COUPLED BARRIER SYSTEM SHORLINE AND SHOREFACE DYNAMICS LOUISIANA, USA

In Louisiana, barrier islands are undergoing rapid morphological change due to shoreface retreat and increasing bay tidal prism driven by high rates of relative sea-level rise and interior wetland loss, respectively. Previous works utilized historical region-scale bathymetry change and shoreline change analyses to assess large-scale coastal evolution. However, more localized assessments considering the role of sediment transport processes and shoreface dynamics in regional evolution are lacking. This is essential to predicting coastal change trajectories and allocating limited sand resources for nourishment. Using historical bathymetric and shoreline data dating to the 1890s for the Louisiana coast, combined with modern data compiled for the Barrier Island Monitoring Program, 100-m spaced shore-normal transects were created to track meter-scale elevation change for 1890, 1930, 1980, 2006, and 2015. An automated framework was used to quantify and track barrier island evolution parameters such as shoreline movement, barrier island width, bathymetric contour migration, and shoreface slope. Average rates of shoreline erosion slowed from 12 to 6 m/yr while mean rates of lower shoreface migration increased from 7 m/yr in 1890 to 20 m/yr in 2006, followed by a decrease to 4 m/yr in 2015. Locally, retreat rates for the Caminada Headland upper shoreface slowed from 14 to 6 m/yr while lower shoreface retreat rates increased from 14 to 20m/yr in 2006 and slowed to 7 m/yr in 2015. The lower shoreface of Timbalier Island transitioned from progradational to erosional and back to progradational, indicating the periodic retreat and localized recovery of the shoreface around a dynamic equilibrium likely corresponding to storm activity. These results illustrate that the shoreface is more sensitive to storm impacts than is evident by the shoreline response and that these fluctuations play a key role in determining regional sediment budget. Our analyses illustrate that monitoring subaerial island erosion rates are insufficient for evaluating regional dynamics of transgressive coastal systems. Advances in understanding these processes will facilitate more informed planning, management, and mitigation of transgressive barrier islands.