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Barrier Island Failure during Hurricane Katrina

Asbury H. Sallenger, Jr.1, C. Wayne Wright2, and Jeff Lillycrop3

1U.S. Geological Survey, 600 4th St. South, St. Petersburg, FL 33701 asallenger@usgs.gov 2NASA, Wallops Flight Facility, Wallops Island, VA 23337 charles.w.wright@nasa.gov 3U.S. Army Corps of Engineers, JALBTCX, Mobile, AL 36602 jeff.lillycrop@usace.army.mil

Classical models of barrier-island response to storms predict that wave runup can periodically overtop an island and transport sand from its seaside to its bayside, forcing the island to migrate landward. While this process can destroy fixed human developments, the island survives with little net change in form or dimensions. In contrast, we find that Louisiana’s Chandeleur Islands during Hurricane Katrina were not periodically overtopped by waves, but were continuously inundated by storm surge. When such inundation occurs locally on a barrier island, it can force the erosion of a narrow breach that connects sea and bay. However, little is known about the response of a barrier island when it is entirely submerged.

Here, we show that the Chandeleur Islands approached complete failure, losing 84 percent of their surface area. Their Gulf of Mexico shorelines retreated landward an average of 268 m, the largest retreat ever reported for a storm. Sand was stripped from the islands, reducing their peak elevation from >6 m to <3 m and exposing them to further degradation and complete failure by subsequent hurricanes of less intensity than Katrina. Further, the islands that survived Katrina were marsh remnants composed of mud and vegetation that relatively small waves diminished following the storm.

The Chandeleur Islands are prone to failure because of their location on the Mississippi delta where small sand supply and large sea-level rise (induced locally by land subsidence) limit natural rebuilding of the islands following a storm. The response of the delta’s barrier islands during Hurricane Katrina provides a warning of how the world’s barrier islands might respond to storm surge inundation should predictions of accelerated global sea level rise prove accurate.

Pre-storm vertical aerial photograph from the northern Chandeleur Islands with post-storm island areas (determined from lidar data) superimposed in red. After Katrina, all non-red areas on the photo were water.

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