W. van de Lageweg1*, M. Boyall1, V. Bax1, J. Stronkhorst1
1 HZ University of Applied Sciences
For decades, sand nourishments have been executed along the coastline of peninsula Walcheren (SW Netherlands) with the goal of improving coastal safety, widening beaches for recreation and reducing the impact of coastal erosion. While the coastal engineering benefits of sand nourishments have become clear, it remains challenging to understand the economic rationale of these nourishments compared to the alternative coastal defence strategy of dike construction and reinforcement. There is demand for direct comparisons between 'soft' building with nature methods like sand nourishments and traditional 'hard' coastal engineering methods such as dike construction. This demand is only heightened by the expectation of rising costs due to sea level rise.
Beach and dune development at Walcheren since 1965 were derived from the JARKUS database. First, the historical trends in volume and position of individual coastal transects were determined over the pre-sand nourishment period 1965-1978. Second, the estimated erosion or accretion trends were applied to the momentary coastline (MCL) to model a 'what-if' scenario in which the sand nourishments had not occurred over the period 1979-2019. Third, dune safety in the what-if scenario was assessed through the identification of weak points, using Dutch flood risk standards in MorphAn. Finally, a comparison was made between on the one hand the costs of 'hard' coastal defences at these weak points (estimated from dike/dune reinforcement costs at Dishoek in 2008) and on the other hand the costs of the conducted sand nourishments (estimated from local sand nourishment costs in the period 2000-2020).
Over the period 1979-2019 some 70 separate nourishments have been executed on the shores of Walcheren at an estimated total cost of €180 million. The sand nourishments resulted in coastal advance along 70% of the 33 km long coastline with an average gain of 48-66 m. This indicates that not only are sand nourishments preventing erosion but are meaningfully expanding the dunes and beaches.
The model simulations indicate that in the absence of sand nourishments over 40 years, volume trends result in a decrease in dune safety. In consequence, a growing length of the coastline would require dike reinforcement. Given a unit price of approx. € 13 million per km, the reinforcement costs are estimated at € 132-155 million. The additional cost of coastal maintenance without sand nourishments of € 0.78 million/y was added to the reinforcement costs over the model period, leading to a cost of €163-186 million. These total costs resulting from the retrospective analysis of a scenario without sand nourishments compares well to the expenditure on the executed sand nourishments for Walcheren over the period 1979-2019.
In conclusion, the overall societal business case of the sand nourishments looks to be very positive as they advanced most of the coastline substantially, reduced reactive maintenance, and increased beach area for recreation at a similar expenditure as 'hard' coastal defences.