LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 119

estimate peak runoff rates and runoff hydrographs using a dimensionless unit
hydrograph derived from many natural unit hydrographs from diverse water-
sheds throughout the country. As discussed below, the NRCS methodologies are
available in several public-domain computer models, including the TR-55 com-
puter model [2], the TR-20 computer program [13], and is an option in the U.S.
Army Corps of Engineers' hydrologic modeling system [14].
6.4 COMPUTER MODELS
The HEC Hydrologic Modeling System
The U.S. Army Corps of Engineers' Hydrologic Modeling System (HEC-HMS)
[14] supersedes HEC-1 as “new-generation” rainfall-runoff simulation
software. According to the Corps, HEC-HMS “is a significant advancement
over HEC-1 in terms of both computer science and hydrologic engineering.”
HEC-HMS was designed for use in a “wide range of geographic areas for
solving the widest possible range of problems.” The model incorporates
several options for simulating precipitation excess (runoff curve number,
Green and Ampt, etc.), transforming precipitation excess to runoff (SCS
unit hydrograph, kinematic wave, etc.), and routing runoff (continuity, lag,
Muskingum-Cunge, modified Puls, kinematic wave). HEC-HMS Version
3.2 (April 2008) and supporting materials can be downloaded free at
http://www.hec.usace.army.mil/software/hec-hms/download.html.
The SCS/NRCS Models: WinTR-20 and WinTR-55
“The WinTR-20 model is a storm event surface water hydrologic model ... .
It can be used to analyze current watershed conditions as well as assess the
impact of proposed changes (alternates) made within the watershed ... . Direct
runoff is computed from watershed land areas resulting from synthetic or
natural rain events. The runoff is routed through channels and/or impoundments
to the watershed outlet” (WinTR-20 User Documentation). TR-20 applies the
methodologies found in the hydrology section of the National Engineering
Handbook [1], specifically the runoff curve number method and the dimen-
sionless unit hydrograph [13]. WinTR-20 Version 1.00 (January 2005) and
supporting materials can be downloaded free at http://www.wsi.nrcs.usda.gov/
products/W2Q/H&H/Tools_Models/WinTR20.html.
Technical Release 55 (TR-55) was originally published in 1975 as a simple
procedure to estimate runoff volume, peak rate, hydrographs, and storage volumes
required for peak rate control [15]. TR-55 was released as a computer program in
1986 and work began on a modernized Windows version in 1998. WinTR-55 gen-
erates hydrographs from urban and agricultural areas and routes them downstream
through channels and/or reservoirs. WinTR-55 uses the TR-20 model for all of
its hydrograph procedures [15]. WinTR-55 Version 1.0.08 (January 2005) and