Initiation and evolution of blowouts within Padre Island National Seashore, Texas

Blowouts once covered the entire northern section of North Padre Island as a result of overgrazing, but are now restricted to a 5.5 km length of beach where driving is permitted between the low water line and the toe of the foredune. A comparison between this driving section and a directly adjacent non-driving section of the beach reveals a significant difference in the ratio of storm run-up (Rhigh) to the elevation of the dune toe (Dlow), suggesting that the foredunes within the driving section are more susceptible to scarping and blowout development. Where driving is permitted, blowouts are located in areas where Rhigh/Dlow ratio is locally large, suggesting that the location of blowouts depends on the alongshore variation in storm surge. Results of a cluster analysis reveal two distinct blowout types, in which the relatively small blowouts of Cluster 1 are characterized by a small throat and limited expansion beyond the foredune. The large and more active blowouts of Cluster 2 tend to be found closer to the access road, where driving on the beach is more concentrated and the larger Rhigh/Dlow suggests a greater potential for scarping during relatively small storms. The development of these blowouts feeds back on the height of the foredune, making it susceptible to scarping and washover during tropical storms and hurricanes. It is argued that historical overgrazing and recent beach driving have significantly altered this landscape and represents a long-term threat to island resilience and have the potential to hasten island transgression with sea level rise.