REGULATORY, TECHNICAL AND MODELING CHALLENGES TO DEVELOPING A FREQUENCY BASED SSO CONTROL PROJECTIN WAYNE COUNTY, MICHIGAN

Wayne County's North Huron Valley / Rouge Valley (NHVRV) interceptor system collects sewage from 15 communities located in Southeast Michigan and transports flows to the Detroit Water and Sewerage Department (DWSD) for treatment and discharge. The County is evaluating a regional approach tocontrolling wet weather sanitary sewer overflows (SSOs). A new methodology called the i3D antecedent moisture (AM) model, was used to perform the hydrologic modeling. The i3D model is a continuous model that produces a good match to observed flow data over time. The accuracy of the model resulted in a high level of confidence in the frequency analysis for SSOs and will serve as the basis for recommending improvements to control wet weather SSOs. The use of the AM model combined with a frequency analysis for sizing improvements eliminated the need to select a design storm event based on "average" conditions. This reduced many of the conservatisms that are frequently included in event models such as the capture coefficient and seasonal effects. The use of spatially varied rainfall also improved the accuracy of the analysis over the use of a point rain gauge. Thispaper presents the modeling and analysis innovations used and the preliminary development of a regional project.