The Cook County Watershed Management Ordinance (WMO) was approved on October 3, 2013, became effective on May 1, 2014 (MWRDGC, 2014), and was amended on February 15, 2018. Section 504.3, Part B specifies that after April 30, 2019, a development’s allowable release rate will be based on a watershed‐specific release rate. The initial goal of this project was to identify a reliable methodology to establish watershed‐specific release rates that would achieve the WMO goals and could be readily applied throughout Cook County, IL, based on available watershed information. The ultimate goal was to determine regulatory release rates in each watershed that mitigate the impacts of development by maintaining 1% annual chance (a.k.a. 100‐year return period) flood event levels at or below current levels throughout the watershed. The Illinois State Water Survey (ISWS), in consultation with the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), developed a methodology to evaluate watershed‐specific release rates. During the investigation, various tools were developed, options were explored, and sensitivity analyses were performed to determine the most reliable and efficient process for selecting watershed‐specific release rates. The methodology was tested using two pilot areas in Cook County to assess the method’s strengths and weaknesses, reliability, and ease of application. The Stony Creek subwatershed and the Upper Salt Creek watershed were selected, as they are both urban, yet typify the land‐use variation across the Cook County urban and suburban areas. Regulatory release rates within the range identified in the WMO were evaluated. Results from applying the methodology in the pilot areas were examined for trends and relationships that might provide insights and simplify future analyses. The release rates were evaluated by comparing results from future scenario models to base model results. The base models were derived from previous Detailed Watershed Plans (DWP) prepared for the MWRDGC using HEC‐HMS and HEC‐RAS software. The previous unsteady‐state DWP models were updated to incorporate major recent stormwater projects. Future scenarios modeled 40% of the land area as developed meeting stormwater management requirements and tested release rates between 0.15 cubic feet per second per acre (cfs/ac) and 0.3 cfs/ac. The hydrologic models simulated future conditions under the various release rates, and the resulting hydrographs were routed through unsteady‐state hydraulic models to analyze impacts to flood water‐surface levels. Results of the two pilot areas were examined for trends that could simplify the selection of release rates in other Cook County watersheds. Factors contributing to increases in the flood water‐surface levels included modified stormwater runoff timing at the watershed scale and increased stormwater runoff volume. No relationship or trend with respect to the stream length or slope was observed that could be used as a basis to identify a release rate that mitigates future flooding from increased development runoff. However, consideration of the pilot area results supports the use of the base runoff rate as an indicator of where hydrologic and hydraulic modeling for each subwatershed should be completed to identify watershed‐specific release rates. Watersheds with high base condition runoff rates tend to show lower flood elevations for release rates lower than the base condition. Watersheds with low base condition runoff tend to require low release rates to avoid exacerbating flood conditions. This observation was used to help identify subwatersheds that may be more sensitive to release rates in the prescribed range and thus warrant detailed consideration. Application of the methodology to the identified subwatersheds provided technical data for consideration for regulatory release rate analysis. Hydrologic and hydraulic modeling of additional subwatersheds supported the observation that lower base runoff rates indicated areas where a lower regulatory release rate is often required to mitigate future flooding. The comparison of the base runoff rate and the release rate cannot be directly interpreted in the same manner when the critical duration event of the subwatershed or watershed is other than that of the regulatory release rate application. Based on the methodology and future condition assumptions, release rates were not an effective tool to mitigate flooding increases along the Des Plaines River with a critical duration event that is significantly longer than the regulatory event. The results of the analysis indicate that in most watersheds and subwatersheds within the study area, increases in 1% annual‐chance flood elevations due to future development can be effectively mitigated through the application of a watershed‐specific release rate. In each of the six studied watersheds, the effectiveness of each studied release rate was evaluated to determine the maximum allowable release rate that could be applied before future development caused increased 1% annual‐chance flood elevations. For the Calumet Sag and North Branch Chicago River watersheds, the highest release rate that was considered reduced the water‐surface elevations across the watershed. For the Poplar Creek and Little Calumet River watersheds, differences in the effectiveness of the release rates occurred between 0.3 and 0.25 cfs/ac with 1.2% and 4.2% more stream length demonstrating increases in water‐surface elevations under future development, respectively, at the higher release rate. For the Upper Salt Creek and Des Plaines River watersheds, differences in the effectiveness of the release rates occurred between 0.25 and 0.2 cfs/ac, with 4.7% and 1.8% more stream length demonstrating increases in water surface elevation under future development, respectively, at the higher release rate. The developed methodology provides an objective, science‐based tool that balances stormwater regulation management with the benefits of the spatial variation of regulatory release rates using available data in identifying management strategies that mitigate increases in the 1% annual chance flood level under future development.
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