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These programs are most useful in those areas where detailed analysis of flow behavior
based on predetermined flow rates is required.
5-5.1.3 Hydrologic Engineering Center of the U.S. Army Corps of Engineers
(HEC). The Hydrologic Engineering Center of the U.S. Army Corps of Engineers
actively maintains a suite of tools for modeling surface water hydrology and hydraulics.
5-5.1.4 EPA. The EPA maintains the Storm Water Management Model (SWMM) that
performs simulations of both water quantity and quality for urban runoff events.11 In late
2002, EPA extensively revised SWMM to include more detailed analysis of small-scale
stormwater management devices. The SWMM algorithm is able to explicitly simulate
storage and, therefore, is particularly appropriate for simulating discrete LID systems.
Obtaining reasonable estimates of storage parameters needed in SWMM is of critical
importance. Creative adaptations of SWMM may be necessary because the model
does not directly model runoff from an impervious surface onto a pervious one.
5-5.1.5 Prince George's County, Maryland. The Prince George's County
Department of Environmental Resources Programs and Planning Division, working
with Tetra Tech, Inc., has developed a BMP evaluation module to assist in assessing
the effectiveness of LID technology. This module uses simplified process-based
algorithms to simulate BMP control of modeled flow and water quality time series
generated from runoff models such as the Hydrologic Simulation Program, FORTRAN
(HSPF). These simple algorithms include weir and orifice control structures, storm
swale characteristics, flow and pollutant transport, flow routing and networking,
infiltration and saturation, evapotranspiration, and a general loss/decay representation
for pollutants. It offers the user the flexibility to design retention style or open-channel
BMPs, define flow routing through a BMP or BMP network, simulate IMPs such as
reduced or discontinuous impervious surfaces through flow networking, and compare
BMP controls against a defined benchmark such as a simulated pre-development
condition. Because the underlying algorithms are based on physical processes, BMP
effectiveness can be evaluated and estimated over a wide range of storm conditions,
BMP designs, and flow routing configurations. Such a tool provides a quantitative
medium for assessing and designing TMDL allocation scenarios and evaluating the
effectiveness of a proposed management approach.
Five basic design aspects were used to develop the methodology for the
module. They are: (1) the incorporation of input runoff data, (2) design and
representation of a site plan, (3) configuration of BMPs of various sizes and functions,
(4) schematic representation of flow routing through a network of BMPs, and (5)
evaluation of the impact of a site design with BMPs. The module interface is the
platform for an interactive linkage between each of the five design features of the
5-5.1.6 Commercial Sources. In addition to the freely available models, there are a
variety of commercial models on the market. Information about these other tools can be
found on the Internet.