Feedback between ridge and swale bathymetry and barrier island storm response and transgression Chris Houser Department of Geography, Texas A&M University, 810 O&M Building, College Station, TX 77843-3147, United States abstract article info Article history: Received 5 May 2011 Received in revised form 21 May 2012 Accepted 22 May 2012 Available online 28 May 2012 Keywords: Ridge and swale Island transgression Hurricane Barrier island Santa Rosa Island 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 eluci- date 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 struc- ture 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 trans- gressed 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 ex- pression of this large-scale feedback and suggests a top-down model in which meso-scale processes and landforms depend on the geologic context. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Barrier islands are elongated accumulations of sediment found along 13% of the world's coastlines (Dean and Dalrymple, 2002), mostly in areas with a small tidal range and an abundance of sedi- ment (Davis and Fitzgerald, 2002). For the most part, modern barrier islands are inherited features that represent the time-integrated deposition of past beach, backshore and dune environments, and are perched on older, eroding layers deposited by shelf and land- based processes (Riggs et al., 1995). Pre-modern features on the inner shelf, such as ancient drainage systems and inlet channels, modify waves and currents, and thereby control patterns of sediment erosion, transport, and deposition along the barrier island shoreline (e.g. Schupp et al., 2006). As a result, these inherited features also control contemporary response of the barrier island to extreme storms, and in turn the rate of island transgression to changes in sed- iment supply or an increase in mean sea-level. Island transgression with rising sea-levels is accomplished during storms capable of over- topping or breaching the dunes washing sediment to the back-barrier shoreline in the form of washover fans and terraces. The threshold surge required for the dunes to be overtopped or breached decreases as sea-level rises, and the probability of island overwash and island transgression increases. A lower overwash threshold is also required where dunes lack height (Sallenger, 2000), creating the potential for rapid transgression and even overstepping. This two-dimensional model of island response to storms does not account for alongshore variability in dune height that can lead to lateral dune erosion through the expansion of washover conduits that develop early in the storm where the dune height is low. Washover is limited and Geomorphology 173-174 (2012) 1–16 E-mail address: chouser@tamu.edu. 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2012.05.021 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph