The importance of storm surge for sediment delivery to microtidal marshes

A supply of suspended sediment from nearby tidal flats is critical for sediment deposition on intertidal salt marshes to keep pace with sea level rise. Recent studies have underscored the tight coupling between marsh accretion and sediment resuspension on tidal flats, but most have not incorporated the effects of storm surge (storm-driven high water levels) on marsh deposition. Storm surge has the potential to significantly increase suspended sediment flux to microtidal marshes by increasing tidal inundation of the marsh platform and elevating suspended sediment concentration on tidal flats, but the overall effects of storm surge largely depend on the exact combination of these two variables. To better quantify the contributions of storm surge to marsh sediment delivery, we applied a spatially resolved and process-based hydrodynamic and sediment transport model to a microtidal bay-marsh complex in Virginia. Our model includes effects of marsh vegetation on flow and wave attenuation, and is well calibrated using spatially distributed hydrodynamic and suspended sediment data from the site. The model is run under different storm surge conditions with/without storm-driven water level changes. Our results show that most storm surge events at our site are associated with northeasterly winds and that they can double suspended sediment flux to marshes compared to simulations without storm surge variations in water levels. Comparison of spatial patterns of marsh sediment deposition with/without storm surge effects indicates that storm surge not only promotes vertical marsh accretion but also significantly increases the inundation area, allowing more sediment deposition in the marsh interior. We calculate the total mass of marsh deposition caused by each storm surge event and show that total marsh deposition increases linearly with the magnitude of storm surge. Our findings have significant implications for predicting future marsh change. Studies that do not resolve storm surge effects on marsh deposition will likely underestimate salt marsh elevations, as infrequently occurring storm surge events can contribute a considerable amount of sediment to marsh accretion.