Quantifying the impact of storm surges on mineral accretion rates of coastal marshes

Coastal marshes represent a highly valuable ecosystem that provides a wide array of ecosystem services. Unfortunately, marshes survival might be compromised by sea level rise, limited sediment supply, and subsidence. Storm surges represent a fundamental source of sediment for starving marshes because of their ability to resuspend bottom material in channels and tidal flats and transport it to the marsh surface. However, their intermittent nature makes the quantification of their effect not trivial. In this study, we selected 11 storm surges with different intensity in Terrebonne Bay, Louisiana, USA and simulated them with the Delft3D-FLOW model coupled with the Simulating Waves Nearshore (SWAN) module. Simulations revealed that the deposition on the marsh platform is correlated with storm intensity and duration. However, when the storm return period is considered, the surge with 1.7 years return period was found to have the highest geomorphological work. This indicates that the most impactful storms are those that balance intensity with frequency. We introduce a new approach to derive long-term vertical inorganic accretion rates based on simulations of real storm surges. We evaluated every possible combination of 11 storms and selected the one that is most closely related to in situ field measurements. Using a linear model, we derived a spatially distributed inorganic accretion rates with values consistent to field measurements. This method has the advantage of considering real scenarios and can be applied in any marsh-bay system. Overall, this study stresses out the central role storm surges have into feeding sediment starving coastal marshes.