Prediction of organic combined sewer sediment release and transport

Accurate predictions of sediment loads released by sewer overflow discharges are important for being able to provide protection to vulnerable receiving waters. These predictions are sensitive to the estimated sediment characteristics and on the site-conditions of in-pipe deposit formation. Their application without a detailed analysis and understanding of the “initial conditions” under which in- sewer deposits were formed normally results in very poor estimations. In this study, in-sewer sediment samples deposited during dry-periods in a combined sewer system were collected, and their properties assessed. Parameters in the sediment transport relationship first proposed by Skipworth for in-pipe deposits were estimated based on simulating the in-pipe deposit formation conditions in laboratory erosion tests. The measured parameters were then used to simulate sediment transport through a small combined sewer network for a number of rain events for which rainfall, hydraulic and water quality data were available. Results showed that the model of Skipworth can provide good predictions of the sediment loads released from such in-sewer deposits. The experimentally-derived calibration parameters used with Skipworth’s model allowed for a realistic simulation of the in-sewer sediment behaviour and so can be used to accurately estimate the sediment load released from combined sewer systems during rainfall events.