Field Investigation of Surcharging in Combined Sewer System

One section of a combined sewer system in the Detroit, Michigan suburbs has experienced high levels of surcharging while experiencing flows that are less than the full conduit discharge predicted by the Manning equation with typical values of the roughness coefficient. Observations during several events also indicated that surcharge levels kept increasing during a portion of a flow event in which the metered discharge was roughly constant. In an attempt to understand the basis for the system behavior, a field investigation was performed by initiating large inflows into an otherwise dry weather flow condition by discharging flow from three fire hydrants into the sewer system. It was possible to achieve maximum flow rates on the order of those experienced during the previous flooding events. A total of 8 flow meters were installed over approximately a two mile length of the sewer and several additional depth gauges were also installed. Because the hydrant inflows were much greater than the existing dry weather flow, it was possible to independently validate the flow meters in the sewer system. Two separate flow events were monitored. Several interesting observations were made from the monitoring data. The results of this monitoring will be presented and the results discussed in order to develop an understanding of the key issues that created the surcharging in this system. SWMM modeling is utilized in an attempt to further this understanding.