VARIABLE RESPONSES OF TEXAS COAST TO SEA-LEVEL RISE

Modern barrier islands and peninsulas of the Texas coast have variable formation histories, spanning several thousand years. Their response to recent accelerated sea-level rise is also quite variable; current rates of landward shoreline migration range from 0 to 4 meters per year. Understanding the underlying causes of this variability is essential to predicting future behavior of these barriers. One of the main controls on barrier stability is sand thickness, which varies from > 10 meters along the central Texas coast to less than two meters along portions of the east Texas coast. This variability is controlled by sand availability and by relief on the Pleistocene surface on which modern barriers rest. Sand supply is predominantly from offshore through transgressive ravinement of falling stage deltas and channels, which has largely been exhausted. The distribution of these sands along the coast has not been uniform as indicated by differences in upper shoreface deposits along the coast. The depth of the transgressive ravinement surface is about -8 meters, so the volume of sand incorporated into the barrier from offshore varies significantly with the thickness of sand in the excavated upper shoreface. Longshore sand transport rates also vary widely along the coast in response to changes in shoreline orientation and sequestration of sand within the backshore environment, in particular flood tidal deltas. Offshore sand transport during storms has been low and consists mainly of very fine sand. Overwash varies with barrier width and height and the relative proportion transported backshore is currently unprecedented along Follets Island and South Padre Island, which are weak links in the Texas coastal barrier system.

Current research focuses on refining numerical models to predict coastal change as a function of key controls on barrier evolution, which include sea-level rise rate, sediment supply, and offshore and overwash fluxes. Such modeling efforts also aim to quantify the relative role of each of these controls on barrier dynamics.