How do storm sequences impact dune erosion? Modelling a 2022 storm group at Egmond aan Zee in XBeach

Abstract

Sandy beaches and dunes form the primary flood defences along a large part of the Dutch coast. Dune safety assessments are formulated in terms of the morphological response to a benchmark storm occurring in isolation. However, a sequence of storms occurring in short succession might lead to increased dune erosion volumes. In this research, the effects of storm sequences on the expected dune erosion volume were evaluated using process-based numerical modelling.

Four storms that made landfall during early 2022 at the beach near Egmond aan Zee were simulated using the numerical model XBeach2DH. The 2022 storm group consisted of one moderate storm (named Corrie) and a triplet of weaker storms (named Dudley, Eunice and Franklin). Modelled dune erosion volumes of the storms simulated in isolation were compared to the dune erosion volume of storms simulated in sequence. In addition, the effects of different storm chronologies (order of arrival) and storm surge heights were tested.

An equilibrium-type morphological response was observed when modelling multiple storms using XBeach. The 2022 storm group simulated as a continuous sequence resulted in ∼15% less dune erosion than the cumulative dune erosion of isolated storm simulations. Altering the storm chronology did not have a significant impact on the total dune erosion volume of the storm sequence. The reduction of dune erosion potential of subsequent storms in a sequence was linked to the tendency of net sediment deposition at the sub-aerial beach in front of the foredune during subsequent storms. The elevation of the peak storm surge level relative to the dune toe affected the morphological impact of a storm and the erosion vulnerability of the dune at the onset of the subsequent storm. Strong storms with a high peak surge level resulted in large dune erosion volumes, which typically coincided with substantial net sediment deposition at the beach in front of the foredune. Conversely, weaker storms with low maximum surge levels were inclined to erode the sub-aerial beach while causing limited erosion of the foredune.

Considering the observed equilibrium-type response to storm sequences and the current safety assessment method of the Dutch dune-system based on the equilibrium beach profile associated with steady peak storm conditions, no evidence was found that the current safety assessment method is at risk of underpredicting dune erosion due to storm sequences. However, dune erosion volume predicted by XBeach was found to be significantly affected by the pre-storm bed elevation of the sub-aerial beach. This calls for validation of sub-aerial beach evolution during storms modelled in XBeach, as the morphological model parameters that were used were primarily calibrated for dune erosion volumes.