DuPage County uses specialized computer modeling programs to plan for and understand flooding in its six watershed planning areas. Unlike standard models like HEC-HMS1 and HEC-RAS--which simulate water flows based on single events and steady stream conditions--the Department uses continuous simulation and dynamic routing models. Here's a breakdown of what each of those mean:
- Continuous Simulation - Instead of just looking at single flood events, the County's approach involves simulating how rainwater flows over time. This allows us to account for factors like moisture levels in the soil before a rain event and how rain is distributed across an area.
- Dynamic Routing - DuPage County's waterways are unnaturally complex due to man-made flood structures and increasing urbanization. Our models are flexible enough to simulate the effects of backwater, floodplain storage, and all sorts of infrastructure that can have an effect on the way water flows through the county.
By using these unique programs, DuPage County can provide a continuous and detailed stream of information about water flow and stage levels. This data isn't just helpful for major flood events but also for understanding water patterns during dry periods and small rainfall events. It's crucial for various Department operations such as floodplain mapping, flood forecasting, water quality protection, environmental projects like wetland creation, and analyzing specific projects related to watershed management.
Below are in-depth explanations of each of the modeling programs used. View the Watershed Model Status Map to check current progress on the modeling of County watersheds.
The Hydrologic Simulation Programs - FORTRAN (HSPF) was originally developed by Hydrocomp International, Inc. and is currently maintained by the United States Environmental Protection Agency (USEPA).
HSPF is a tool designed to help us understand the intricate dynamics of water movement within watersheds. It achieves this by simulating the flow of rainfall across different types of land cover (e.g., forests, farms, urban areas) during an extended period of rainfall and weather records. This simulation takes into account various hydrological processes, including infiltration (how water enters the ground), interflow (subsurface water movement), depressional storage (temporary storage in low areas), soil storage, snowmelt, overland flow (water running on the surface), evapotranspiration (water loss due to plant activity), and changes in soil moisture, which all influence the amount of runoff.
The model relies on data from five long-term precipitation gages, to create a continuous meteorological input file covering a span of sixty years. Four of these gages are maintained by the National Oceanic and Atmospheric Administration (NOAA), while one is managed by Argonne National Laboratory.
HSPF has been calibrated for six land cover categories: including impervious surfaces (like concrete), flat grassland, medium grassland, steep grassland, forests, and agricultural areas. Calibration means that the model's parameters have been adjusted to match observed water flows at stream gauges operated by the United States Geological Survey (USGS) throughout the county.
The result of running HSPF is a detailed time series file (TSF) that shows how much runoff occurs for each land cover type and for each rain gage.
The Full Equations (FEQ) model, designed by Dr. Delbert Franz of Linsley, Kraeger Associates, Ltd., is a program used to understand how rivers behave during flood events. Its development is verified and supported by the U.S. Geological Survey (USGS).
FEQ specializes in illustrating the dynamics of unsteady water flow in channels and reservoirs, based on numerical solutions to the Saint-Venant equations which describe one-dimensional flow in open channels. FEQ integrates the Time Series Files (TSF) generated from the HSPF model in its hydraulic analysis. The detailed data from these TSFs allow FEQ to account for various factors influencing water flow, including buildings, on-line and off-line flood storage, diversions, channel improvements, bridges, culverts, dams, weirs and other obstacles. FEQ can easily represent even more complex hydraulic structures, like time or stage-dependent gates, and intricate flow paths like split flow.
FEQ's integration with HSPF and its flexibility in representing a wide variety of hydraulic features make it invaluable for comprehensive flood risk assessment and management, particularly tailored to the characteristics of rivers and streams in DuPage County.
PVSTATS is a computer program designed to help us understand and predict how high floodwaters might rise during significant flood events, such as a 100-year flood. It does this by analyzing historical data and using a technique called "peak-to-volume."
Here's how PVSTATS works:
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Data Collection: The program starts by collecting data on past floods in a specific area. It looks at how much water these floods carried and how often they occurred. This information helps establish a baseline for understanding the nature of flooding in that region. Data from large storm events in the Midwest are also added to fill in the high end of the dataset.
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Probability Distributions: PVSTATS then estimates the likelihood of different flood volumes happening. Think of it as trying to figure out how often floods of various sizes might occur. This is important because different floods can vary significantly in their severity.
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Regression Analysis: PVSTATS looks at the relationship between flood peak (the highest water level) and flood volume (the amount of water). This helps predict how high the water might get during a flood based on how much water is involved.
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Integration: Finally, PVSTATS combines all this information. It uses the probability estimates of different flood volumes and the relationship between peak and volume to predict the water level (or stage) and flow during floods of various sizes and recurrence intervals in the area of interest.
In simpler terms, PVSTATS uses data and mathematical relationships to tell us how high the water might get during different types of floods in a specific area, helping communities plan for and mitigate the impacts of potential flood events.
