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Diversions are being used to encourage Missippi River delta growth via increased sediment availability to coastal wetlands. USGS studies hydrodynamics and sediment transport in Louisiana to better understand how marshes and deltas respond to these sediment inputs.
Science Issue and Relevance: Across the Mississippi River delta plain, a series of Mississippi River diversions are being implemented to stimulate delta growth through increased sediment supply to coastal wetlands. Although river diversions deliver significant quantities of sediments to their receiving basins, the ability of these projects to augment sediment deposition and subsequent vertical accretion in marsh settings has not been clearly demonstrated. Though modeling efforts are currently underway to predict land building in response to diversion-induced sediment introduction, a major limitation of those models is that the rate at which entrained sediments settle through the water column and subsequently accumulate on the marsh surface remains unknown. This process is governed by the balance between turbulent energy in the water column and settling velocity of entrained sediment particles. These parameters vary with wave and flow conditions, vegetation density, and sediment concentration and grain size distributions. Our ability to accurately model marsh and delta geomorphological responses to sediment inputs from river diversions hinges on developing an improved understanding of how hydrodynamics and sedimentological properties interact to govern particle settling in vegetated marshes and delta flats.
Methodology for Addressing the Issue: This project aims to quantify changes in sediment concentration and grain size distribution, wave- and current-induced turbulence, and deposition at five stations along a 150-meter transect across Mike Island in the Wax Lake Delta (29.4948°N, 91.4471°W) in south central Louisiana. Each station is equipped with an acoustic Doppler velocimeter, a wave gauge, and an automated water sampler, all collecting data every quarter hour. We aim to address how sediment deposition rates, grain size distributions, and turbulence vary along the length of the flume, and how these factors vary between vegetated and non-vegetated settings. We will also address how canopy hydrodynamics and particle characteristics interact to determine if conditions favor sediment deposition, transport, or erosion, and how these conditions vary in time (hours to weeks to months) and space (tens of meters).
Future Steps: We are currently conducting similar time-series measurements in a newly-reclaimed wetland in north San Francisco Bay (38.0550°N, 122.4992°W). Unlike the Wax Lake delta, this subtidal, non-vegetated area receives little freshwater inflow and has a large tidal amplitude. We aim to make comparisons between the two sites, then to draw inferences on the relative importance of vegetation, tidal forcing, water depth, and buoyancy forcing as driving mechanisms of sediment transport through wetland settings.
Location of the Study: Throughout the Louisiana coastal zone, Latitude: 29.678°, Longitude: -91.552°