TRANSGRESSIVE SUBMERGENCE OF BARRIER ISLANDS: THE ROLE OF MULTIPLE HURRICANE IMPACTS ON THE SOUTHERN CHANDELEUR ISLANDS, LOUISIANA

Increased tropical cyclone frequency has resulted in an increased rate of transgressive submergence of barrier islands along the southern portion of the Chandeleur Islands barrier system in southeast Louisiana. Multiple storm impacts have forced parts of the system to exceed submergence thresholds at rates an order of magnitude greater than previously predicted. Overwash features are present along the island chain, however the dominant hurricane-associated process appears to be offshore sediment transport during the storm, followed by onshore transport to the north during recovery. Whereas northern portions of the island arc now show signs of beach and dune recovery after storm impacts, the southern portion has become increasingly sand deficient. The result in the southern portion is conversion from a barrier island with vegetated dune field, to a subaqueous shoal after Hurricane Katrina in 2005. During the last 2 years of recovery the shoal sand has been reworked into a broad sand wave field. On the basis of recent bathymetric surveys and aerial reconnaissance flights, the shoal morphology is characterized by strike-parallel, landward-oriented sand waves with amplitudes of ~ 1 m and wavelengths of ~ 20 m . The consistent sand wave spacing signals that sand bar amalgamation, welding, and island reexposure are not imminent. Because the dominant longshore transport direction is to the north, the southern islands lack a significant sediment source, possibly inhibiting a full recovery to subaerially exposed islands. Inner-shelf shoals bordering the Mississippi River delta plain have been suggested to represent submerged barrier islands on the basis of geomorphology, faunal assemblages, lithology, the presence of beachrock, and historical maps. This is however the first case of observed barrier island transgressive submergence and evolution toward an inner shelf shoal.