Sand nourishment strategies to mitigate coastal erosion and sea level rise at the coasts of Holland (The Netherlands) and Aveiro (Portugal) in the 21st century

Coastline retreat is an evident sign of the vulnerability of low-lying sandy coasts to erosion and sea level rise (SLR). Sand nourishments are commonly used to maintain foreshores, beaches, and dunes to preserve their role in flood protection, beach recreation and biodiversity. The feasibility of sand nourishments for long-term maintenance of large-scale coastal systems is hard to determine with detailed numerical morphological models. This study presents a method using a sand nourishment impact model (Ntool) that accounts for repeated sand nourishments over time as well as erosion and SLR impacts on the shoreline position. In Ntool the redistribution of sediment alongshore is based on detailed simulations of the current situation of waves (Delft3D-WAVE) and shoreline development (UNIBEST-CL +). Two sand nourishment strategies for the period 2010-2100 were evaluated and compared for the ca. 100 km long sandy shores of the Dutch coast (the Netherlands) and the Aveiro coast (Portugal): i) the hold-the-line strategy, in which coastline retreat is directly restored by means of local sand nourishments and ii) the sand balance strategy, in which sand nourishments are applied each five years at high-value locations with volumes equal to the sand deficiencies due to sea level rise plus other known structural sediment losses in a particular sediment cell. Regionalised SLR projections of up to 0.5, 0.7 and 1.9 m by 2100 were taken into account. Results show that sand nourishments remain a cost-effective method of flood protection for the Dutch coast and are feasible for the centuries to come, even under the high-end SLR scenario, because large sand resources are available in the North Sea. For the Aveiro coast, large-scale sand nourishments might be feasible in combination with a policy of managed retreat for low-value areas to avoid expensive locked-in situations. The sand balance strategy is considered the most practical and has the advantage of economy of scale.