THE ISLAND AND THE BERM: INTERACTIONS BETWEEN THE SAND-STARVED CHANDELEUR ISLANDS AND A SAND-RICH BERM CONSTRUCTED TO CAPTURE SPILLED OIL

In response to the Deepwater Horizon oil spill, the State of Louisiana began construction of a sand berm parallel to and immediately offshore of the ~30-km-long Chandeleur Islands to keep oil from reaching mainland marshes. The berm was built to a height of approximately 2 m above mean sea level. In this presentation, we focus on the potential contribution of sand from the berm to the sand-starved Chandeleur Islands. Specifically, we test whether the introduction of new sand from the berm has significantly changed peak elevations, D_high, along the natural islands and hence changed island vulnerability to being overtopped by storm-driven water levels, such as still-water level (η, due to tides, surge, and wave setup) and runup (R, due to swash). Vulnerabilities to overwash, where R > D_high, and inundation, where η > D_high, will be identified. During storms over the past decade, overwash on the Chandeleurs has driven island migration landward but with little degradation of D_high (e.g. during Hurricane Lili), while inundation has reduced island surface areas and significantly degraded D_high (e.g. during Katrina and Gustav). Construction and subsequent response of the berm has been monitored with satellite imagery, oblique aerial photography, airborne lidar, and acoustic bathymetry. Waves and sea level are being monitored with models and in-situ sensors; and coastal-change models, such as Xbeach, are being used to simulate changes to the berm and islands. As of January 12, 2011, the berm reached over 11 km in length, with the last several kilometers constructed on the sub-aerial beach. During a storm in early January 2011, significant wave heights of 4.9 m generated runup on the berm where R > D_high. Four breaches were cut through the berm, the largest 590 m wide. Over the past decade, the Chandeleurs have lost significant surface area, 82% during the passage of Hurricanes Ivan and Katrina. We will address how much the introduction of new sand to the islands replaces these losses and resets the countdown to ultimate disappearance of the islands.