Reproduction of groynes-induced river bed morphology using LES in a 2-D morphological model

摘要

The bed topography of rivers trained with groynes such as the Rhine branches in the Netherlands, display a typical pattern of scour holes and ridges that are forced by the groynes towards the main channel. The ridges, often referred to as "groyne flames" are noticeable as shoals, which are critical for navigation during low flow conditions. Ordinary two-dimensional depth-averaged morphological models are found to reproduce this pattern if the computational grid is sufficiently fine. Yet the resulting pattern is not accurate enough. The use of Horizontal Large Eddy Simulation (HLES) in the same models is found to make the pattern more realistic. It yields spatial variations in the groyne flames shapes as well as the occurrence of typical bimodal groyne flames. Furthermore, the application of HLES improves the simulation of the temporal formation of the groyne flames substantially. This implies that large eddies which form at the interface between the main channel and the groyne fields are the basic mechanism, which underlies this bed pattern. In this paper the bed topography resulting from the numerical simulation for an idealised straight channel with groynes is compared with field measurements in the Rhine River. The time-scale of the formation of the groyne flames, important for their recovery after dredging, is studied as well.