Updated Hydrologic Model Development for the Little Calumet River Watershed

This paper describes how hydrologic information was recently updated to represent current flooding conditions within the Little Calumet River watershed in Northeastern Illinois. The Metropolitan Water Reclamation District of Greater Chicago (District) is currently developing a Detailed Watershed Plan (DWP) for the Little Calumet River Watershed in Cook County, Illinois. The objective of the DWP is to identify regional stormwater related concerns and develop alternative solutions to address those concerns. The District is coordinating the planning effort with the Little Calumet River Watershed Planning Council (WPC). This presentation includes an overview of the objectives of the planning effort and a detailed description of the hydrologic and hydraulic modeling performed. The planning process consisted of several steps, including gathering information on current watershed conditions; analyzing the suitability of existing information; obtaining required new data; developing detailed hydraulic and hydrologic models of the watershed; identifying potential projects to address stormwater related concerns, such as flooding and water quality; quantifying benefits; and estimating the costs of potential improvement projects. The HEC-HMS model was used in the hydrologic evaluation of actual storms and design storms. To provide for the most representative conditions, Illinois State Water Survey Bulletin 70 frequency distributions were used. Watershed sub-basins were delineated using GIS and HEC-GEOHMS tools. A total of 540 sub-basins were delineated. Initially, the NRCS method was used for determining rainfall loss and unit hydrograph shape, but ultimately the Clark Unit Hydrograph method was chosen as more representative of the watershed area. The Muskingum-Cunge hydrologic routing method was used for sub-basins outside of Cook County, whereas the modified Puls channel routing and diversion was used to represent combined sewer areas. Detailed "unsteady flow" hydraulic modeling was performed with HEC-RAS due to unique characteristics of the Little Calumet River system, including complicated inter-basin flow situations. Comprehensive field survey work was performed to verify every structure and channel dimension within all tributaries, and only the most current existing modeling and supporting data were used to build a robust model. While several large, discrete storms were available and were used for calibration and verification, a storm event in mid September 2008 provided the storm of record for several areas within the watershed. Models were recalibrated to this event. Based upon the updated area hydrology and detailed hydraulic modeling, flooding inundation maps were generated from the models that showed several areas with increased predicted floodplains over existing DFIRM maps. The latest versions of HEC-RAS and HEC-HMS modeling tools were used in this effort. The calibrated and validated models were used to evaluate almost 200 individual flooding issues and develop a DWP that contains information on the watershed's areas of concern and a listing of potential regional capital improvement projects to address flooding concerns based upon benefit / cost analysis and other factors.