Sediment nourishments in the River Waal to mitigate bed degradation

Over the past century, the bed level of the Waal has degraded by 1 to 2 metres (Blom, 2016). The high rate of degradation and its spatial non-uniformity places the various functions of the river under stress. Rijkswaterstaat (RWS) therefore seeks solutions in their Integral River Management (IRM) programme to stop the degradational trend of eroding river sections and to elevate the bed level where needed (Klijn et al., 2022). Previous studies have shown that solely implementing measures in the current floodplains of the river, that reduce the erosivity of the flow, such as side channels, is not enough and that other measures like sediment nourishments are required to fully counterbalance bed degradation (Barneveld et al., 2019; Welsch, 2021; Barneveld et al., 2022). This study (De Lange, 2022) investigates the effect of sediment nourishments of various characteristics on the bed of the Waal. This is done by modelling sediment nourishments in a 1D-model of the Rhine branches developed by Deltares for RWS (Chavarrias et al., 2020). The results show that individual nourishments can reduce bed degradation in the Boven- and Midden-Waal by orders of centimetres. Varying the characteristics of a nourishment influences its behaviour. Coarsening the composition causes a reduction in dispersion rate. This also leads to additional erosion downstream of the nourishment, which can be prevented by dividing the nourishment into multiple parts. By placing the nourishment further upstream, the period in which the nourishment affects the degrading section of the Waal increases. Finally, the relative effect increases with larger volumes. It is found that a coarse nourishment that is distributed into multiple parts and that is placed as far upstream as possible mitigates bed degradation most effectively. It is shown that nourishments can mitigate bed degradation, although this requires large volumes and repeated nourishments. It is expected that in reality, the optimal characteristics of the nourishment are not leading in the design of a nourishment. Instead, practical aspects like the availability of sediment will largely determine the possibilities. The insights gained in this study can be used in the early stages of designing a nourishment strategy. Moreover, the non-linear effect of changes in volume that is found implies that performing nourishment pilots with small volumes of sediment may lead to an underestimation of the effect of large-scale nourishments. In a successive research project, the effect of nourishments with a finer composition than the bed will be investigated.