The effect of storms on sediment resuspension and transport on the outer continental shelf, Northwest Gulf of Mexico

Abstract

Time series measurements of current velocity, transmissivity and water temperature, along with meteorological data obtained during the winter of 1980-81 and the spring/summer of 1980 were analyzed in order to determine the effect of winter storms and hurricanes on suspended sediment concentration on the outer continental shelf (average water depth 100 meters) of the northwest Gulf of Mexico. During the winter deployment, seven northers and Hurricane Jeanne passed over the study area. The time series data indicate that the passage of these storms disrupted the strong easterly flow (at 50 meters depth) present for much of the year in this area and caused the entire water column to oscillate north/south across the shelf edge. The passage of the storms did not cause an increase in current speed. Transmissivity data obtained from 2 levels above the bottom (four and eleven meters above the bottom) during the winter deployment showed that, in general, the passage of storms did not directly cause an increase in suspended sediment concentration. Regression analyses indicated that increases in current speeds were not linearly correlated with increases in suspended sediment concentration at this height above the bottom. Visual examination of the records indicates that suspended sediment is being advected into the area from onshore. Spectral analyses performed on the current meter data from both the winter and spring deployments indicated the presence of: (1) oscillations at the inertial/diurnal period, (2) shelf waves with periods of two days, and (3) a 28-30 hour natural mode for the Gulf. In addition, during the winter, a three day oscillation was observed in the spectra. Analysis of the wind and current meter data indicates that strong wind events which occur in winter may either directly or indirectly be forcing this oscillation. Coherence square estimates computed between the suspended particulate matter and the current meter records indicate that, during the winter, the v (north/south) component of the current and the suspended particulate matter concentration are coherent at periods of 28-30 hours, 2 days, and 3 days. Phase relationships suggest that when the flow is to the south (offshelf) the suspended particulate matter concentration increases, implying that sediment is advected into the area. When the flow is to the north (onshore) warm, clear water from off the shelf is brought into the area and the suspended particulate matter concentration begins to decrease. This advection of suspended sediment over the shelf edge may be an effective mechanism for the removal of sediment from the shelf.