The representation of combined sewer areas in the SWMM RUNOFF model is an important issue as it influences the prediction of peak flows and overflow volumes in subsequent combined sewer overflow (CSO) system modeling. Careful consideration should be given to the proper selection of rainfall input, computational time step and RUNOFF elements used to simulate the combined sewer area response. Whenever possible, monitored flow data should be used to calibrate the timing and distribution of the RUNOFF generated hydrograph. If monitored flow data is limited or unavailable, a synthetic hydrograph method may be used to support representation of the hydrograph shape. This paper details the work performed to improve the Greater Detroit Regional Sewer System Model (GDRSS) RUNOFF model for simulation of the complex combined sewer area response from its watershed.
This chapter describes how long-term precipitation and infiltration records were developed for use in the performance evaluation of a proposed regional drainage tunnel shared by the cities of Detroit and Dearborn, Michigan. The precipitation record includes homly precipitation data (rainfall plus snowfall) plus a calculation tor snowmelt. Methodologies were applied to this record to develop 36 y of rainfall and snowfall plus snowmelt at 15-min intervals. Infiltration parameters are varied monthly based on long-term rainfall/nmoff records, rather than assumed to be constant throughout the year. In addition, allO\vances have been made for incorporation of spatially non-uniform precipitation into the evaluation. The application of these precipitation and infiltration records has allowed an improved representation of the hydrologic factors contributing to combined sewer overflow (CSO) in Southeast Michigan over an extended simulation length. The implementation of these records has improved the capabilities of the continuous model for computing the long-•term performance of proposed CSO facilities in Southeast Michigan, including the Proposed Regional Tunnel Project.
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