Sensitivity of dune erosion predictions to tidal currents

Currently the dune safety assessments for the Dutch coast are performed using a 1-dimensional XBeach model to assess the primary failure mechanism, dune erosion. This 1D approach has been extensively validated for dunes along an alongshore uniform coast (Arcadis/Deltares, 2023a). However, more complex coastal stretches require a 2-dimensional approach to capture alongshore variations in hydrodynamics and morphodynamics. As part of this more comprehensive approach, alongshore tidal currents could be included in the numerical model XBeach. This would inherently lead to a more complex probabilistic assessment due to the need for additional physical parameters in the hydraulic boundary conditions and well-tested assumptions on the local phasing and magnitude of the tide with respect to the storm conditions.

To support decision-making on the need to include tidal currents in the dune safety assessment framework for complex coasts, this study tested the sensitivity of dune erosion predictions by XBeach for alongshore tidal currents. The sensitivity was investigated for three schematised coastal types: 1) a simple, elementary dune coast similar to the Holland coast (simulated with and without a coastal protrusion); 2) a curved coast similar to the tip of an island (simulated with and without a nearshore tidal channel); and 3) a dike-to-dune transition. To drive an alongshore tidal current in the model, an alongshore water level gradient was imposed on the lateral model boundaries. In total, three different water level gradients were assessed (0, 1 and 4 cm/km), leading to alongshore tidal flow velocities of 0–1.5 m/s, representing the expected range of velocities that may occur along the Dutch coast.

Based on the results of the sensitivity simulations, it can be concluded that simulated dune erosion volumes in the XBeach model are minimally sensitive (erosion volume difference generally less than 10%) to the imposed tidal currents, even when very large tidal gradients are imposed. In this analysis, the physical interpretation of the sensitivity to tidal currents is hampered by model implementation limitations on lateral flow boundary conditions, specifically for the curved coast test case. This leads to smaller differences in simulated nearshore velocity between different water level gradients than initially expected. Notwithstanding this limitation, there is no consistent trend of increasing, or decreasing, simulated dune erosion due to the presence of alongshore tidal currents in the model simulations. Instead, the presence of bathymetric (e.g., nearshore tidal channels) and topographic (e.g., hard structures) features in the model are of greater importance for the safety assessment methodology.

Based on the results of the sensitivity simulations, there is no clear utility (no additional usefulness) in including alongshore tidal gradients in the development of a (semi-) probabilistic model for the safety assessment of sandy flood defences based on the XBeach model. Dune erosion volumes simulated by the XBeach model are minimally sensitive to imposed tidal gradients. Although the sensitivity study is affected by limitations in representing the physics of tidal currents on curved coasts, results from other test cases in this study do not suggest that simulated dune erosion sensitivity would change significantly for the curved coast case if tidal physics were better represented in the model. Differences in dune erosion volume between simulations with and without alongshore tidal currents are of similar order (10%) as those found to be acceptable to be ignored during the development of the 1D safety assessment framework (BOI development 2020-2023; Deltares, 2021) and are substantially smaller than the model uncertainty. Furthermore, there is no consistent trend of increasing or decreasing simulated dune erosion due to the presence of alongshore tidal currents in the model, which implies that omitting this variable would not unduly bias a probabilistic dune erosion calculation.