Samenvatting

Sand nourishments have been executed throughout the coastline of Walcheren for decades with the goal of improving coastal safety, widening beaches for recreation and reducing the impact of coastal erosion on dune habitats and the landscape. While the coastal engineering benefits of sand nourishments have become clear, it remains challenging to understand the economic rationale of these nourishments compared to other coastal defence strategies. There is demand for direct comparisons between ‘soft’ building with nature methods, such as sand nourishments, and traditional ‘hard’ coastal engineering strategies, such as dune and dike construction or reinforcement. This demand is only heightened by the expectation of rising coastal erosion and costs due to sea level rise, land subsidence and increasing storm intensity.

In this thesis, the MorphAn coastal modelling tool was applied to the coastline of Walcheren to determine historical trends in volume and position of individual coastal transects. These trends were then applied to the coastline to model a ‘what-if’ scenario in which the sand nourishments had not occurred over the last 40 years. Dune safety was then assessed with weak points identified. A final calculation of the necessary dune and dike reinforcements was made along with the estimated costs. It was shown that sand nourishment costs were comparable to the estimated costs of dune/dike reinforcements and also provided other important economic benefits.
Furthermore, a recent important development has been the design and construction of mega sand nourishments. These mega nourishments present an opportunity to improve the cost/benefit profile of sand nourishments even further. Cost per cubic meter of sand is reduced due to the economies of scale with the additional benefit of reduced ecosystem interference. Given the benefits of mega nourishments, it is logical to investigate whether historical sand nourishments could have instead been executed as a single mega nourishment for the equivalent or less cost.
The second part of this thesis outlines a feasibility study for a mega nourishment at the Walcheren coastline. This showed that a particularly vulnerable part of the coastline, Domburg, could have been protected by a single mega nourishment executed in 2000 with a half-life of 14.6 – 29.2 years. This would have an equivalent cost to the multiple nourishments that occurred in the 2000-2019 period. An optimised design also demonstrated the feasibility of a smaller, cheaper mega nourishment that would have achieved a dry beach width of at least 50m with a half-life of 10 – 20 years.