Sediment transport on a river-dominated shallow water shelf: Atchafalaya Bay region, Louisiana
Master of Natural Sciences (MNS)
Natural Sciences (Interdepartmental Program)
Cold front passage impacts on sediment transport on the inner continental shelf near Atchafalaya Bay were studied, combining satellite imagery and in-situ meteorological and oceanographic measurements. An empirical model relating atmospherically corrected MODIS imagery reflectance to surface measurements of suspended sediment concentration was developed and successfully applied to the coastal waters. An equation for predicting total suspended sediment from Nephelometric Turbidity Units was also applied to the water column at 1, 2, and 3 meters from the seabed. Five case study weeks were selected to examine the dynamics of pre-frontal and post-front forcing mechanisms on sediment transport in a fine-grained sediment environment, with the major objective to determine whether changes in surface sediment concentration revealed characteristics of sediment transport in the vertical structure of the water column. Remote-sensing data showed that great spatial variation occurred in the surface mud-plume during pre-frontal, post-front and inter-frontal conditions. Southeasterly winds confined the plume nearer shore and often within the bays during pre-frontal conditions. Post-frontal northwesterly and northerly winds increased plume area by 3 to 4 times, extending past the 10 meter isobath and reversing the dominant transport direction from westward to southeastward. A comparison of the surface sediment concentrations at WAVCIS station CSI-3 (seaward of Marsh Island) to simultaneous maxima elsewhere in the mud-plume revealed that the main plume sediment concentrations were often an order of magnitude greater south of the Atchafalaya River mouth. The presence or absence of the plume over the CSI-3 site was not indicative of the level of sediment flux throughout the water column at CSI-3. High rates of surface and upper water column sediment transport were primarily in response to strong currents from wind-forcing. In contrast, the dynamics of near-bottom transport at CSI-3 showed high rates of sediment flux in the post-front period attributed to high sediment concentrations. Total suspended sediment in the bottom meter sustained peaks which were 2 - 10 times that of the surface and mid-depth concentrations, reaching maxima of 1,100 mg/l due to resuspension and/or the advection of fluid mud from the Chenier Plain coast.
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Bellotte, Renee T., "Sediment transport on a river-dominated shallow water shelf: Atchafalaya Bay region, Louisiana" (2007). LSU Master's Theses. 3307.
Nan D. Walker