Coupled modelling of flow and non-capacity sediment transport in sewer flushing channel

Abstract

Flushing is cost-effective for mitigating sediments and constraining environmental problems in sewer systems. Previous mathematical models are almost exclusively built upon simplified governing equations evoking the assumptions of slow bed evolution and sediment transport capacity, of which the applicability remains open to question. Here a 1D coupled non-capacity model is presented for non-uniform sediment transport in sewer flushing channels, as adapted from recently established shallow water hydro-sediment-morphodynamic models for fluvial processes. The present model is tested for an experimental flushing case in Paris's Des Coteaux catchment system. The computational results agree with observed data more closely than those of a previous decoupled capacity model. While the differences between decoupled and capacity models and the present coupled non-capacity model are minor for flushing processes of short-durations, they are more pronounced for sustained long-duration flushing processes. Physically, the adaptation of suspended sediments to capacity regime cannot be fulfilled quickly, though bedload sediments can adapt to capacity regime instantly. Also, the bed deformation rate is comparable to its counterpart of the flow depth. Therefore, coupled non-capacity modelling is suggested for general applications to sewer flushing channels, of which the computing cost is essentially equivalent to simplified models built upon decoupling and capacity assumptions.