Exploring the role of vegetation and sediment supply to coastal dune states using integrated process-based modelling

Coastal dune morphology is largely controlled by the availability of sand to be transferred from the beach and the capacity of the vegetation to trap and retain the moving sand grains. The resultant coastal dune morphology is, in turn, key to achieve maximum efficiency of nature-based solutions that plan the construction of such aeolian features. Therefore, developing approaches that integrate key processes becomes crucial, especially in order to efficiently design and test solutions that meet the timescale requirements of coastal management. The process-based XBeach-Duna model has been developed to integrate nearshore, aeolian and ecological processes across the beach-dune profile, thus allowing long-term simulation of complex coastal features and feedbacks. Here, we explore the potential of this coupled modelling solution to simulate the morphological response of coastal dunes to changes in sediment supply and vegetation cover over decadal timescales. Simulations show the capacity of the approach to reproduce the natural response to changes in sediment supply, shifting the shoreline position and simultaneously modifying the overall shape of the dune, within a range of dimensions that are in agreement with observations. In general, narrow and low dunes are formed under high supply conditions, wide and high dunes develop if sediment supply is low and the shoreline position stable, while narrower and higher dunes are created after a relative drop in sediment supply that induces a negative budget. Denser vegetation coverage, on the other hand, favours taller dune morphologies, however the influence of sediment supply and receding shoreline positions to plant growth are non-linear and, in turn, produce feedbacks that cascade to the morphology of the dune itself. These results demonstrate the capacity of the approach to reproduce different dune states, resulting from alternative evolutionary pathways, and its potential to identify coastal dune (in)stability domains and critical morphological shifts, factors that are key to better understand the efficiency of dunes as nature-based solutions for coastal management.

This work was supported by the project PTDC/CTA-GFI/28949/2017, funded by the Portuguese Foundation for Science and Technology.