Abstract

Examination of current meter measurements from fair-weather periods and storm events on the central Texas shelf provides information on the means by which sand is transported across the shelf to be deposited as thin (< 10 cm), discrete beds. Weak southerly winds characteristic of fair-weather periods are incapable of generating shelf currents or waves with sufficient combined strength to entrain the fine sand of these beds. In contrast, measurements of near-bottom shear stresses during storms such as Hurricane Allen in 1980 and an extratropical event in September 1984 suggest that shelf-sand transport occurs during periods of coastal downwelling. In the case of the central Texas shelf, this takes place under conditions of strong northeasterly wind, such as are associated with approaching tropical storms. The downwelling flow typically consists of surface-water flow toward the coast, water-column interior flow directed along-shelf toward the southwest, and near-bottom current motion oriented along-shelf and obliquely offshore to the south. These geostrophic storm flows are driven primarily by the wind-induced hydrostatic pressure gradient forces. The bottom currents interact with wave-orbital motion to produce a combined flow with sufficient magnitude and with the proper kinematics to transport sand and silt from shoreline sources. Building upon observations from these smaller storms and available wind, tide, and wave data, a model of the kinematics of sand transport was developed for Hurricane Carla, which crossed the Texas shelf in September 1961. In this reconstruction, combined flow is oriented obliquely offshore on the immediate right and left side of the storm track (looking toward the coast) and onshore on the storm's far left side. This combined-flow transport pattern is consistent with the observed distribution of the sand bed deposited during the passage of the storm, as revealed by cores of the near-surface modern sediments. This model also differs considerably from an earlier explanation for the Carla sand bed which invoked turbidity currents generated during the ebb of the storm surge following hurricane landfall.

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