2022 CSDMS meeting-086

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Modeling an Exceptional Barrier-Island Erosion Event with an Ocean Model

Chris Sherwood, (he/his),US Geological Survey Woods Hole Massachusetts, United States. csherwood@usgs.gov
John Warner, U.S. Geological Survey Woods Hole Massachusetts, United States. jcwarner@usgs.gov
Christie Hegermiller, SoFar Ocean San Francisco California, United States. christie.hegermiller@sofarocean.com
Alfredo Aretxabaleta, U.S. Geological Survey Woods Hole Massachusetts, United States. aaretxabaleta@usgs.gov
Jin-Si Over, U.S. Geological Survey Woods Hole Massachusetts, United States. jover@usgs.gov
Andy Ritchie, U.S. Geological Survey Santa Cruz California, United States. aritchie@usgs.gov
Christine Kranenburg, U.S. Geological Survey St. Petersburg Florida, United States. ckranenburg@usgs.gov

North Core Banks, a long (36-km), low (2.6-m mean elevation), narrow (~1200-m) barrier island in the Outer Banks of North Carolina, was inundated from the sound side and severely eroded by outwash during Hurricane Dorian (September 2019). As the fast-moving Category-1 hurricane moved offshore after a brief landfall at Cape Hatteras, winds shifted to the northwest, forcing a ~2.5-m surge onto the back side of the island. Deeply incised drainages were cut into the island as water ran from the washover platform to the ocean through gaps in the primary dune line, removing ~16% of the island volume. This style of storm impact is less common than typical ocean-side attack by waves and storm surge, and rarely modeled. Model simulations may provide insight into the fate of sands eroded during these unusual and difficult-to-measure events. We used the COAWST modeling system to simulate conditions during Dorian for a typical segment of the island using topography and landcover derived from pre-storm mapping using aerial imagery. The high-resolution (~2-m horizontal grid spacing) model was forced by output from a coarser-resolution model that provided water levels, incident waves, and alongshore currents. The model reproduced the steep cross-island water-level gradients inferred from high-water marks and wrack deposits, and generated washout channels ~2 m deep, cut through pre-existing low spots in the primary dune line. We evaluated model performance by comparing the simulated topography with post-storm topography derived from aerial imagery. The location, depth, and width of the simulated channels matched observations well, but the inland portion of the modeled channels were more linear than the observed dendritic drainages. Model simulations were sensitive to water-level forcing, sediment size, and vegetation patterns. The simulated channels extended into the surf zone and deposited sediments in relatively deep water. This transfer of sand from the island core to the nearshore has implications for barrier island evolution, and the ability to model it with COAWST demonstrates the generality of its morphology components.