Volumetric Analysis of Overwash Fans Resulting from Tropical Storms on North Hatteras Island, North Carolina

In 1999, Hurricanes Dennis, Floyd, and Irene affected the North Carolina coastline. Although none of these storms made landfall north of Cape Hatteras, the northern Hatteras Island shoreline suffered severe beach erosion and widespread overwash. The volume of three overwash fans located within a 3.75 km section of shoreline in Rodanthe was determined by measuring depth of deposition at selected locations on the fans. Fan area was determined from measurements taken from aerial photographs. The depths of deposition on the fans ranged from 0.2-1.2m, and 30-50m3 per meter length of fan were deposited at the sites. The volumes deposited exceed the amounts recorded during previous storms at locations along the U.S. East Coast. Subaerial sedimentation on barrier islands depends on high magnitude/low frequency events because their higher storm surge and wave heights produce dune breaching. Overwash is not widespread during lower magnitude storms because the water levels are not high enough to result in flow across the barrier. The total volume of overwash on North Hatteras Island was estimated from fan areas measured on aerial photographs in conjunction with the average depth of deposition measured on the Rodanthe fans. The total amount of sediment deposited was estimated to be 48 m3 per meter of fan length. Comparing these overwash volumes to amounts deposited by wind on the barrier island north of Hatteras Island shows that, on an annual basis, overwash far exceeds wind as a mechanism for transferring sediment inland. However, an extrapolation over a longer period to accommodate for temporal variations reveals that the volumes of sediment transferred by overwash and by wind are nearly balanced.