2D numerical modeling of intense bedload-transport processes at confluences of mountain rivers and steep tributaries

The presented work deals with the numerical modeling of intense bedload-transport processes at confluences of mountain rivers and steep tributaries. Steep tributaries are characterized by having high transport capacities which supply large amounts of sediments to the confluence zone, where due to the sudden change in slope and the local hydraulic conditions, intense deposition can occur. The objectives of this study are to understand the potential applications and limitations of 2D numerical simulations for modeling these processes. The calibration of the applied 2D numerical model (BASEMENT, Basic Simulation Environment, v2.8) is based on comparing high-density point clouds of the confluence morphology at the end of flume experimental runs with numerical results. The calibrated numerical model is then used to test different discharge ratios and to investigate depositional patterns and mechanisms. The results show that key morphological features such as the confluence fan, the bank-attached bar and the scour hole can be accurately reproduced, showing that 2D numerical simulations are a valuable tool for modeling the complex interactions between morphodynamics and hydraulics at river confluences. Additionally, it is shown that the confluence morphology of steep tributaries and mountain rivers is strongly influenced by the sediment concentration in the tributary channel and the discharge in the main channel.