BARRIER ISLAND DEGRADATION FROM MULTIPLE HURRICANE IMPACTS: RELEVANCE TO BARRIER ISLAND FAILURE DURING RAPID SEA LEVEL RISE

The impacts of individual storms on barrier islands have been categorized in terms of wave runup elevation, R, and still-water elevation, η (which includes storm surge, wave setup, and astronomical tide elevations), relative to the peak elevation of the foredune, D_high. As R/D_high and η/D_high increase, thresholds will be crossed that define increasing intensity of impacts, progressing from runup colliding against the dune face and eroding it landward, to runup overwashing the dune (R/D_high > 1), to still water level completely submerging the beach system (η/D_high > 1). The most extreme barrier island changes have been observed during this latter inundation regime. For example, during Hurricane Katrina, the Chandeleur Islands in Louisiana were completely inundated and the average shoreline erosion was ~ 250 m. In this talk, I will discuss how a sequence of six hurricanes (Lili, Ivan, Katrina, Rita, Gustav, and Ike) over six years (2002 to 2008) drove the Chandeleur Islands toward a persistent inundation regime. The specific impacts during each storm varied depending upon storm intensity, proximity to the islands, whether the track was to the east or west of the islands, and the islands’ geomorphic condition that was left after the previous impact. During the sequence, D_high was decreased and kept relatively small by successive storm impacts, which increased the probability of repeated inundations even by relatively low intensity storms. The resulting degradation of the islands pushed them toward failure and permanent submergence. These islands are conditioned for failure because of small sand supply and rapid sea-level rise induced on the Mississippi delta by the river switching channels and by land subsidence. Should global sea-level rise accelerate in the future as predicted, other sand-starved barrier islands, like some of those lining U.S. Atlantic and Gulf Coasts, may respond similarly to multiple storm impacts and may ultimately fail.