Morphodynamic Modeling of Supply-Driven Sediment Nourishments to Mitigate Bed Degradation and Discharge Skewing in the Dutch Rhine Branches

The Waal, the primary distributary of the Dutch Rhine, has been experiencing significant bed degradation of 1-2 cm per year, reaching up to 1-2 meters over the last century. Among others, this process threatens infrastructure stability and navigation. Furthermore, uneven erosion rates alter the discharge distribution at the Pannerdensche Kop bifurcation where the Rhine splits into the Waal and Pannerdensch Kanaal (which then bifurcates into the Nederrijn and the IJssel). This skewed discharge threatens flood safety standards and limits the vital freshwater supply to the IJssel Lake in the north of the Netherlands. While sediment nourishments are a known possible countermeasure, their implementation can be constrained by the availability and cost of sediment with a coarseness similar to the local bed material. This study evaluates the morphological efficacy of supply-driven nourishment strategies using locally available sediment, which is typically finer than the local bed material. Using a 1D morphodynamic model of the Dutch Rhine branches, we simulated several nourishment strategies over a 50-year period. These strategies were based on four identified nearby sediment sources with varying volumes, sediment compositions, and temporal applications (singular vs. repeated nourishments) in the upstream part of the Waal, close to the Pannerdensche Kop bifurcation. Our results show that nourishments with volumes comparable to the annual sediment transport capacity effectively counteract local erosion and mitigate the skewing of the discharge distribution. However, all strategies induced a fining of the riverbed, potentially increasing its erodibility. For an equivalent total volume, a single large nourishment achieves the desired mitigation effects more rapidly than smaller, repeated applications. This approach, however, also increases sediment deposition in the downstream aggrading section, increasing the need for future maintenance dredging. Ultimately, this research confirms the feasibility of using finer, supply-driven sediments but underscores the critical trade-offs between nourishment strategy, mitigation rates, and reach-wide morphological impacts.