Exploring the bio-geomorphological evolution of mega nourishments with a cellular automata model

Mega nourishments are innovative Nature-based Solutions to protect against coastal erosion and flooding. By upscaling the volume of sand nourishments, mega nourishments enhance coastal safety while creating new space for recreation and nature development. The application of such large nourishment volumes can significantly alter natural beach-dune morphology, with an unclear effect on the long-term evolution of the beach-dune system. The DuBeVeg model, a bio-geomorphological Cellular Automata model, was extended to incorporate the typical surface armouring and longshore coastline development of mega nourishments. The extended model was successfully validated with morphodynamic data from the Sand Motor mega nourishment and was used to explore the long-term dune development of idealized bell-shaped mega nourishments. Over a 50-year time span, the typically artificially high and wide beach of a mega nourishment resulted in the seaward expansion of the foredune zone whilst a more natural, lower beach elevation resulted in a greater volume of the foredune zone in the long-term. A faster diffusion of the initially bell-shaped mega nourishment gave rise to a lesser advance and a more alongshore-uniform width of the foredune zone. Furthermore, the armour layer that develops when the nourishment material is less well-sorted and coarser than native beach sand, led to the emergence of more scattered and isolated dunes compared to a nourishment with native beach sand. These model results show potential for investigations into design optimisation of the subaerial development of Nature-based Solutions for sandy coasts.