Long-term beach-dune evolution of mega-nourishments: a Cellular Automata approach

Coastal sand nourishments have become a widespread strategy in response to coastal erosion and the need to strengthen coastal dunes for flood protection. Recently, a new strategy has been tested by upscaling from frequently recurring small-scale sand nourishments to a one-off local mega-nourishment (e.g. the Sand Motor). Such large volumes can significantly modify the natural beach-dune morphology, leading to an uncertain impact on its long-term evolution. Mega-nourishments may result in elevated berms and steeper cross-shore profiles that can alter the aeolian sand transport from the beach to the dunes, with the elevated berm leading to the formation of scarps through wave erosion (van Bemmelen et al., 2020). Furthermore, a mega-nourishment creates a planform perturbation of the coastline that will spread out in the alongshore direction, retreating locally whilst feeding the nearby coastline (Arriaga et al., 2017). Lastly, the use of coarser sediments in mega-nourishment construction may lead to the development of an armour layer on the beach, limiting aeolian transport (Hoonhout and de Vries, 2017). It is challenging to quantify and predict the development of mega-nourishments due to the diverse feedbacks involving aeolian dynamics, beach hydro- and morphodynamics, and vegetation development.