BEACH-DUNE INTERACTION IN A DISTURBED SYSTEM

Barrier island response to extreme storms depends on the height and extent of the foredunes, which in turn depends on the availability on an adequate sediment supply and the aeolian transport potential. Disruption to the exchange of sediment between beach and dune can limit dune height, leading to shoreline erosion and washover that limit the ability of the dunes to recover. A number of recent studies suggest that driving on the beach has a significant biophysical impact and it has been suggested in a number of recent studies that driving on the beachface leads to a net loss of sediment from the beach-dune system. Identifying a conclusive link between beach driving and beach erosion is, however, complicated by the natural variability of the environment in both space and time, and it has proven difficult to distinguish the driving signal from this background noise. In the present study, LiDAR data from both Padre Island National Seashore and Assateague Island National Seashore are used to determine if the differences in beach and dune morphology between restricted and open access sections of the beach are associated with beach driving. Results from Padre Island National Seashore suggest that beach driving does not affect the height and volume of the foredunes, but is responsible for a statistically significant decrease in the elevation of the dune crest and base compared to the control section of beach. The decrease in elevation is ascribed to the compaction and pulverization of seaweed rack that accumulates along the Texas coast in the spring and summer months, and is responsible for the anchoring of sediment of new vegetation seaward of the foredune. At Assateague Island National Seashore, driving on the beach is shown to cause a statistically significant change in the beach-dune morphology, with smaller dunes set further back from the shoreline within the open access sections of the beach. Despite the change in dune morphology, there is no difference in beach-dune volume on either side of the beach access road at either study site, which suggests that driving on the beach does not lead to a net loss of sediment from the beach-dune system. The greater exchange of sediment from beach to dune does, however, make the foredune at both sites susceptible to scarping and overwash during tropical storms and hurricanes.