Developing dynamic river boundary conditions for historic and design storms combined sewer modeling

The Great Lakes Water Authority (GLWA) has expanded and calibrated the Storm Water Management Model (SWMM) of the City of Detroit West Side combined sewer system.  This system includes approximately 22,000 acres of combined sewer area that can discharge to the Main Rouge River through 17 untreated outfalls.  The updated SWMM model will be used for planning long term combined sewer overflow (CSO) control alternatives to mitigate the impact of CSO on the Main Rouge River. 

Evaluation of wet weather data has shown that the Main Rouge River regularly imposes a backwater impact at the outfalls.  These backwater conditions impact the frequency and magnitude of CSO to the Main Rouge River by preventing the outfall backwater gates from opening thereby preventing or minimizing overflow from the CSO’s to the Main Rouge River. Previous model versions of the West Side Collection system did not address these outfalls with a dynamic boundary condition.  To correctly represent the timing and magnitude of these backwater impacts from the Main Rouge River on the 17 outfalls, the revision of the West Side model included a dynamic boundary condition representation.  Although the boundary condition for historic events could be represented with a combination of measured flow and rating curves, design storms required an actual representation of Main Rouge River. 

Flow rates along the Main Rouge River were generated based on a model representation of the 450 mi² Rouge Watershed.  To streamline development of the hydraulic representation of the Main Rouge River, stream crossing was not represented in the model.  Further simplifying the model development process, in-line pumps were added to river model to precisely simulate river levels at the 17 CSO locations.  The use of in-line pumps avoided the need for intensive model calibration and improved the accuracy of the river level representation. Pump curves used at 17 CSO location along the river were based on river rating curves also developed for this project.

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