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INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
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The National Stormwater Calculator
shows users how land use decisions and
green infrastructure practices affect the
amount of stormwater runoff produced.
Green infrastructure, such as the street
planter and porous pavers shown above
(Image 1), are low impact development
controls that promote the natural
movement of water within an ecosystem
or watershed, instead of allowing it to
wash into streets and down storm
drains, as it does with traditional grey
infrastructure shown above (Image 2),
These practices allow the stormwater to
be used as a resource rather than a
waste product. Having less water runoff
into storm drains and roadways can help
prevent contamination of waterways,
infrastructure degradation, flooding, and
overwhelming of treatment plants.
KM» \A
stormwater-calculator
Contact: SWC@EPA.gov
Download and Additional Material:
epa.eov/water-research/national-
National Stormwater Calculator (SWC)
Tool that helps users control runoff to promote the natural movement of water
Stormwater discharges continue to cause impairment of our Nation's waterbodies. In
order to reduce impairment, EPA has developed the National Stormwater Calculator
(SWC) to help support local, state, and national stormwater management objectives
and regulatory efforts to reduce runoff through infiltration and retention using green
infrastructure practices as low impact development (LID) controls. The primary focus of
the SWC is to inform site developers on how well they can meet a desired stormwater
retention target with and without the use of green infrastructure. It can also be used by
landscapers and homeowners.
Platform. The SWC is a Windows-based desktop program that requires an internet
connection. A mobile web application version that will be compatible with all operating
systems is currently being developed.
Cost Module. An LID cost estimation module within the application allows planners and
managers to evaluate LID controls based on comparison of regional and national
project planning level cost estimates (capital and average annual maintenance) and
predicted LID control performance. Cost estimation is accomplished based on user-
identified size configuration of the LID control infrastructure and other key project and
site-specific variables. This includes whether the project is being applied as part of new
development or redevelopment and if there are existing site constraints.
Climate Scenarios. The SWC allows users to consider how runoff may vary based both
on historical weather and potential future climate conditions. To better inform
decisions, it is recommended that the user develop a range of SWC results with various
assumptions about model inputs such as percent of impervious surface, soil type, sizing
of green infrastructure, as well as historical weather and future climate scenarios.
Please check with local authorities about whether and how use of these tools may
support local stormwater management goals.
The SWC is comprised of ten tabbed pages:
1-Location.	This step has an address lookup feature that allows the user to easily navigate to a
site selected anywhere within the United States.
2-Soil	Type. In this step, soil type is identified and is used to infer infiltration properties. It can
be selected based on local knowledge or from the online database.
3-Soil	Drainage. This step identifies how quickly water drains into the soil. Conductivity can be
selected based on local knowledge or retrieved from the online database.
4-Topography,	Here, the site's surface topography is characterized, as measured by the surface
slope. The user can rely on the slope data display as a guide or can use local knowledge to
describe the site's topography.
5-Precipitation.	A National Weather Service rain gauge relevant to the site is selected. A long-
term rainfall record is used to replicate storm events that might occur.
6-Evaporation.	This step is used to select a nearby weather station that will supply evaporation
rates for the site.
7-Climate	Change. Users can elect to apply different future climate scenarios that modify the
historical precipitation events and evaporation rates normally used.
8-Land	Cover. For this step, impervious land cover and the different types of pervious land
cover are assigned to the site for the condition the user wants to analyze.
9-LID	Controls. These controls can be selected for use throughout the site, and consist of seven
green infrastructure practices (see page 2). LID cost estimation factors are included in this tab.
10-Runoff.	An analysis of the site is run and the results are displayed for review.
(2) Traditional Grey Infrastructure
Office of Research and Development
EPA/600/F-13/095c | Revised April 2017

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Low Impact Development Controls (Green Infrastructure Practices)
The SWC allows users to apply any mix of low impact development (LID) controls by simply selecting what percentage of the
impervious area is treated by each type of control. Each LID control has been assigned a reasonable set of design parameters,
but these can be modified by clicking on the name of the control. This will also allow the user to automatically size the control
to capture a 24-hour design storm that is specified. The SWC includes seven different green infrastructure practices:
Rooftop (Downspout) Disconnection allows rooftop rainwater to discharge to rain barrels, cisterns,
or landscaped areas and lawns, instead of directly into storm drains. The stormwater can then be
stored or allowed to infiltrate into the soil. Downspout disconnection could be especially beneficial
to cities with combined sewer systems.
Rainwater Harvesting (Rain Barrels or Cisterns) are containers that collect roof runoff during storm
events and can either release or re-use the rainwater during dry periods. Cisterns may be located
above or below ground and have a greater storage capacity than a rain barrel.
Rain Gardens are planted depressed areas that collect rain water from a roof, driveway, or street
and allows it to infiltrate into the ground. Rain gardens can also help filter out pollutants in runoff
and provide food and shelter for butterflies, song birds and other wildlife. Rain gardens with
drainage systems and amended soils are often referred to as bioretention cells.
Infiltration Basins are narrow ditches filled with gravel that intercept runoff from upslope
impervious areas. They provide storage volume and additional time for captured runoff to infiltrate
the native soil below.
Porous Pavements are excavated areas filled with gravel that are paved over with a porous
concrete or asphalt mix. Normally, rainfall will immediately pass through the pavement into the
gravel storage layer below where it can infiltrate at natural rates into the site's native soil. Block
paver systems consist of impervious paver blocks placed on a sand or pea gravel bed with a gravel
storage layer below. Rainfall is captured in the open spaces between the blocks and conveyed to
the storage zone and native soil below.
Green Roofs, a variation of a bioretention cell, have a soil layer laying atop a special drainage mat
material that conveys excess percolated rainfall off the roof. They contain vegetation that enables
rainfall infiltration and evapotranspiration of stored water. Green roofs are particularly cost-
effective in dense urban areas where land values are high and on large industrial or office buildings
where stormwater management costs are likely to be high.
Street Planters are urban rain gardens constructed with vertical walls and either open or closed
bottoms. They collect and absorb runoff from sidewalks, parking lots, and streets, and are ideal for
space-limited sites in dense urban areas.
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Office of Research and Development

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