SEDIMENT REMOBILIZATION BY WIND, WAVES, AND CURRENTS DURING METEOROLOGICAL FRONTAL PASSAGES IN A SHALLOW, MICRO-TIDAL BAY

초록

Sediment transport in the bays and estuaries of the Northern Gulf of Mexico can be significantly impacted by episodic meteorological events. This study investigated the impact of cold fronts on the seabed of the shallow (~ 2 m), micro-tidal Galveston Bay located along the Texas coast in the northwestern Gulf of Mexico. Two instrument deployments, which included an Acoustic Doppler Velocimeter (ADV), Acoustic Wave and Current meter (AWAC), CTD with Optical Backscatter Sensor (OBS), and sonar altimeter; collected flow and suspended sediment concentration data during two separate cold front passages. Results show that wind stresses increased as a result of the frontal passage, where wind speed had increased following the shift in wind direction from the south/southeast to the north/ northwest. This also resulted in a 2-3 fold increase in near-bottom current speeds, and waves in the bay. Sediment resuspension was induced due to enhanced shear velocities near the bed caused by waves generated in the bay. These results show that sediment resuspension requires wind stresses strong enough, and from the proper direction to generate waves. Therefore sediment remobilization does not occur throughout the frontal passage, but rather at discrete times when the proper conditions persist. Maximum sediment resuspension observed occurred after the front had passed through the area. During this period an order of magnitude increase in suspended sediment concentrations were measured. In this microtidal bay, with a negligible freshwater discharge, where cold fronts impact the area on average 30-40 times a year, the cumulative effect of these events may represent the dominate mechanism for sediment transport in this bay, and similar bays throughout the region.