROSEY JENCKS Rain garden at Glencoe Elementary SchoolCase Study: Portland, OR 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.ReferencesCity of Portland. 2007 [cited 2007 Jun]. Design Report: Rain Garden at Glencoe Elementary School. Portland,OR: Bureau of Environmental Services. Available from: Garden Network. 2007 [cited 2007 Jun]. Local, On-Site Solutions for your Local Stormwater Issues. Availablefrom: Gardens of West Michigan. 2007 [cited 2007 Jun]. Raingardens: Qualities and Benefits. Available from:www.Raingardens.orgCity of Maplewood. 2007 [cited 2007 Aug]. Rain Water Gardens. Available from: O W I M PA C T D E S I G N T O O L K I T 21
  26. 26. 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 overallPHOTO BY ROSEY JENCKS 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 Case Study: Portland, OR 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. 22 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
  27. 27. Bioretention PlantersPHOTO BY ROSEY JENCKS Bioretention planter in Portland, OR Cost and Maintenance 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 • Improves water quality watered until the pants are established. Semi-annual • Improves air quality 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 • Creates habitat and increases biodiversity the porosity of the soils, some raking of the mulch in the city and soil surface may also necessary to maintain • Provides aesthetic amenity 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 • Vegetation requires maintenance and can 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, PHOTO BY ROSEY JENCKS 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 L O W I M PA C T D E S I G N T O O L K I T 23
  28. 28. Bioretention PlantersDesign DetailsDuring a storm event, runoff may temporarily pond in a bioretention depression as it percolates through themulch layer and engineered soil mix. Plant material provides water quality benefits as the roots and soilsuptake some pollutants from stormwater. Bioretention areas can either infiltrate a portion of or all of thestormwater 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 thestormwater. The primary considerations in siting a bioretention planter are space availability, suitability ofthe 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, whichincludes mulch and wet-tolerant vegetation. A minimum of 4 feet of engineered soils and a gravel drainagelayer beneath the vegetation allow for proper infiltration. To ensure proper functioning, the maximumdrainage 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 tothe 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 feasible24 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
  29. 29. Bioretention Planters 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 StreetsCase Study: Seattle , WA 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 2007 [cited 2007 May]. Components. Design Details. Maintenance. Retrieved at www.biore-tention.comCity of Portland. 2007 [cited 2007 May]. Sustainable Stormwater Management Green Solutions: StormwaterSwales and Planters. Portland, OR: Bureau of Environmental Services. Available from: and County of San Francisco. 2006. Low Impact Development Literature Review. Prepared by Carollo En-gineers [Unpublished Memo].City of Seattle. 2007 [cited 2007 May]. High Point Development: Healthy Environment. Seattle, WA: Seattle Hous-ing Authority. Available from: Sound Action Team (PSAT). 2005 [cited 2007 Jul]. Low Impact Development: Technical Guidance Manualfor Puget Sound. Olympia, WA: Puget Sound Action Team and Washington State University Pierce County Exten-sion. Publication No. PSAT 05-03. Available from: O W I M PA C T D E S I G N T O O L K I T 25
  30. 30. 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 26 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
  31. 31. Permeable PavingBenefits Limitations • 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 PHOTO BY ROSEY JENCKS hour Cost and Maintenance 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 PHOTO BY ROSEY JENCKS 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, ORCase Study: Seattle , WA 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.L O W I M PA C T D E S I G N T O O L K I T 27
  32. 32. Permeable PavingDesign DetailsPermeable paving consists of a series of layered elements that allows stormwater to penetrate through thepaved surface, be stored, and then either infiltrate into the soils or be slowed and conducted to the sewersystem. The top layer is the permeable paving material, below which is a gravel or sand bedding that filterslarge particulates. If a storm event exceeds the capacity of the storage layer, a perforated overflow pipedirects 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 gravelPermeable pavements are best suited for runoff from impervious areas. If non-paved areas will drain topervious pavements, it is important to provide a filtering mechanism to prevent soil from clogging thepervious 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 todetermine whether infiltration is appropriate, and to ensure that excessive sediment and pollutants are notdirected onto the permeable surfaces. Permeable paversPavers with openspaces filled withgravel or sandFilter layer: finegravel or sandStorage layer:coarse gravelPerforated pipeflows to sewerOptional geotextilefabricSubgrade Infiltration where feasible28 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
  33. 33. Permeable PavingCase Study: Portland, OR 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 pavementPorous asphaltFilter layer: finegravel or sandStorage layer:coarse gravelPerforated pipeflows to sewerOptional geotextilefabricSubgrade Infiltration where feasibleReferencesLos Angeles County. 2002 [cited 2007 Jun]. Development Planning For Stormwater Management: A Manual ForThe Standard Urban Stormwater Mitigation Plan (SUSMP). Los Angeles, CA: Department of Public Works. Avail-able from: York City. 2005 [cited 2007 Jun]. High Performance Infrastructure Guidelines: Best Practice for the PublicRight-of-Way. New York, NY: Department of Design and Construction. Available from: Sound Action Team (PSAT). 2005 [cited 2007 Jul]. Low Impact Development: Technical Guidance Manualfor Puget Sound. Olympia, WA: Puget Sound Action Team and Washington State University Pierce County Exten-sion. Publication No. PSAT 05-03. Available from: 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: O W I M PA C T D E S I G N T O O L K I T 29
  34. 34. 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 flushPHOTO BY ROSEY JENCKS 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.) 30 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
  35. 35. 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 SHORTERPHOTO BY ROSEY JENCKS Detention basin in Seattle, WA Big Creek multi-purpose detention basin in Roswell, GA Benefits Limitations • 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) L O W I M PA C T D E S I G N T O O L K I T 31
  36. 36. Detention BasinsDesign DetailsSurface detention basins generally consist of a depressed area of land, or an area that is surrounded bybuilt up berms, where stormwater is directed and stored during storm events. There is a spillway to allowflows that exceed the designed capacity of the system to reenter the sewer system. Detentions basinsshould not be constructed within 25 feet of existing structures and new structures cannot be built on top ofthem. Detention basin sizing is important because if runoff exceeds the holding capacity, excess water isdischarged back into the normal conveyance system. Underground detention fills up during rain events andstores 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 BasinUnderground detention basin Maintenance hatch Parking lot Overflow drains to sewer Trash racks Designed storm elevation Outflow Sediment32 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
  37. 37. Detention BasinsCost and MaintenanceTypical construction, design and permitting costs for an above ground,extended detention basin are estimated at $41,000 for a one-tenth ofan acre basin, which can manage stormwater from 2 acres of land. PHOTO BY ROSEY JENCKSDetention basins require periodic maintenance and monitoring ofconditions to make sure that sediment accumulation is not a problem.Underground detention basins must have a maintenance access hatchthat allows system monitoring. Periodically, sediment may need to beremoved to maintain the continued efficiency of the system. Detention basin in Seattle, WACase 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 DepartmentReferencesCalifornia Stormwater Quality Association. 2003 [cited 2007 Jun]. Extended Detention Basin. In Stormwater BestManagement Practice Handbook. Available from: of Seattle. 2000 [cited 2007 Jun]. Flow Controls Technical Requirements Manual. Seattle, WA: Departmentof Planning and Development. Available from:, HA. 1995 [cited 2007 Jun]. Reclamation Art: Restoring and Commemorating Blighted Landscapes.Available from: Angeles County. 2002 [cited 2007 Jun]. Development Planning For Stormwater Management: A Manual ForThe Standard Urban Stormwater Mitigation Plan (SUSMP). Los Angeles, CA: Department of Public Works. Avail-able from: York City. 2005 [cited 2007 Jun]. High Performance Infrastructure Guidelines: Best Practice for the PublicRight-of-Way. New York, NY: Department of Design and Construction. Available from: O W I M PA C T D E S I G N T O O L K I T 33
  38. 38. 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.34 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