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.
Method
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
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
Conclusions
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.
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Project
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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
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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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