Threshold Conditions for Episodic Beach Erosion along the Southeast Texas Coast

By

GIBEAUT, JAMES C., GUTIERREZ, ROBERTO, and HEPNER, TIFFANY L.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Changes in beach and dune topography along the Gulf of Mexico shoreline in Galveston County, Texas, were documented by comparing beach profiles measured from 1994 through 1998 and by analyzing LIDAR surveys. During this time, although eight tropical storms and hurricanes affected the northwestern Gulf of Mexico, only Tropical Storms Josephine in October 1996 and Frances in September 1998 caused significant changes in the dunes and beaches of the southeast Texas coast.

Conditions generated by Josephine appear to have just exceeded the threshold above which significant episodic erosion occurs, resulting in dune erosion, overwash, and damage to structures. It is estimated that the threshold conditions are ocean levels that exceed 0.9 m above sea level and coincident wave heights that exceed 3 m for at least 12 hours. The amount of episodic change caused by Josephine and Frances at a particular location is related to the dimensions and shapes of beaches and dunes, which are partly controlled by relic geomorphic features and rate of long-term (50 yr) shoreline change.

During Frances, ocean level exceeded 1 m above sea level, with coincident wave heights greater than 2.3 m for 60 hours. Frances, however, did not completely erode or wash over dunes that were more than 3 m above the beach berm tops or where the dune system was more than about 30 m wide. Comparisons of water levels, wave heights, beach morphology and dimensions, and resulting erosion are providing a basis for devising a storm-susceptibility classification.