Barrier island overwash and breaching captured by immediate post-hurricane reconnaissance and simulated with a numerical model

This study investigates the morphodynamics of a developed barrier island during a hurricane, focusing on overwash processes and breach initiation. XBeach is used to simulate morphological change on Dauphin Island, AL during Hurricane Nate (2017). The study area includes structures and a pit on the north side of the island. The XBeach model setup is calibrated and compared to post-storm surveys, achieving a Brier Skill Score of at least 0.84 and RMSE of at least 0.30 for survey transects within the high-resolution model domain. Using a storm impact scale to characterize morphological changes, overwash is predicted for most of the study area, except for a single dune that reaches only the collision regime. However, post-storm imagery shows this dune is overwashed and its sediment redistributed across the island. While XBeach underpredicts overwash of this dune, it accurately predicts a breach through the pit. On the north side of Dauphin Island, a pit is breached into Mississippi Sound by flow from the Gulf Mexico channelized along a ridge. The pit breach is captured by drone and aerial photographs taken immediately after the storm. Satellite imagery collected less than ten days after the storm show the breach is closed, highlighting the importance of immediate post-event reconnaissance of dynamic barrier systems.