Computing near-bed solids transport in sewers and similar sediment-carrying open-channel flows

Near-bed solids (NBS) in sewers are known to convey a large part of the particulate polluting material and to introduce serious bias in sewer flow quality modelling when their existence is not properly acknowledged. It is proposed that a suspended-load approach should be used to characterize this material and the related flow pattern. A two-layer suspension model (proposed as an alternative to the classical Rouse equation) is applied to combined sewer flows displaying a NBS pattern, but also to capacity suspension flows carefully controlled under laboratory conditions. Both applications are conclusive and stress the importance of a good knowledge of particle settling velocity in ambient (turbulent) conditions. In combined sewers, the marked increase of effective shear velocity associated with rain runoff events is understood to lift the coarse settleable material which ordinarily remains in a concentrated NBS flow zone, into the upper part of the flow.