Spatially and Temporally Variable Impacts of Hurricanes on Shallow Sediment Structure

Sediment dynamics are fundamental to understanding coastal resiliency to climate change in the coming decades. Tropical cyclones can radically alter shallow sediment properties; however, the uncertain and destructive nature of tropical cyclones make understanding and predicting their impacts on sediments challenging. Here, grain size sampling in conjunction with continuous hydrodynamic data provided an unprecedented perspective of the impacts of two tropical cyclones, including Hurricane Sally (2020), in which the inner core of the storm passed directly over the field sites, on shallow coastal sediments in Alabama (USA). Sampling directly before and after Sally as well as out to similar to 7 months after the second storm event, Hurricane Zeta, showed that the changes in sediments following storm events exhibited notable site-to-site variability. This variability during the first storm event was consistent with low sand supply and flow interactions driven by local bathymetry that led to sand transport and deposition at some previously-muddy sites, near-surface mud loss at some sandy sites, or little change at others. Post-Sally impacts to grain size were well preserved 8 months after the storm, despite passage of Zeta as well as seasonal winds and riverine inputs during winter and spring. Overall, high temporal-resolution sampling over a relatively large area (<500 km2) revealed relatively small-scale spatial variability (on the order of 5-10 km) of hurricane impacts to sediment structure. These observations demonstrate a critical limitation for accurately predicting changes to coastal sediment dynamics in the face of a changing climate and its impact on tropical cyclones.