The First Line of Defense: Louisiana's Barrier Islands

Much of Louisiana's mainland coasts are separated from the Gulf of Mexico by barrier islands that have long been considered the first line of defense to protect wetlands and infrastructure from storm surge and waves. Many of these islands are severely eroding, with some experiencing over 20 m/year of Gulf-shoreline retreat; and many are not following the classic model of roll-over barrier islands that migrate landward with no change in form. Rather, the Louisiana barriers are diminishing in size and will eventually disappear because of rapid sea level rise (mostly from subsidence), low sand supply, and impacts by extreme storms. During Hurricane Katrina, the Chandeleur Islands lost 85% of their surface area and peak elevations on the islands decreased from about 5-6 m to 2-3 m.

The utility of a barrier island as a first line of defense depends on a number of factors, including the island's elevation, its distance from the mainland shore, and the magnitude of storm surge. For example, if the island is high enough to contain waves and surge, it will limit impacts farther landward. If, however, the surge elevation is much greater than the island's elevation, the island's effectiveness diminishes. Further, if the barrier is far offshore, leaving a broad, shallow bay between island and mainland, waves propagating across that bay will become depth limited because of breaking; the shallow depths will limit how high the waves can grow and how much they will impact the mainland.

Each of Louisiana's barrier islands is unique and each needs to be evaluated separately to determine the utility of a potentially restored island as a first line of defense. Maintaining such restored islands over the long term will, however, be difficult, because of climate induced sea level rise and diminishing availability of sediments for re-nourishments.