GEOLOGIC CONTROL ON BARRIER ISLAND STABILITY: SHIP AND HORN ISLANDS, MISSISSIPPI

The Gulf Islands National Seashore (GUIS) is a series of barrier islands in Florida and Mississippi, which comprise the longest national seashore in the country. Over 80% of GUIS resources are submerged, and include 20 km² of sea-grass habitat. The sea-grass beds are in a state of decline, and the island land areas have been severely impacted over the past century by storms, sea-level rise and human alteration. The most notable recent impact to the islands was from Hurricane Katrina, which hit the Mississippi portion of GUIS with over 160 kph winds and 9 m storm surge.

Offshore Mississippi, littoral processes over the past 4-5 ky transformed a narrow shoal platform into a barrier system that prograded westward across Pleistocene fluvial deposits. This older stratigraphy contains textural variability that interacts with modern coastal processes, such as wave action, to drive the morphologic response of the barrier system. Although all of the barrier islands within GUIS are undergoing rapid land-loss and translocation, there are large differences in relative change from one island to another. Studies show that since the mid-1800s Horn Island has lost 24% of it's land area, whereas neighboring Ship Island has lost a staggering 64%. Since the two islands are subjected to the same modern coastal processes, it is likely that the substrate variability influences island stability.

Over the past few years, the U.S. Geological Survey (USGS) and the National Park Service, which manages GUIS, have been mapping the land, seafloor and substrate to assess change in land area and habitat. To characterize geologic control on the islands, the USGS conducted subbottom (CHIRP) and seafloor (swath/single-beam) surveys around Ship and Horn Islands. The data identifies buried alluvial deposits and modern regressive features, such as flood-tide deltas and spit progradation. One month after completion of the survey, Hurricane Gustav passed nearby, bringing over 2.5 m of surge to Ship Island. To monitor storm impact, the USGS reoccupied the previous survey tracklines. Seafloor change included shoreface reduction, overwash, and breach enlargement. This presentation reports these findings and identifies nearsurface stratigraphic relationships at a chronic breach location on Ship Island (Camille Cut).