Morphological Response of the Dutch Sandy Coast to Accelerated Sea Level Rise

Abstract

Accelerated sea level rise (SLR) is predicted to have multiple adverse impacts on the coastal zone, aggravating phenomena such as coastal erosion on sandy coasts. For climate change adaptation planning and informing policy, morphodynamic changes occurring at coastlines are becoming increasingly important. In this study, a calibrated Delft3D model forced by real-time wave conditions, was applied to simulate and assess the morphological behaviour of the Delfland coast in response to accelerated SLR over a 30-year time period. The calibrated Delft3D model uses a novel acceleration technique called brute-force merged (BFM) proposed by Luijendijk et al. (2019), which enables the modelling of multi-decadal predictions, with significant gain in computational effort. An assumption of the study was that no nourishments take place, i.e. no additional sediment supply. The Sand Engine (Zand Motor in Dutch), currently located along this coast was also excluded from the model, thereby assuming a straight unnourished coastline. A selection of six SLR scenarios was simulated, including a no SLR scenario used as the reference case. The chosen scenarios covered the full bandwidth of accelerated SLR projections translated for the Dutch coast up to 2100, assuming increased mass loss from the Antarctic ice sheet, a hypothesis proposed by DeConto and Pollard (2016). These projections therefore exceed those presented in the IPCC AR5. Based on the recent literature, the SLR rates selected, ranging from 3 mm/year to 120 mm/year, are assumed plausible and useful for the modelling study. Model outputs that are assessed include erosion and sedimentation plots, and volume changes, particularly erosion volumes. Analysis shows that no major change to the general coastal system behaviour occurs due to accelerated SLR; erosive sections in the south remain erosive and accretive regions in the northern part remain accretive. This is influenced mainly by gradients in alongshore sediment transport and presence of structures. Erosion volumes increase with higher SLR rates, indicating an increase in erosion rate due to accelerated SLR. Volume changes were calculated in different alongshore sections and in different depth/elevation zones in the cross-shore direction in order to identify regions more vulnerable to accelerated SLR. It is determined that the southern section is most impacted by SLR causing increased erosion, particularly in the subtidal zone. Processes driving the observations and trends are discussed in the study. With significant SLR, dune erosion also occurs due to water levels and waves being able to reach higher elevations. The dunes along the Delfland coast are the primary sea defence which protects the hinterland from flooding, therefore it is critical to consider potential dune erosion due to accelerated SLR. Implications of the model study’s findings are briefly discussed, in the context of coastal maintenance policy and implementation of nourishments. Evaluating the added value of Delft3D as a coastal impact model is another objective of the study. Delft3D shows a number of benefits, including detailed analysis at multiple spatio-temporal scales. Another is the inclusion of transport processes in both alongshore and cross-shore directions, which is not the case for the Bruun Rule or 1-D coastline models. A limitation of Delft3D is that it does not include aeolian transport processes, and so dune growth cannot be simulated.