Redford Township, MI, is updating its combined sewer overflow (CSO) hydraulic and hydrologic model in the development of a CSO Control Plan for eight uncontrolled CSO areas to meet its National Pollutant Discharge Elimination System permit requirement. During wet weather, inflows from combined sewer areas can overwhelm the capacity of the system, which causes CSOs to local streams. CSO control in the Township’s northern areas has been provided since 1997 through construction of a retention treatment basin. In 1999, a CSO model was developed to address CSOs in the central and southern areas but financial constraints delayed implementation. The model is being revised to represent the current system and its response to wet weather for use in developing control alternatives for the Township’s remaining CSOs.

Rainfall and sewer flow data was collected and analyzed using meters installed at 10 flow monitoring locations to measure flow during dry and wet weather periods over 7 months. The data will be used to calibrate the model to verify inflow volumes, peak flow rates, and hydrographic information. The model will also be converted to new software. Once calibration is complete, the model will be used to quantify CSO peak flow rates and volumes that must be controlled under design storm conditions.