Three-dimensional numerical modelling of levee depositions in a Scandinavian freshwater delta

Abstract

A three-dimensional numerical model was used to compute the water flow and the sediment deposition for a 100-year flood in the delta of Lake Øyeren in Norway. The model solved the Reynolds-averaged Navier-Stokes (RANS) equations with the k-ε turbulence model on a three-dimensional unstructured grid. The sediment transport was computed solving the convection–diffusion equation for sediment concentration using an empirical formula for the bed pickup rate.The results were tested against sedimentation data obtained from field measurements in a qualitative way. Both the mapped sedimentation patterns on the delta islands and the computed ones showed the typical levee-like structures along the deltaic channels. A series of sensitivity tests showed the important role of uncertainties in the bed roughness parameter, the incoming sediment concentration, the grain size distribution on the channel bed, and the vegetation parameters. The presence of vegetation strongly affected the position and intensity of the sediment depositions, and its proper parameterization plays a key role for the modelling.According to the computations, the levee deposition during the 1995 flood appeared as a process where fine sand mainly originating from the channel bed was brought into suspension and deposited on the floodplain where the flow velocity was lower and the flow became overloaded with sediment. An inspection of the simulated flow patterns and velocity vectors suggested that advective sediment transfer out of the channel and into the floodplain probably was the dominating process for the levee depositions.