Development and Progression of Aeolian Blowouts in Padre Island National Seashore

This study characterizes the development and migration of blowouts within Padre Island National Seashore (PAIS). A combination of aerial photographs and Light Detection and Ranging (LIDAR) are used to track the migration of eighteen blowouts, while Ground-penetrating Radar (GPR) is used to investigate the subsurface at two smaller sites in the study area. This data, coupled with beach morphology and changing anthropogenic factors, helps understand why the dune blowouts develop and are restricted to a particular section of the National Seashore.

Aerial Photographs taken at least twice a decade since 1969 were used to track blowouts. Each blowout was digitized in order to understand its morphometric characteristics by studying its length, width, area, segmentation, perimeter, and the width of the neck, when present, through the foredune. The velocity and direction of movement were also calculated. Cluster analysis was used to analyze the blowouts using these morphological variables. Based on this data, blows appear to group into two morphologically different clusters. Blowouts grouped into Cluster 1 are longer, thinner, have smaller perimeters and areas, smaller throat widths, and are furthest from the beach access road. A lower dune elevation leads to a larger wave runup to crest height ratio. A larger ratio suggests that the dunes are more easily overtopped during large storms, thus scarping, a precursor to blowout development, is increased. Cluster 2 blowouts tend to be longer, wider, and stabilized faster leading to a more undulated perimeter in addition to a smaller wave runup potential due to a higher dune elevation.

Historically blowouts covered the entire northern portion of PAIS. In the 1970?s the portion of the beach north of Park Road 22 was designated as non-driving. Since then all blowouts in this section have revegetated, while, blowouts in the driving section are still active. Beach driving pulverizes seaweed leading to less deposition along the dune toe and therefore a lower elevation of the backshore. As a result there is a greater wave runup in storms leading to an increase in susceptibility to scarping, and therefore, blowouts.

Despite the fact that storms are the primary mechanism for blow development, anthropogenic effects, such as vehicle traffic, flatten the beach profile allowing for lower areas to become inundated during storms. This, along with decreased sediment budget and increased storm frequency increases the potential for blowouts to form events and leave the island vulnerable to an increased rate of sea level rise.

GPR surveys were completed at two sites; an active blowout with a foredune that is not completely reestablished (Site 1) and a blowout that is stabilized by vegetation (Site 2). Six GPR surveys were completed at Site 1 and four surveys were completed at Site 2 that show the preservation of historic phases, surfaces, and facies used to interpret sequences and compare to aerial photography and LiDAR data. Site 1 moves through five phases that begin in 1969 and end at the present location, while Site 2 moves through three active phases and then ends in a fourth phase by becoming completely stabilized with vegetation in 2010.