INVESTIGATING THE GEOMORPHOLOGY OF STORM-INDUCED OVERWASH ON MASONBORO ISLAND, NC FROM HURRICANE FLORENCE

Storm surge and wind-driven waves produced by Hurricane Florence, which made landfall as a Category 1 storm on September 15, 2018 near Wrightsville Beach, NC, resulted in the overwash of Masonboro Island, NC immediately south of Wrightsville Beach. Masonboro Island is an undeveloped barrier island that provides an ideal location to study the natural geomorphological response of barrier islands to storm impacts. Storm-induced overwash is a critical component of island sediment budgets providing pathways for sediment transport from the beach system to the back-barrier. In order to determine overwash morphology, texture, and extent, two storm-induced overwash sites located along the central portion of Masonboro Island were selected for characterization. Site 1 (34.10466, -77.86179) and Site 2 to the north (34.10617, -77.86105) both extend ~10–30 m into the back-barrier marsh and exhibit a general flattening trend of the dune system as a result of sediment transport from the berm and foredune to the back-barrier. Gouge cores from both sites and unmanned aerial system (UAS) imagery were combined to determine the areal extent of overwash through the comparison of the visible depositional extent and depositional contact between the relict surface and overwash sediments. Mini-push cores were taken along transects extending from the boundary between the marsh and overwash fan into the marsh to determine the extent of deposition into the back-barrier marsh. Overwash deposition extended 6 meters into the marsh at Site 1 while Site 2 is characterized by a depositional extent of 3–8 m into the marsh. The maximum thickness of deposition over the marsh exceeded the depth of the push cores (10 cm) 2 meters into the marsh at Site 1 and 4 meters at Site 2. Preliminary textural analysis of core data indicate that storm deposits have a higher percentage of coarser grain sediment (>2mm to 2 phi) compared to pre-storm deposits at depth which have higher percentages of finer grained sediments (2 phi and finer). Overwash deposition at both sites resulted in considerable transportation and deposition of coarse-grained sand to the back-barrier marsh aiding in back-barrier aggradation as a morphological response to the impacts of Hurricane Florence on Masonboro Island.