Geophysical fluid dynamics

Abstract

The shoreface of Santa Rosa Island in northwest Florida is characterized by a ridge and swale bathymetry that forces an alongshore variation in beach and dune morphology. The alongshore variation in dune morphology in turn controls the modern island response to and recovery from tropical storms and hurricanes, and is therefore, an important control on island transgression with relative sea-level rise. Field sampling and remote sensing are used in the present study to describe the geologic framework of Santa Rosa Island, and to elucidate on the origins of the shore-attached ridge and swale bathymetry. Vibracores and seismic and GPR surveys were completed along 42 cross-shore transects and 3 shore-parallel transects to examine the structure of the 21 ridge and swale structures found along Santa Rosa Island. The shore-parallel seismic surveys reveal strong near-horizontal reflectors through the ridges at depths consistent with thick back-barrier muds extracted from vibracores taken across and along the ridges. Near-horizontal reflectors are identified in ground-penetrating radar (GPR) surveys between the ridges and cuspate spits along the back-barrier shoreline, but are not present in the narrow sections of the island landward of the swales. Continuation of the seismic surveys in the back-barrier also reveals near-horizontal reflectors at the cuspate spits that are characterized by seagrass beds, salt marsh and maritime forest. Consistent with the GPR survey, there is an absence of horizontal reflectors between cuspate spits where the washover deposits extend to the back-barrier shoreline. It is argued that the ridge and swale bathymetry is a transgressive surface and the remnants of cuspate spits that are present along the back-barrier shoreline. In this respect, the cuspate spits had to first develop along the back-barrier shoreline and eventually evolve into the mud-cored ridges as the island transgressed with relative sea-level rise. Once the ridge and swale bathymetry emerged on the Gulf of Mexico shoreface it was able to reinforce the alongshore variation in dune height and storm response. It is further argued that the cuspate spits are reinforced by the shoreface ridges through alongshore transport of sediment from adjacent washover fans although the ridge orientation suggests that the spits have migrated westward by ~ 750 m. In this respect, the alongshore variation in beach and dune morphology on this island is the expression of this large-scale feedback and suggests a top-down model in which meso-scale processes and landforms depend on the geologic context.