Effect of upstream meanders on bifurcation stability and sediment division in 1D, 2D and 3D models

摘要

At river bifurcations, water and sediment are divided over two branches. The dynamics of the division determine the long-term evolution of the downstream branches, which can be studied by 1D models. For such models, a relation describing the sediment division is necessary, but this is poorly understood. We study the division of sediment and the morphodynamics on a time scale of decades by idealised 2D and 3D modelling of bifurcations with upstream meanders and dominantly bed load transport. Initially, migrating alternating bars in the models caused damped quasi-periodic fluctuations in bed levels, water and sediment division until the bars are near equilibrium. Varying the radius of upstream meanders and the slope of one of the downstream channels led to subtle changes in the sediment transport direction and the location of bars and pools. This caused large differences in which branch becomes dominant and in the rate of change in discharge asymmetry. The effects of wider downstream branches or of an overall narrower or wider river are dramatic, again demonstrating the importance of bars. The resulting division of sediment, on the other hand, is similar to the division of flow discharge in all runs after the initial fluctuations have damped out and until the discharge division becomes very asymmetrical. We conclude that bifurcations are extremely sensitive to local conditions affecting the secondary currents and the sediment transport direction, and to the downstream boundary conditions. Although most combinations of parameters lead to the development of an asymmetrical discharge division, some combinations lead to a quasi-stable symmetrical division. Finally we discuss the limitations of the models and the applicability to natural meandering rivers.