Posthurricane airflow and sediment transport over a recovering dune

During Hurricane Ivan (2004), dunes in undeveloped sections of northwest Florida were significantly reduced in height through direct wave erosion and the transfer of sediment to the backbarrier by overwash. The poststorm morphology of the island consists of a narrow beach face, multiple breach corridors, a washover terrace, and a remnant secondary dune that was further eroded in 2005 by Tropical Storm Arlene and Hurricanes Dennis and Katrina. The redevelopment of this dune will direct the response of the island to future storms, but the mechanisms of posthurricane dune recovery are unclear, particularly in this low-energy coastal environment. The recovery is further complicated by a swale that was created through a knickpoint effect on an adjacent road. The swale was reinforced when the road surface was removed and as a berm developed during the recovery of the beach face. Field observations suggest that sediment transport across the narrow beach face and berm is limited by a lag of shells and gravel from the pre-Ivan roadbed. As a consequence, sediment input to the dune is relatively minor with onshore winds, despite the presence of fans deposited during frontal storms in the swale behind the berm. The resulting sediment transport gradient across the seaward slope of the dune is more reflective of the expanding fetch than topographic acceleration and the drag imposed by the sparse vegetation. In contrast, there is a larger sediment input to the dune during offshore winds in response to the larger fetch across the washover terrace, despite the presence of a lag. It is concluded that posthurricane dune recovery in this area is strongly dependent on the deposition of sediment during frontal storms and the resulting development of a sand ramp through the swale. This will increase both the availability of sediment and the fetch for winds directly or obliquely onshore.