Measures for Mitigating the Ongoing Bed Degradation in the Rhine River-A First Assessment Using Idealized Models of the Waal Branch

The Waal branch of the Rhine River is currently degrading by around 3 cm per year, which may have severe negative consequences for river functions like navigation, water supply and ecology. Various measures to mitigate the bed degradation are considered by the Dutch water management authority. Examples are sediment nourishments (SN), the construction of longitudinal dams (LD), and side channels (SC). However, no indicative information has been provided yet on how these measures affect the morphodynamic equilibrium state of the river and, eventually, on how effectively they would serve the goal of mitigating the bed degradation. The present work seeks to fill this knowledge gap in providing a first assessment using both idealized analytical and numerical models of the three measures named above. We have focused on studying the effects of the measures on the equilibrium channel slope and bed surface texture of the main channel and on how the river bed adjusts to the changes over time. Aiming at comparing the orders of magnitudes of the effects, we have chosen an idealized way of modeling the measures. We have assumed decreased discharge and sediment load values in the LD and SC case, an increased sediment supply value in the SN case, and furthermore, a reduced main channel width in the case of LD. The way of schematizing the measures has been decided upon based on the findings of literature research and the consultation of various experts. Given the limitations of the approach, the results of the models indicate that SN increase the equilibrium bed slope of an idealized Waal River most effectively. The bed adjustments may, however, take several centuries until an equilibrium state is reached. According to the results of the models, SN also lead to coarsening, whereas LD and SC, as they have been modeled here, almost have no effect on the equilibrium surface texture of the main river channel. A negative aspect of SN is the fact that they may be accompanied by initial degradation, which could negatively affect the river functions in the short term. Another disadvantage is that SN require that suitable material has to be provided continuously to nourish the river. Depending on the design, also LD and SC may serve to mitigate the bed degradation. This may, for instance, be the case if they lead to an extraction of one fourth of the water discharge from the main channel. Yet, this condition is unlikely to be fulfilled in the Waal River due to its navigation function and associated drought restrictions. The resulting slope and surface texture values have been proven to be particularly sensitive to the assumption of the gravel supply parameter. At current, the effect of mitigation measures on this parameter is of high uncertainties and so are, hence, the results of the models. In view of the results of the models, we conclude that a combination of different measures may be promising for solving the bed degradation problem in the Waal River and that further research should be done regarding the assumptions that underlie the models.