Sedimentary Features of the Coastal Wetlands of Breton Sound and Barataria Bay in the Mississippi River Delta and their Implications for Sediment Transport and Coastal Restoration
Cody Johnson1, Qin Jim Chen1,2, Arash Karimpour3, Samuel J Bentley2,4, Crawford White4, Kehui Xu5,6, Jiaze Wang7 and Marisa Fanguy1, (1)Louisiana State Univeristy, Department of Civil and Environmental Engineering, Baton Rouge, LA, United States, (2)Louisiana State Univeristy, Coastal Studies Institute, Baton Rouge, LA, United States, (3)Louisiana State Univeristy, Louisiana Sea Grant, Baton Rouge, LA, United States, (4)Louisiana State University, Department of Geology and Geophysics, Baton Rouge, LA, United States, (5)Louisiana State Univeristy, Department of Oceanography and Coastal Sciences, Baton Rouge, LA, United States, (6)Louisiana State University, Department of Oceanography and Coastal Sciences, Baton Rouge, LA, United States, (7)University of Maryland, Center for Environmental Science, Horn Point Laboratory, Cambridge, MD, United States
Abstract:
Louisiana is experiencing some of the highest land loss rates in the world. The majority of this occurs in its coastal wetlands and is partly attributed to exacerbated conditions which influence erosive mechanisms, e.g. increased fetch, reduced sediment delivery, ecosystem stress. Wetland soils and their underlying substrates are an integral component of these systems’ morphological response to erosional forcing, but one that is notoriously difficult to characterize. To this end, a multi-disciplinary field campaign was undertaken to capture essential soil mechanical and geological data over a wide expanse of coastal wetlands in both Barataria Bay and Breton Sound in Louisiana. Here we present the results of our data collection and a synthesis focused on elucidating the study area’s subsurface geotechnical features with reference to the Mississippi’s deltaic depositional environment.
A total of 100 data collection sites were subjected to vibracore sampling and in-situ tests to determine bulk density, organic content, water content, grain size, undrained shear strength, and erosive critical shear stress at discrete subsurface intervals up to a maximum depth of 6 meters. The sites in total encompassed approximately 500 km2 and are distributed on the eastern side of Barataria Bay, LA and the western portion of Breton Sound, LA. Two general landscape features, i.e. bay bottom and vegetated marsh edge, were sampled. In general, this controlled the vertical distribution of our data and influenced the observed stratigraphic sequence. Typically, the muddy bay bottom cores exhibited less variability, displayed interbedded sand layers less frequently, and lacked the upper organic layer seen in the marsh edge cores. This confirms the conversion of marsh to open water model proposed by Wilson et al. (2008) for eroding coastal wetlands. This study has implications for the sediment diversions planned in the region. It is likely that the increased hydrodynamic forcing and sediment loading associated with a sediment diversion will preferentially erode and compact the younger, softer, and less dense sediment rather than the older deltaic sediments.
