UNDERSTANDING FEEDBACKS BETWEEN STORM-INDUCED WATER LEVELS AND THE MORPHOLOGICAL RESPONSE OF MASONBORO ISLAND, NORTH CAROLINA

Extreme storm events can cause significant, permanent morphological changes to coastal areas over short periods of time. The destruction of dune systems caused by extreme storms makes coastal environments and communities more vulnerable to future storm-induced changes. When storm water levels reach the elevation of the dune base, sediment is transported seaward resulting in a narrower, and more vulnerable dune system. As storm water levels begin to exceed the elevation of the dune crest, more significant impacts to dune systems are caused either by overwash transporting sediment landward, or inundation events that can lead to breaches. While the prediction of these coastal responses is often simplified into comparing total water level elevation to dune elevation, specific factors including beach slope, varying sediment characteristics, and the occurrence of small erosive events that precede the storm peak can influence the probability and magnitude of collision, overwash, and inundation. The intent of this research is to better understand how storm-driven variations in total water levels (primarily setup and swash) and the diversity of existing topography can influence the post-storm morphology of barrier islands. This was done by using a combination of in situ sensors, remote sensing, and a 2D XBeach model developed for Hurricane Florence. The study area encompassed Masonboro Island, a 13.5-kilometer-long undeveloped barrier island off the coast of Wilmington, North Carolina, and located just south of where Hurricane Florence made landfall in 2018. Beachface and dune water levels were obtained from two cross-shore arrays of pressure sensors on the northern and southern sections of the island that are characterized by dunes with high and low elevations, respectively. Aerial drone imagery was also collected to derive pre- and post-storm elevation data to quantify spatially varying erosion and overwash. These data were used to validate the model which was then used to evaluate dominant drivers of the spatially varying morphological response. A more complete understanding of what drives storm-induced changes in coastal environments is necessary for identifying vulnerable areas and supporting resilient coastal management plans.