DuPage County’s Watershed Modeling and Mapping Methodology
DuPage County has six watershed planning areas including: Salt Creek, East Branch DuPage River, West Branch DuPage River, Sawmill Creek, Des Plaines River Tributaries, and the Fox River Tributaries. The County is unique in its development of the hydrologic
and hydraulic models used in its watershed planning and flood plain mapping. Rather than using single event, steady state models such as HEC-HMS1 and HEC-RAS, the Stormwater Management Department utilizes continuous simulation and dynamic routing
models. The reasons DuPage County selected this methodology was; (1) the continuous simulation hydrologic model is used to capture the effects of antecedent moisture on runoff volumes and peaks, and to account for a non-uniform precipitation distribution
over the watershed; (2) the effects of backwater, flood plain storage, and complex urban stream systems have a significant impact on the hydraulics of DuPage County streams.
This approach produces continuous flow and stage information based on precipitation that has occurred in the past. From this data, flow and stage duration information is readily available for not only large events, but also for time of dry conditions
and small runoff events. This methodology also provides the County with the necessary information to properly implement stormwater programs such as flood plain mapping, flood forecasting, water quality protection and enhancements, wetland creation,
and project analysis.
The following presents detailed explanations on how each model program is used:
The Hydrologic Simulation Programs - FORTRAN (HSPF) was developed by Hydrocomp International, Inc., and is currently maintained by the USEPA. HSPF is used to create the hydrologic inputs for the hydraulic analysis. The model simulates continuous runoff
for various land cover types for a continuous period of precipitation record. The model incorporates infiltration, interflow, depressional storage, soil storage, snowmelt, overland flow, evapotranspiration, and changes in soil moisture in determining
the runoff. Five long-term precipitation gages are utilized to develop a sixty-year continuous time series meteorological input file, with four of these gages being maintained by National Oceanic and Atmospheric Administration (NOAA) and one by Argonne
National Laboratory. Six land cover categories defined as impervious, flat grass, medium grass, steep grass, forest, and agricultural have been developed and calibrated throughout the county to observed flows recorded at stream flow gages operated
by the United States Geological Survey (USGS). The resulting output from the HSPF is a time series file (TSF) of runoff for each land cover type and each rain gage.
The hydraulic analysis is achieved using the dynamic flood routing model known as Full Equations (FEQ). FEQ is used for both project analysis and flood plain mapping. FEQ was developed by Dr. Delbert Franz of Linsley, Kraeger Associates, Ltd., and verified
and supported by the USGS. The FEQ model represents unsteady flow in channels and reservoirs and is based on the numerical solution of the Saint-Venant equations describing one-dimensional flow in open channels. FEQ uses the TSF’s created from the
HSPF output and can represent the effects of flood plain encroachment, on-line and off-line storage, diversions, channel improvements, bridges, culverts, dams, weirs, and other flow impediments. Complex hydraulic structures such as time or stage dependent
gates, and complicated flow paths such as split flow, can be represented readily in FEQ. The model is flexible enough to represent the complex hydraulic nature of DuPage County streams.
PVSTATS, "peak-to-volume", was developed for use with the County’s continuous simulation approach to determine the 100-year and other frequency flood elevations used for floodplain mapping. The computer program that is PVSTATS is used to compute the peak-to-volume
calculations. The basis for the method is to estimate both the probability distribution of flood volume and the regression relationship between flood peaks conditioned on flood volume. At the site of interest, flood volumes are extracted, and a probability
distribution is fit to this series yielding a frequency estimate of volume. This exploits the fact that flood volumes often conform to commonly assumed probability distributions, even when flood peaks are affected by flood control projects. Many locally
historical storms are used to define the regression relationship between flood peak and volume. Additional "big" mid-west storm events were used to fill in the upper end of the peak-to-volume relationship. The peak-to-volume curve is then integrated
with the frequency estimates of volumes curve to produce the stage and/or flow at the site of interest for different recurrence intervals.
For further information on PVSTATS, please contact the DuPage Stormwater Department.