Bottom Boundary Layer Parameters and Sediment Transport on the Louisiana Inner-Shelf During Cold Front Passages

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Increasing focus has been placed on the critical role the inner-shelf plays in coastal sediment dynamics. During storms, bottom sediment may be mobilized by waves and currents and transported along or across the inner-shelf, affecting the sediment budget of the adjacent coast. During a twelve-day period that included two cold front passages, we measured waves and near-bed currents on the Louisiana inner-shelf (depth ~ 8m) using a sophisticated bottom-mounted instrumentation system. We then calculated bottom boundary layer parameters using wave-current interaction models, including those of Grant and Madsen (1986) and Glenn and Grant (1987). Finally, we predicted sediment transport by assuming steady state turbulent diffusion within and above the wave boundary layer.

Results indicate that the second front (Event 2) was the more energetic of the two, and maximum significant wave height (1.33 m), and current speed (0.22 ms-1), occurred during this time. Additionally, mean current-induced shear velocity (1.9 cms-1) and wave-current shear velocity (4.4 cms-1) were highest during this event's frontal and prefrontal stages (respectively). During the postfrontal stage, currents were strong and well organized although combined shear velocities were low as a result of reduced wave height. Predicted sediment transport varied considerably in direction and magnitude throughout the deployment, but was highest (0.13-0.14 gcm-1s-1 SE) during the prefrontal and frontal stages of Event 2. Fair weather transport was low and to the west. We conclude that cold fronts are an important mechanism for sediment movement on the Louisiana inner-shelf, although the associated transport direction and magnitude require further quantification.

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Gulf Coast Association of Geological Society, v. 49, p. 432-439