Theoretical model of suspended sediment transport capacity in submerged vegetation flow

Abstract

Quantifying suspended sediment transport capacity (SSTC) in submerged vegetation flow is vital to predicting the topographical changes of a river channel with vegetation and the effect of ecological restoration projects. Based on the definition of suspended sediment transport rate (SSTR) and the assumption that the energy maintaining sediment suspension (suspension work) comes from the turbulence kinetic energy (TKE) of flow, two models for estimating SSTC in submerged vegetation flow were proposed in this study. The SSTR-based model calculated SSTR by taking the depth-integral average of the product of local velocity and suspended sediment concentration and dividing it by discharge per unit width. The Energy-based model introduced suspension work to the double-averaging TKE budget equation as a sink term. That is to say, the production of TKE is used for sediment suspension and viscous dissipation. In the open channel without vegetation, a fixed proportion of the TKE production term is used to maintain the sediment suspension, but the proportion falls and is no longer a constant for the reason that the presence of submerged vegetation raises the viscous dissipation term. To represent this influence, an equivalent ratio coefficient was introduced. Thus, the SSTC model was derived. Then, a series of experiments in suspension capacity flow through submerged vegetation were carried out to verify the two models. Results showed that the two models could estimate the SSTC in flow with submerged vegetation well. Moreover, it was found that the suspended sediment is mostly transported by the water flow above the vegetation layer, and the proportion of which rises as discharge and vegetation density increase. Furthermore, compared with the bared-bed channel, the increase of TKE dissipation term in submerged vegetation flow leads to the decrease of SSTC, which is why submerged vegetation promotes sediment deposition.