THE EVOLUTION OF BEACH-DUNE MORPHOLOGY: PADRE ISLAND NATIONAL SEASHORE, TEXAS

Response and recovery of barrier islands to extreme storms depends on the height and extent of the foredunes relative to the storm surge. Our understanding of barrier island response to storms and rise of sea level is complicated by the tendency of dune morphology to vary alongshore at a range of spatial scales, suggesting that alongshore variation in beach-dune morphology will be preserved in the sub-surface structure. This study used ground-penetrating radar (GPR) and vibra-cores to investigate sub-surface structures at the beach-dune interface for foredunes of different heights within a 2.5 km section of beach at Padre Island National Seashore, Texas. Dune normal and parallel GPR profiles were collected for dunes that would be in the collision, overwash and inundation regimes for a storm surge of ~2 m. Results from the GPR surveys suggest that each site contains laterally continuous radar reflectors at depths of ~1.2 m that are associated with a distinct shell layer. Below the storm-layer are low-angle, seaward-dipping reflectors characteristic of swash deposition topped by short, seaward-dipping clinoforms characteristic of swash bar migration at the smallest dune.

A similar transition from swash to swash bar deposits is also observed for the intermediate dune, but the beach-dune interface at the intermediate dune is also dominated by convex reflectors that are characteristic of embryo dunes that migrated landward to form the dune core. Stacked, horizontal reflectors across the largest dune profile suggest swash deposition on the beach and a relatively constant supply of sediment from the beach to the dune. The storm layer does not extend through the dune and with the exception of a large scour at the toe, there is no evidence that this dune was eroded by the storm that overwashed and inundated the adjacent smaller dunes. In other words, the response and recovery of these adjacent but morphologically distinct dunes over the same sequence of storm activity is quite different. It is further argued that the extent and form of beach-dune recovery is dependent on the storm impact scale and may represent a reinforced process once alongshore variations in dune form develop. The results of this study suggest further that it is difficult to identify a representative transect along a barrier island with natural dune height variability.