Page 202 - Biosystems Engineering

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three-dimensional full Richards’ equation. Stream–groundwater
interactions are considered. In general, depending on the size of the
watershed, simulations can be computationally very intensive. MIKE
SHE can be used in combination with MIKE-11 for river hydraulics.
This modeling package, however, is proprietary. The GIS interface for
MIKE SHE uses ArcView 3.X GIS.

SWMM
The USEPA’s Storm Water Management Model (SWMM) model
(Huber and Dickinson 1988) is a comprehensive computer model for
analyzing water quantity and quality issues associated with urban
runoff. Both, single events and continuous simulation can be performed
on catchments having storm sewers, or combined sewers and natural
drainage, to predict flows, stages, and pollutant concentrations. The
structure of SWMM contains several computational blocks including
Runoff, Transport, Storage/Treatment, and Extended Transport or
Extran. The Runoff Block is used for generating runoff and quality
constituents from rainfall (plus simple routing of flow and quality),
the Transport Block for kinematic wave routing and for additional
dry-weather flow and quality routing, the Storage/Treatment Block
for reservoir routing and simulation of treatment and storage quality
processes, and the Extended Transport or Extran Block for hydraulic
routing of flow (no quality routing) using the Saint–Venant equations.
All aspects of the urban hydrologic and quality cycles, including
rainfall, snowmelt, surface and subsurface runoff, flow routing
through the drainage network, storage, and treatment can be simu-
lated using the SWMM model. The Rain Block is used to process
hourly and 15-min precipitation time series for input to continuous
simulation. Although historically designed to address urban runoff
quality issues, the model is often used for hydrologic and hydraulic
analyses. SWMM Version 4 is microcomputer based (DOS compati-
ble). For hydrologic simulation in the Runoff Block, data require-
ments include area, imperviousness, slope, roughness, width (a shape
factor), depression storage, and infiltration parameters for either the
Horton or Green–Ampt equations for up to 100 subcatchments (num-
ber of subcatchments, pipes, etc. are variable depending on the com-
pilation). Flow routing can be performed in the Runoff, Transport,
and Extran Blocks in increasing order of sophistication. Extran can
also simulate dynamic boundary conditions (e.g., tides). Quality pro-
cesses, initiated in the Runoff Block, include options for constant
concentration, regression of load versus flow, and buildup/washoff.
Options such as street cleaning, erosion, and quality contributions
from precipitation, catchbasins, adsorption, and base flow can also be
simulated. USEPA Nationwide Urban Runoff Program data are often
used as starting values for quality computations. Basic SWMM out-
put consists of hydrographs and pollutographs (concentration vs. time)
at any desired location in the drainage system. Depths and velocities