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. The removal of vegetation along the dune line causes an initial breach to develop, focusing wind velocity and sediment transport into the dune field which covers more vegetation. As this process continues, more sediment erodes from the foredune increasing the size of the blowout until vegetation stabilizes the entrance and begins to rebuild. With the front stabilized, the blowout begins its movement across the island. Aerial photographs, LIDAR data, and ground penetrating radar 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. 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. Geophysical analysis was completed on two smaller sites within the study area; 1) a blowout whose foredune is not completely reestablished and 2) a former blowout now stabilized by vegetation. Ground penetrating radar surveys completed at both dune normal and average direction of travel for each blowout show the bounding layers, cross stratification, and bedding planes of the migrating blowout without physical disruption. Comparisons of structures and burial ages are compared to geographic extent described the frequency of storms, vegetation extent, and anthropogenic affects over the past 70 years. These factors are integral for understanding how blowouts within the National Seashore have changed and how these variables will affect their evolution and spatial distribution.