HOLOCENE DEVELOPMENT AND PROGRESSION OF AEOLIAN BLOWOUTS ON PADRE ISLAND NATIONAL SEASHORE

Recent evidence suggests that development of dune blowouts along Padre Island National Seashore, Texas, and migration of parabolic dunes to the backbarrier shoreline are the primary mechanisms by which the island transgresses in response to relative sea level rise. This study characterizes the development and migration of blowouts at decadal and century scales in order to understand these changes. An initial breach, caused by the removal of vegetation, develops along the dune line allowing sediment to be funneled into the dune field. The entrance of the blowout focuses the wind velocity, allowing sediment transportation into the dune field, covering any vegetation present. This process continues as sediment erodes from the foredune increasing the blowout size until the foredune rebuilds and vegetation stabilizes the system. Aerial photographs, LIDAR data, ground penetrating radar and optically stimulated luminescence were used to track and date the migration of these blowouts. Photographs and satellite images were used to track blowouts from conception to their final stabilization by vegetation using digitization. For a greater understanding of the system at the decadal scale, LIDAR data collected by the USGS and other agencies was used to create an elevation model in order compute the volumetric changes within the northern portion of the National Seashore. Within the larger study area, three smaller sites: a blowout where the foredune is becoming reestablished, a blowout detached from the foredune and a former blowout now stabilized by vegetation, were selected for geophysical analysis . A ground penetrating radar survey was completed to show the bounding layers, cross stratification, and bedding planes of the migrating blowout without physical disruption. Vibracoring and optically stimulated luminescence provided a physical comparison of the stratigraphy to the data returned in the GPR survey. By using this comparison, sediment type, structures and burial age are compared to geomorphic properties to determine the abundance of storms, sediment budget and vegetation extent, key characteristics in understanding blowouts. These factors are integral in understanding how blowouts have changed over the past 70 years and how these variables will affect evolution and spatial distribution of the shoreline.