Characterizing the near-bottom dispersion of drilling mud on three Canadian offshore banks

摘要

The three locations provide a dramatic contrast in drift, dispersion, and potential concentration levels. The drift rates and diffusivities are greatest at NEP, least at Hibernia and intermediate at North Triumph. The differences in magnitude and seasonal cycle can be explained in terms of the regional oceanography and meteorology. The relatively weak tidal currents and mean flows at Hibernia means that events such as low-frequency current variability and winter storms are an important component of the drift and dispersion environment. At NEP on Georges Bank the strong tidal currents and mean flows result in conditions where events are less important to the characterization of the drift and dispersion. At North Triumph the bottom stress regime is comparable to Hibernia but the near bottom currents and current shear are larger. As a result the overall levels of drift and diffusivity are closer to NEP than Hibernia but there is more variability than at NEP. Overall, the Northeast Peak of Georges Bank is the least likely to experience large concentrations from drilling discharges, with expected near-bottom concentrations about an order of magnitude less than at N. Triumph. The typical concentrations at Hibernia are expected to be a factor of 2-5 larger than at N. Triumph, when the effect of the limited water column coverage of the velocity observations is accounted for. The largest near-bottom concentrations at Hibernia and N. Triumph are predicted for the summer when the surface wave forcing is a minimum, however this is also the season when the vertical stratification is the greatest and this may limit the amount of material that reaches the seafloor during the initial descent phase. This would tend to reduce the near-bottom concentrations. The variations in the simulated near-bottom concentrations at all the sites can be explained by the scaling F(z_1)/D which captures the effect of both the differences in the concentration profile, F(z_1), and differences in diffusivity, D. Figs. 11 and 12 can be used to relate bb1t simulations at other locations to the results presented here or to predict near-bottom concentrations given prior information on the vertical profile and the diffusivity. These scaling arguments provide the potential to account for variability in currents, waves and stratification in estimating near-bottom concentration of discharge drill muds.