Depth-Averaged Turbulence Modelling Controls on Fine Sediment Deposition in Compound Channel

In rivers with a compound channel geometry, the flow extends beyond the main channel into the floodplains during high water. Thus, floodplains show distinct properties regarding hydro- and morphodynamic but also ecological processes. In the case of flooding, fine sediments are conveyed into the floodplain by the water and can be deposited, with potential contrasting effects on floodplain ecosystems and functionality. Fine sediment can promote the establishment of different species, but on the other hand, sediment depositions and vegetation encroachment can lead to a decrease in channel conveyance and thus increase flood risk. We present numerical simulations of a compound channel with and without main channel levees using a 2D depth-averaged hydro-morphodynamic model to identify turbulence modelling controls on suspended sediment exchange and deposition between main channel and floodplains. The numerical model includes a k-e turbulence model, shear stress partitioning, and suspended load is governed by the advection-diffusion equation considering exchange terms with the riverbed. The results show that capturing emerging shear layers and the cross-sectional velocity distribution in combination with appropriate sediment transport model parametrization contol the shape fine sediment deposion in the transition of the main channel to the flood plain. ... mehrThe findings are useful for the planning of river restoration projects with the help of numerical models as used in this application.