Geomorphic and hydrodynamic impacts on sediment transport on the inner Louisiana shelf

Abstract

To investigate the interactions among geomorphology, hydrodynamics, and sediment dynamics on the inner shelf offshore Louisiana, multiple acoustic and optical sensors were deployed during a 58-day intermediate-energy period from May 23 to July 22, 2016. Time series results show that an elongated bathymetric “trough” between Ship Shoal and Isles Dernieres partially confines flow in the E-W (shore-parallel) direction. Warm water with lower salinity was observed in the mid to upper water column with cool water with higher salinity in the lower water column. High sediment concentrations of 1–10 g/L were observed in the bottom boundary layer during intermediate-energy conditions in response to sustained winds of up to 11 m/s, significant waves heights of up to 1.5 m, occasional 8 s period swells, and a spring tidal range of 0.6 m. The dominant current and sediment transport directions were westward during the study period. About 77% of the sediment flux occurred during three 2-day-long periods (only 10% of the observation period), revealing the nonlinear and episodic nature of sediment transport in this study area. Although intermediate-energy conditions are less energetic than hurricanes and storms, they occur more often and contribute greatly to the long-term net sediment transport. Based on preliminary estimates, ~51.0 million tons of sediment passes along the Louisiana inner shelf annually, comparable with the annual sediment exiting the Mississippi Delta and sourced from marsh edge erosion in coastal Louisiana combined. The inner shelf sediment flux is an integral part of the coastal sediment budget and may provide important mineral sediment for wetland accretion if transported onshore during storms.