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Figures 2a and 2b (Photo: Maisie Hughes, ASLA, Casey Trees Endowment Fund, Special Projects Coordinator, Shefali Ranganathan, Casey Trees Endowment Fund, Special Projects Coordinator)








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The Green Build-out Model: Quantifying Stormwater Benefits of Trees and Greenroofs in Washington, DC
Casey Trees Endowment Fund, Washington, DC

"Water is a global problem and this sets forth a model for cities to prove the benefits of trees and green roofs. It's so impressive, easily understandable, and clear in its intention. It's particularly impressive that there were multiple agencies involved. This is much needed and important work!"

— 2007 Professional Awards Jury Comments

Project Statement

The Green Build-out Model uses the District of Columbia Water and Sewer Authority’s hydrologic and hydraulic model to estimate the ability of tree cover and greenroofs to reduce stormwater runoff. Findings show reductions in stormwater volume of up to 10 percent across the city, with up to 54 percent reductions in individual sewersheds. These results demonstrate how green infrastructure can reduce stormwater in urban settings. As a result, trees and greenroofs will be integral components of the strategy to meet the District’s stormwater management goals.

Problem Description
Water infrastructure is aging in many cities in the United States. Capacity issues due to growth increase the stress on pipes, pumps, and treatment facilities. In addition, the requirements of the Clean Water Act are becoming more stringent. Municipalities and wastewater utilities are increasingly asked to do more with less.

Nearly all of the waters in the District of Columbia are listed as impaired by the US EPA for a number of reasons. The chief sources of pollution are sewer overflows and stormwater. Approximately one-third of the District is served by a combined sewer system (CSS) (Figure 2a) and two-thirds by a municipal separate storm sewer system (MS4) (Figure 2b). Both systems operate under permits administered by EPA.

In both the CSS and MS4 areas, costs are high for pipes and tunnels and space is limited for traditional stormwater controls such as detention and retention ponds, infiltration controls, grassed swales, and rain gardens.

Both greenroofs and trees decrease the volume of runoff, reduce peak rates of runoff, and improve water quality. To date, these benefits have not been evaluated nor sufficiently quantified on a cumulative, sewershed and city-wide basis. This has made it difficult for engineers, landscape architects, and planners to integrate these options as solutions to the city’s CSS and stormwater permitting requirements.

Relationships Investigated
Research shows that the leaves of trees are like cups and can hold up to one-tenth of an inch of rainwater, and that an extensive greenroof with three to four inches of soil media will store on average one inch of rain. In the District, a combined sewer overflow (CSO) and stormwater discharge occurs, on average, every time it rains as little as one-tenth of an inch.

The research therefore asks the question, “How many greenroofs and trees are needed to make a significant contribution to stormwater management in the District?” It investigated the relationships between tree cover, greenroof coverage, and larger tree boxes, and key hydrologic and hydraulic variables including storm water and CSO volume, flow rate, and frequency. In addition, pollutant load benefits were quantified, and operational savings from reduced pumping and treatment within the CSS were estimated.

An Advisory Team of key stakeholders from EPA, DC WASA, the District Government, and NGOs was formed to review and comment throughout the research process. WASA’s Mike Urban hydrologic and hydraulic model served as the platform to integrate GIS information about the sewer systems and green infrastructure. The Mike Urban model has been peer reviewed and successfully applied by WASA in the development of an EPA-approved Long Term Control Plan (LTCP) for the CSS.

Interception storage amounts for trees and greenroofs were added to the same Mike Urban model which builds from the basic run-off equation:

Runoff = Precipitation – potential evapotranspiration – infiltration – storage

The model was applied for an average rainfall year using hourly precipitation recorded at Washington National Airport for 1990. Potential evapotranspiration rates are published by the Virginia Climatology Office. Infiltration is for pervious areas only and measures water holding capacity by soil type.

The amount of interception storage provided by trees was determined using the same methodology used by the USDA Forest Service in its UFORE Hydro Model whereby:

Storage = LAI * 0.2mm * Tree Cover Area = 0.032 inches
(LAI = 4.10 (the average LAI for all live DC Street Trees from the 2002 Inventory))

The model also took into account the seasonality of tree cover (leaf-on and leaf-off). All greenroofs were assumed to be three to four inch extensive greenroofs with one inch of storage based upon data found in peer reviewed literature.

Two scenarios were used to determine tree and greenroof cover. The first, “high end”, or “Green Build-out”, scenario considered putting trees and greenroofs wherever it was physically possible. The “Low-end” scenario looked at putting trees and greenroofs where it was practical and reasonable.

Greenroof and tree cover assumptions were determined by making assumptions for each land cover type as identified by the 2005 planimetric data from the District of Columbia Office of the Chief Technology Officer (OCTO) (Figure 3) (Table 1). Greenroofs were assumed to cover a maximum of 75 percent of the rooftop to account for HVAC and access. Greenroof coverage assumptions for building sizes are summarized in Figure 4 and Table 2.

Stormwater benefits were determined for the incremental difference between the existing tree or greenroof coverage and the proposed coverage scenario.

Scenarios were run for an average year (1990) wet weather continuous simulation, and a 1-year, 6-hour (1”) design storm. Stormwater and outfall volume/ frequency analysis were determined for the CSS and MS4 areas in the Anacostia, Potomac, and Rock Creek watersheds.

A Tree Box scenario was also run to estimate the stormwater benefits of increasing the minimum tree box dimensions from 3 x 5 feet to 6 x 20 feet in the downtown core where sidewalks average 20 feet in width (Figure 5).

In addition, pollutant load benefits were quantified by associating the storm water and CSO volumes with mean concentrations for pollutants. Operational savings from pumping and treatment were derived from the literature.

Findings are summarized in Tables 3-10 and Figures 6-9. Key findings include:

  • For an average year, the low-end scenario prevented over 310 million gallons of stormwater from entering the sewer system resulting in a reduction of 2.6 percent or 282,000,000 gallons in discharge volumes to DC’s rivers and a 1.5 percent reduction in cumulative CSO frequency (16 discharges)
  • For an average year, the “Green Build-out” or high-end scenario prevented over 1.2 billion gallons of stormwater from entering the sewer system resulting in a reduction of 10 percent or over one billion gallons in discharge volumes to DC’s rivers and a 6.7 pecent reduction in cumulative CSO frequency (74 discharges)
  • Nearly one in seven sewersheds experienced stormwater volume reductions greater than 10 percent
  • Discharge volume reduction in the entire CSS area is 6 percent for the low-end scenario and 22 percent for the green build-out scenario
  • Using one cent per gallon, DC WASA would realize between $1.4 and $5.1 million per year in operational savings in the CSS area
  • Green roofs and increased tree cover keeps thousands of pounds of nutrients, metals, and other pollutants out of area waterways
  • Per unit area, greenroofs intercept and store almost four times more rainwater than trees
  • Per unit area, runoff reductions from trees over impervious areas are more than three times greater than runoff reductions from areas with trees over pervious soil types
  • Increasing the size of tree boxes provides substantial stormwater benefits because it both reduces impervious surface and allows trees to grow larger

The Research and Advisory Teams conclude that trees and greenroofs can provide:

  • Substantial reductions in CSO and stormwater volume
  • Limited reductions in CSO and stormwater discharge frequencies
  • Significant reductions in runoff for small storms (85 percent of all rain events in DC are < one inch)
  • Significant operational savings in pumping and treatment from reduced volumes
  • Moderate reductions in peak flow and velocity
  • Significant reductions in pollutant loadings
  • Important Best Management Practices (BMPs) for CSS and MS4 areas where options are limited

EPA, WASA, the District Government, and key stakeholders agree this research demonstrates the efficacy of tree cover and greenroofs as stormwater BMPs on a citywide scale for the District of Columbia, and that trees and greenroofs should be a significant component of any solution to the long-term management of stormwater in the District of Columbia. These conclusions are being used as a basis to evaluate planning, design, regulatory, and incentive policies and practices in Washington, DC.

Comparisons with Past Research
Research in the past has quantified the hydrologic benefits of trees and/or greenroofs. The Green Build-out Model is unique because in addition to quantifying the reduction in stormwater volumes, the hydraulic benefits of trees and greenroofs are quantified by modeling stormwater flows spatially through the actual existing combined and separate storm sewer systems in the District of Columbia, thus seeing the effects of discharges on receiving waters, pumping stations, and the wastewater treatment plant.

Applicability to Landscape Architecture Practice
From the city as a whole, to individual rooftops, streetscapes, and tree boxes, this research provides innovative approaches, collaborative and strategic leadership opportunities, and tools for landscape architects to set policy, plan, design, and implement green infrastructure to improve the quality, health, and welfare of our cities. While the hydraulic information is unique to the District of Columbia system, the planning principles, assumptions, and methods are transferable to other locations and scenarios.

The Need for New or Further Research
While it is believed that trees and greenroofs provide the greatest low impact development (LID) opportunities in cities for stormwater management, the benefits of other LID solutions, such as rain gardens and rain barrels, could be quantified and positioned accordingly. And while stormwater benefits from trees and greenroofs are perhaps most direct and immediate at this time in DC, it would be helpful to quantify the other benefits trees and greenroofs provide, such as air quality improvements, reductions in the Urban Heat Island Effect, and potential carbon offsets for climate change strategies.

Project Resources

Client: United States Environmental Protection Agency (EPA)
Jenny Molloy, EPA, Water Permits Division, and Grant Project Officer, Robert Goo, EPA, Non point Source Control Branch, Jonathan Essoka, EPA Region 3, Anacostia Watershed Program

Partners: Casey Trees Endowment Fund
Mark Buscaino, ISA, Executive Director, Barbara Deutsch*, ASLA, ISA, Senior Director and Principal Investigator, Heather Whitlow*, Director Planning and Design, Holli Howard, ISA, Director GIS, Dan Smith, Senior Director Communications, Meredith Upchurch, ASLA, Green Infrastructure Designer

Limno-Tech, Inc.:
Michael Sullivan*, Vice President,
Anouk Savineau*, P.E., Senior Engineer,
Brian Busiek, P.E., Senior Engineer, Tadd Slawecki, Senior Engineer

Advisory Team:
David Bardin, Esq., DC Water and Sewer Authority Board of Directors, Member,
Mohsin Siddique, Ph.D., DC Water and Sewer Authority, Supervisor Environmental Plan,
Hamid Karimi, Ph.D., DC Department of Environment, Deputy Director Natural Resources,
Alexi Boado, DC Department of Environment, Urban Watershed Project Officer & LID Coordinator,
Chris Shaheen, ASLA, DC Office of Planning, Program Manager, Neighborhood Planning and Revitalization/Design,
David Berry, DC Office of Planning, Revitalization Planner / Urban Design,
Nancy Stoner, NRDC, Clean Water Project,
Neil Weinstein, Low Impact Development Center, Executive Director,
Chris Kloss, Low Impact Development Center,
Pete Johnson, Chesapeake Bay Foundation, Anacostia River Initiative,
James L. Sherald, Ph.D., National Park, Service, National Capital Region,
Center for Urban Ecology, Chief, Natural Resources & Science ,
David J. Nowak, Ph.D., USDA Forest Service, Northeastern Research Station, Project Leader,
Jun Wang, Ph.D., USDA Forest Service, Northeastern Research Station









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