Currents and water characteristics around the West Flower Garden Bank

Abstract

The West Flower Garden Bank is a coral reef on the Texas continental shelf. The corals on the bank are vulnerable to sediment contamination and to excess turbidity in the overlying water column. Concern for the environmental impact on this and other banks in the region exposed to nearby hydrocarbon production prompted the Bureau of Land Management to fund a data collection effort on the Texas/Louisiana shelf which provided the data analysed here. Data analysed includes profiles of velocity, temperature and salinity taken around the Bank in Oct., 1980 and March, 1981. Fixed current meter moorings and a dye experiment conducted in the bottom boundary layer provided additional input. Mean flow was eastward across the bank with perturbations occurring in the flow as the bank was approached. Flow was generally directed around the bank with intensification in velocity over the bank. Modification in the isotherms was observed during October on the East side of the bank when isotherms were compressed and depressed in an apparent indication of lee waves. The complexity of the bank topography resulted in additional modification in the water characteristics. Flow across a moat at the base of the bank resulted in a separated bottom nepheloid layer. Isotherms within the trough, which bisects the bank, were inclined relative to those outside the trough. The presence of gravel waves on the flanks of the bank indicates that significant current speeds occur. Bottom nepheloid layers occur at depths below 85m around the bank but none are observed up on the bank. No fine sediment is found on the top or flanks of the bank. The dye experiment revealed a complex bottom boundary layer on the terrace around the bank where many roughness scales are present. This layer was not logarithmic and consisted of two distinct regimes. The data reveals a very complicated flow regime around the bank, with some intensification of flow around and over the bank but no movement of water from the bottom of the surrounding shelf up onto the bank.