Modeling Past and Potential Future Evolution of the Chandeleur Islands of Southeastern Louisiana in Response to Relative-Sea Level Rise

Observations of the 72 km-long Chandeleur Island arc in southeastern Louisiana since August 2005 indicate that large volumes of sediment were removed from the islands during and following Hurricane Katrina. Whether or not this indicates that Hurricane Katrina initiated a threshold crossing in the Chandeleurs, causing the subaerial, landward-migrating barrier islands to begin evolving as submerged sand shoals, remains unclear.

To assess the combination of factors likely to cause a threshold crossing in this environment and to better understand the potential future evolution of the Chandeleurs, we run a series of model experiments using the morphological-behavior model GEOMBEST. This model simulates the evolution of coastal morphology and stratigraphy resulting from changes in relative sea level and sediment supply, and provides insight into how barriers evolve over time scales ranging from decades to millennia. Vibracores, geophysical records, bathymetric surveys, and lidar surveys provide data necessary to design the model domain, while sediment budget studies, estimates of sea-level rise rates, and measurements of shoreline change rates provide input and calibration parameters. To calibrate the model and to initialize the model for forward experiments, we first simulate the late-Holocene evolution of 42 km-long North Chandeleur Island as it migrated from the distal end of the St. Bernard Delta to its modern position over the last 1500 years as suggested by the conceptual model of transgressive submergence first proposed by Shea Penland. Building on the late-Holocene simulation, we present a series of initial, multi-decadal, forward model experiments that assess the combination of factors likely to initiate a threshold crossing. We also consider the effect of potential management scenarios on the future evolution of North Chandeleur Island. Model results suggest that barrier island viability depends on relative sea-level rise rates, sediment supply rates, the geologic framework and the rate of shoreface evolution.