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 the 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 dune 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 to be transported into the dune field, covering any vegetation that is present. This process continues as sediment is eroded from the foredune increasing the size of the blowout until the foredune is rebuilt and vegetation stabilizes the entrance. With the front stabilized, the blowout begins its movement across the island. 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, taken at least twice a decade since the 1940s, were used to track blowouts from their initial conception to their final stabilization by vegetation. Each consecutive blowout was digitized to understand the surface characteristics of the feature. 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 young blowout that had just begun to close as the foredune is reestablished, a "middle age" blowout that was detached from the foredune and become an active dune field, and a former blowout now stabilized by vegetation, were selected for geophysical analysis . A Trimble GX 3-D scanner was used to determine the current volume of the blowouts and to create an end point for the elevation model at the smaller scale. Then a ground penetrating radar survey was completed at both dune normal and the average direction of travel for each blowout 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, which are key characteristics in understanding blowouts. These factors are integral in understanding how the blowouts have changed over the past 70 years and how these variables will affect evolution and spatial distribution of the shoreline.