SEDIMENT DYNAMICS AND GEOMORPHOLOGY OF ERODING MARSH SHORELINES IN SOUTHEASTERN LOUISIANA

Louisiana coastal marshes are experiencing the highest wetland loss rates in the U.S. due to subsidence-driven relative sea-level rise. These marshes are also vulnerable to the erosive power of wave attack 1) on the marsh edge adjacent to open water bodies and 2) after the marsh platform is submerged. Marsh shorelines in Barataria Bay, Breton Sound, and the active Balize delta of southeastern Louisiana were examined in areas where the subaerial marsh platform had disappeared since 1932. Vibracore transects of marsh and adjacent bay surface sediments (to ~2m depth) were analyzed using geotechnical, stratigraphic, and radiochemical (¹³⁷Cs and ²¹⁰Pb) methods, and the subaerial-to-subaqueous elevation transition of the marsh was examined using standard stadia rod transit and fathometer measurements. Results indicate that marsh-edge erosion of the platform continues subaqueously until depths of 1.5-2 m are reached: this is observed even in interior pond regions but the balance of erosion versus subsidence-caused elevation loss varies with setting. Core stratigraphy reveals that at some point after erosion and subsidence of the marsh surface, processes in the shallow bay areas switch from erosional to depositional, covering marsh platform deposits unconformably with estuarine shelly muds. ¹³⁷Cs and excess ²¹⁰Pb activity indicates that these muds are deposited within a few decades. These results suggest that a single profile of equilibrium can approximate the morphology of eroding marsh edges in southeast Louisiana: platform stratigraphy and resistance to erosion has little effect on profile shape. This profile is a combination of lateral erosion that predominates at the subaerial marsh edge, and subsidence. At the outer edge of the profile, subsidence creates accommodation space that is filled with estuarine muds. The profile of equilibrium can be used to predict the morphology of future marsh loss in Louisiana, as well as sediment yield to adjacent estuarine areas.