Modelling suspended sediment wave dynamics of reservoir flushing

Flushing of fine sediments from reservoirs is one of the most effective techniques to reduce reservoir sedimentation, but the sudden release of high suspended sediment concentration can have adverse effects on the receiving water body. Field observations of sediment flushing operations have shown that the released volume of water and sediments propagate downstream through different types of waves. In particular the hydrodynamic signal travels faster than the sediment signal resulting in the splitting of the two waves. Since the sediment wave lags behind, deposition is enhanced in the tail of the hydrodynamic wave, where velocity decreases and cannot sustain the sediment in suspension: the separation phase of the two waves controls the deposition process of the released suspended sediments. The separation and the interaction between the two waves, especially in the transport of suspended sediments, is controlled by the sediment wave celerity. Neverthless in many sediment transport models the sediment wave celerity is assumed to be the mean flow velocity. We developed a simplified one-dimenional numerical model to study the interaction between the two waves. In the model we introduced a celerity factor that corrects the depth-averaged sediment transport velocity as a function of the shape of the vertical velocity profile and suspended sediment concentration. We observed that the use of the celerity factor in one-dimensional sediment concentration transport enhances the deposition because of the reduced celerity of the sediment wave, which separates sooner from the hydrodynamic wave.