LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 120

supporting materials can be downloaded free at http://www.wsi.nrcs.usda.gov/
products/W2Q/H&H/Tools_Models/WinTR55.html.
The Stormwater Management Model
The U.S. Environmental Protection Agency's storm water management model
(SWMM) is a dynamic rainfall-runoff simulation model used for single-event
or long-term (continuous) simulation of runoff quantity and quality from primar-
ily urban areas. The runoff component of SWMM operates on a collection of
subcatchment areas that receive precipitation and generate runoff and pollutant
loads. The routing portion of SWMM transports this runoff through a system of
pipes, channels, storage and treatment devices, pumps, and regulators. SWMM
tracks the quantity and quality of runoff generated within each subcatchment, and
the flow rate, flow depth, and quality of water in each pipe and channel during
a simulation period comprised of multiple time steps [16, 17].
SWMM is a powerful model capable of simulating areas ranging from a single,
uniform subcatchment to the drainage system of an entire city. Although typically
not used to evaluate a single development site, the recently released Version
5 is more user-friendly and should promote an increase in use among design
professionals.
Rainfall excess is calculated in SWMM by subtracting infiltration (based
on Horton [8], Green and Ampt [9], or the NRCS curve number method [3])
and/or evaporation from precipitation. Rainfall excess is converted to runoff
by coupling Manning's equation with the continuity equation [18] SWMM
Version 5.0.013 (March 2008) and supporting materials can be downloaded at
http://www.epa.gov/ednnrmrl/models/swmm/.
The Source Loading and Management Model
The source loading and management model (SLAMM) is designed to provide
information about the sources of critical pollutants in urban runoff and the effec-
tiveness of stormwater BMPs for controlling these pollutants. SLAMM was
developed primarily as a planning-level model to predict flow and pollutant
discharges from a wide variety of development conditions using many com-
binations of common stormwater BMPs. Development of the model began in
the mid-1970s and was supported by the U.S. Environmental Protection Agency,
the Wisconsin Department of Natural Resources, and the Ontario (Canada) Min-
istry of the Environment [19]. Because of their importance for pollutant loading,
SLAMM places special emphasis on small storms and utilizes the small storm
hydrology method to calculate surface runoff. According to Pitt and Voorhees
[19], “SLAMM also calculates correct NRCS curve numbers that reflect spe-
cific development and control characteristics. These curve numbers can then be
used in conjunction with [other] available urban drainage procedures to reflect
the water quantity reduction benefits of stormwater quality controls.” The latest
version of SLAMM, WinSLAMM version 9.3.1 (July 2008), can be purchased
through http://www.winslamm.com/.