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. ... Read more

Relative sea level rise and climate change represent long-term threats for the ecosystems and flood defence system in the Wadden Sea. Moreover, the flood-dominant tidal basins in the area lead to sedimentation in the navigational channels used by the ferry services that connect the mainland to the Dutch Wadden islands. This report provides flood safety and coastal zone management solutions for the village of Holwerd, in the province of Friesland, Netherlands. High morphological dynamics and biodiversity characterizes this environment. Within this context, the Holwerd municipality, Wetterskip and Rijkswaterstaat were identified as key players to this study from a stakeholder perspective.

Alternative preliminary designs were presented to increase the resilience of the local sea dike and to recommend solutions for improving the ferry connection between Holwerd and the island of Ameland due to current dredging operations considered as excessive and which caused ferry delays. Careful consideration was given to designs due to the Wadden Sea being a UNESCO World Heritage Site and therefore protected under environmental legislation. Therefore, minimal negative environmental and ecological impacts were strived for in design of both the alternative dike designs and ferry connection alternatives. The impact that these alternatives may have on the socio-economy of Holwerd was also assessed where applicable.

Some alternatives were initially eliminated due to fatal flaws and the remaining options were evaluated by means of a multi-criteria analysis. The main criteria considered were the environmental impact, practical implementation, socio-economic impact, safety risk and reliability. Costs were evaluated through a subsequent cost-value assessment. The Wide Green Dike concept was the selected preferred solution as a reinforcement design to withstand extreme wave heights and water levels with a 1000-year return period, including sea level rise projections for the next century. This was achieved by performing an extreme water level analysis and wave transformation using the software Delft3D Wave. The Wide Green Dike concept would therefore ensure that the Holwerd sea dike still sufficiently meets the Dutch flood safety standards in the future. In relation to the navigability of the ferry, channel bend cuts are considered the feasible short-term solution to reduce the sailing time by 10 minutes. The long-term solution, adjusting vessel fleet sizes, potentially decreases the current maintenance dredging volume by 20%. The beneficial use of the dredged material for dike reinforcement and salt marsh development are suggested as integrated Building with Nature solutions to provide additional ecosystem services. Environmental Impact Assessments were conducted to identify potential risks related to each of the preferred alternatives and provide mitigation measures to ensure appropriate project implementation. ... Read more