Experts from the Illinois State Water Survey have been working since 2015 with the Metropolitan Water Reclamation District of Greater Chicago (MWRD), supporting MWRD in making informed watershed management decisions for its vast service area that includes 128 suburban communities in Cook County.
The first two phases of this long-term project have had a major impact on development throughout the region and helped define one of the most visible aspects of stormwater management: detention requirements.
Development changes water flow, particularly when impervious surfaces, like parking lots and buildings, supplant open land. If less water is being absorbed into the soil, this can increase the risk of urban flooding, so communities and stormwater management authorities typically require developers to limit how much stormwater their properties “release” -- either by installing landscaping that retains water and allows it to be absorbed into the soil, such as rain gardens, or by holding water in detention basins and releasing it at a controlled rate.
“If you’re trying to plan development, you want to know how impervious surfaces change water flow and how you mitigate potential increases in flood risk,” explained water resources engineer Greg Byard, who leads phase III of the MWRD project and is a member of the Water Survey’s Coordinated Hazard Assessment and Mapping Program (CHAMP).
MWRD previously used a single release rate for all of the six watershed planning areas in its territory but had realized that the same release rate could be too rapid for one area, increasing flood risks, but be overly restrictive in another region, making development costs needlessly steep. MWRD asked the Water Survey to develop a way to customize detention pond release rates for all six watershed planning areas.
To do this, the Water Survey adapted unsteady state hydrologic and hydraulic models, analyzing how water elevation changed under different development scenarios and a range of release rates (from a low of 0.15 cubic feet per second per acre up to 0.3 cubic feet per second per acre).
Steady-state modelling uses a consistent flow rate--for the type of intense rainfall event that has a 1% chance of occurring in any year, how much water discharge would be expected over 24 hours? Unsteady-state modelling looks not just at the peak flow, but at variable flow through the system and how flow interacts with floodplain and detention storage. This more realistic modeling requires more detailed terrain data. Fortunately, the Water Survey could tap into high-resolution elevation data available from the Illinois State Geological Survey
The Water Survey’s analysis also included projections of population and land-use change, provided by researchers from the University of Illinois' Urban Planning Department and the Chicago Metropolitan Agency for Planning, enabling the team to anticipate the magnitude of development and redevelopment.
Using this method, the Water Survey summarized how different release rates would affect flood risk in each watershed. (See Watershed-Specific Release Rate Analysis: Cook County, Illinois, 2019).
After the Water Survey’s assessment was reviewed by the MWRD’s Technical Advisory Committee, the MWRD board of commissioners revised the Watershed Management Ordinance to define a customized release rate for each watershed.
MWRD recently commissioned the Water Survey to extend this multi-year project by investigating how different stormwater management scenarios could impact communities that may be disproportionately affected by flooding, potential erosion and sedimentation impact on streams, and how the watershed management activities of surrounding counties could impact the area overseen by MWRD.
For this phase of the project, the Water Survey is collaborating with scientists from U of I’s departments of Civil and Environmental Engineering, Geography and Geology, and Natural Resources and Environmental Science. Initial results are anticipated in 2022.
