Evaluating coastal dune vulnerability to tropical cyclone forcing using numerical modeling in a data-scarce region

Abstract Sandy coastlines are becoming increasingly susceptible to cyclone-induced flooding and erosion as a result of global change. The assessment of these processes, however, is particularly challenging in data-scarce regions such as Mozambique, where limited topographic and hydrodynamic data restrict robust model calibration and validation. This study evaluates the protective capacity of dunes against a range of storm and scenarios of Mean Water Level (MWL) variation, using Cyclone Eloise in Mozambique as a case study. An XBeach 1DV model in surfbeat mode was employed to simulate the hydrodynamic and morphological response of two distinct beach profiles, one reflective and one more dissipative, under different scenarios that combined varying dune heights, storms with different return periods, and MWL variations, resulting in 400 combinations. The results revealed non-linear threshold relations of dune stability, identifying a critical dune height of approximately 3 m, below which dune failure is likely under most MWL-increase conditions. The capacity of dunes to withstand storm impacts is largely determined by their height. However, higher water levels dramatically increase the risk of failure, transforming a medium-sized (3 m) dune from collision regime to inundation. Although dissipative beach profiles possess a greater intrinsic ability to resist erosion and maintain their form, this advantage weakens during extreme cyclone events. The findings in the study area provide critical thresholds for nature-based coastal protection, underscoring the necessity of maintaining dune heights above 3 m to ensure long-term defence in a changing climate.