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PEPPER, DAVID A., Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA; GREGORY W. STONE, and PING WANG, Coastal Studies Institute, Louisiana State University, Baton Rouge, LA

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

Increasing focus has recently 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, thus affecting the sediment budget of the adjacent coast. During a 10-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 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 during the second, and stronger, of the frontal passages (Event 2), significant wave height and near-bottom current speed reached 1.34 in, and 0.22 ms-1, respectively. Maximum wave-current shear velocity (u*cw=6.57 cm s-1) and current-induced shear velocity (u*c=3.08 cm s-1) occurred during Event 2, although the former preceded the latter by several hours. Predicted sediment transport rates were highest during the frontal passages, particularly Event 2, although transport direction varied widely during these periods. Predicted suspended sediment transport closely followed the direction of mean currents, which rotated in response to changes in wind direction, while predicted bedload transport was in the direction of maximum combined wavecurrent stress, which also varied. We conclude that cold fronts are probably an important mechanism for sediment movement on the Louisiana inner-shelf, although the associated net transport requires further quantification. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana