The effect of nourishments on dune erosion during a storm sequence
Summary
The Dutch coast is one of the most heavily nourished coasts in the world, with an average of 12 mln. m3 of sand being added to the coast annually. This strategic coastline maintenance
is necessary to ensure flood safety for the hinterland, by maintaining the nearly uninterrupted
sandy beaches and, in particular, wind-blown dunes. Still, the effect of localized nourishment on beach-dune development and dune erosion during storms remains unclear. Besides feeding the coast, nourishment may act as a direct buffer against dune erosion during a storm. Moreover, sequences of storms during a single storm season may lead to different responses of the nourished beach-dune system, compared to a non-nourished stretch of coast.
The objective of this study is to provide insight into the effect of nourishments on dune erosion during a storm sequence, in particular the difference between a nourished and non-nourished stretch of coast (sub-aim 1) and the effect of nourishment designs (sub-aim 2). For this, the numerical model XBeach was used to simulate littoral hydrodynamic and morphodynamic responses during a storm sequence at Egmond aan Zee, the Netherlands. For this study, offshore data measured during a sequence of storms in early 2022 (with storms Corrie, Dudley, Eunice, and Franklin) were used as input for the model. Six measured bathymetries from 2020 - 2023, each spanning 6 km alongshore with a nourished and non-nourished section of the coast, were utilized to investigate the potential persistent effects of nourishments on dune erosion during the storm sequence (sub-aim 1). Additionally, different nourishment designs were added to the most recently acquired bathymetric data of October 2023 to investigate the effects of nourishment design on dune erosion (sub-aim 2). Configurations included the actual design of the 2023-2024 Bergen-Egmond shoreface nourishment, various beach nourishments, and the removal of the outer subtidal bar.
The results showed consistent decreased dune erosion during the storm sequence in the nourished site compared to the non-nourished site from 2020 - 2023, attributed to increased wave dissipation over the shallower and further onshore-positioned subtidal bars in the nourished site. Additionally, infragravity wave heights over the beach during the peaks of the storms were larger at the non-nourished site compared to the nourished site. Spatial variability (alongshore) of dune erosion strongly correlated with beach slope, whereas steeper beaches correlated with more dune erosion. Furthermore, a persistent erosional hotspot in the non-nourished site corresponded to the alongshore location of a deeper area (up to -6m) in the nearshore bathymetry, where short waves propagated further onshore before breaking. The addition of various shoreface nourishment designs resulted in limited impact on dune erosion rates during the storm sequence, suggesting that shoreface nourishment effects on dune erosion develop over timescales longer than that of a single storm season. Conversely, beach nourishment designs had a more direct impact on dune erosion since the entire beach and the height of the dune toe were raised. In conclusion, this study demonstrates that nourishment has a positive effect on coastal protection during a storm sequence. The findings contribute to encouraging nourishment in the context of coastal safety.