Morphological controls on barrier-island response and recovery following natural and anthropogenic perturbations, northern Chandeleur Islands, Louisiana

Prevailing oceanographic climate, sediment supply, the magnitude and frequency of storm events, and anthropogenic modifications interact to drive the geomorphic evolution of barrier systems at varying spatial and temporal scales. The Chandeleur Islands east of the Louisiana mainland receive little external sediment input, and alongshore currents generally transport sediment away from the nearshore and littoral system to flanking depositional centers. We analyzed Landsat satellite imagery and lidar datasets from the northern Chandeleur Islands to quantify morphological changes that resulted from storm impacts and human-induced sediment input at intra-annual to decadal time scales. Since 2001, the study area was impacted by multiple tropical systems, including Hurricanes Lili (2002), Katrina (2005), and Isaac (2012). Additionally, between June 2010 and April 2011, in response to the Deepwater Horizon oil spill, the State of Louisiana constructed a 2-m high sand berm extending more than 12 km along the northern Chandeleur Islands platform. Berm emplacement provided a unique opportunity to study how anthropogenic sediment input affects the morphologic response of a naturally evolving barrier system. Land-cover and elevation metrics were utilized to test the hypotheses that (1) island geomorphology, in particular marsh extent, significantly influenced both “instantaneous” and longer term morphologic change and recovery following storm events and (2) redistribution of berm sediment depended on both antecedent morphologic controls as well as spatial variability in berm placement relative to the island platform. Despite the rapid post-construction degradation of the berm, imagery and elevation data suggest that some berm sediment remained in the system. Where the barrier-island was backed by healthy marsh platform, shoreward translation of the berm crest and increased elevations landward of the berm provide evidence of berm sand redistribution onto the emergent island. At the northern end of the study area, new sub-aerial islands were observed on the submerged island platform within the footprint of the pre-Katrina island extent. These data indicate that berm sediment may contribute to continued island recovery under normal climatic conditions.