Morphologic changes from sound-side inundation of North Core Banks, NC during Hurricane Dorian

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Morphologic changes from sound-side inundation of North Core Banks, Cape Lookout National Seashore, North Carolina, during Hurricane Dorian

Hurricane Dorian tracked just offshore of the Core Banks and Ocracoke Island in North Carolina as a Category 1 hurricane on Sep 6, 2019, making landfall briefly at Cape Hatteras before moving northeast into the Atlantic Ocean. Winds, initially from the southeast at >40 m/s, drove ocean waves and storm surge against the islands and forced water across Pamlico Sound, elevating water levels in the northwestern tributaries and bays. As the storm moved offshore, winds switched to the northeast and the combination of a relaxation seiche and wind forced a ~2.5-m sound-side surge against the back of the islands. This water inundated most of North Core Banks and flowed through gaps in the dunes toward the ocean, incising channels across the beach. Images taken from aircraft by NOAA and the USGS before and after the storm have been analyzed using structure-from-motion photogrammetry, allowing for quantitative evaluation of the morphological change resulting from the ebb surge. More than 90 cross-beach channels were formed along 28.5 km of North Core Banks island, eroding about 99 m3/m from the island above mean high water (MHW). Most of this loss came from dendritic channel networks that formed in the locations of previous overwash fans and widened and deepened existing cuts through the primary dune line. This loss represents about 16% of the sand volume in the beach/dune/overwash portion of the island, and about 11% of total island volume above the MHW contour. The eroded sand was deposited offshore in small ebb deltas, which were quickly reshaped by ocean waves. Recovery began immediately as small wave-formed spits grew into and across the newly cut channels. The most recent images (November 26, 2019) show a nearly intact berm enclosing ponds and low spots in the former channels. We speculate that the low spots will persist for years before being infilled by overwash, and may cause a long-term (years) change in habitat distributions and vulnerability of the dune system.