Alan J. Elliott

Modelling tides and surface drift in the Arabian Gulf—application to the Gulf oil spill

Abstract A tide and surge forecasting model capable of predicting conditions for up to 5 days ahead has been developed to provide environmental data on tides, currents and particle trajectories in the Arabian Gulf. A two-dimensional depth-integrated model on a 5′ × 5′ grid of the entire Gulf, driven by a 10 constituent tidal forcing at the mouth near the Strait of Hormuz and by meteorological forecasts from the United Kingdom Meteorological Office global numerical weather prediction model was used to provide hourly distributions of the depth-averaged tidal and wind-driven currents. Following the discharge of oil from Mina Al Ahmadi into the Gulf in January 1991 an oil spill model was interfaced to the tide and surge model, providing forecasts of the movement and spread of the oil slick. The oil spill model used a three-dimensional particle tracking algorithm to model the dispersion of the oil so that surface and sub-surface concentrations could be determined. The effects of surface evaporation and decay of the oil were included in the model.

Diffusion in Irish coastal waters

Results are presented from dye-diffusion experiments performed at a selection of near-shore and estuarine sites around the coastline of Ireland. The data from discrete dye releases have been analysed to derive along- and across-patch horizontal diffusivities, and estimates are presented of both Fickian and non-Fickian diffusion coefficients. Most of the sites were characterized by strong tidal currents so that the dye became vertically well-mixed soon after release. Consequently, the mixing was found to be tidally dominated and, to a reasonable approximation, the magnitude of the one-dimensional (radial spreading) horizontal diffusivity (m 2 s -1 ) was equal to the tidal current speed (m s -1 ). Wind-induced mixing was only of secondary importance, but this may be a consequence of the experiments having taken place during conditions of generally light winds. In general, the observed mixing rates agree with those predicted by oceanic diffusion diagrams.

Baroclinic tidal currents in the Gareloch, Scotland

Abstract Acoustic doppler current profiler, moored current meter and thermistor data were collected in the Gareloch, a sea loch on the west coast of Scotland, for 4 days during June 2001. The loch is approximately 8 km long, 40 m deep in the interior and is separated from the Clyde estuary by a sill at a depth of 16 m with respect to mean sea level. The sill is situated in the Narrows at the mouth of the loch where the tidal currents are barotropic in character and reach maximum speeds of about 0.3 m/s. In the interior of the loch, where the barotropic tidal currents should not exceed 0.02–0.03 m/s, the flow was markedly baroclinic and reached speeds of 0.15 m/s in the surface layer. The surface and bottom layer flows displayed a 180° phase difference, with the lower layer flow being directed landward during the flood tide. The tidal currents did not display the expected semi-diurnal motion that is characteristic of the Clyde Sea and estuary. Instead, the motion was dominated by short period oscillations that appeared to be seiche related rather than due to shallow water tidal effects. A strong inflow event, observed at the mouth of the loch during a flood tide, was consistent with the presence of a density-driven current superimposed on the tidal flow. The thermistor data showed that the loch was thermally stratified with a surface layer of thickness 8–10 m. Simulations with a two-layered model that allowed for variable channel width and water depth reproduced the observed phase difference between the surface and bottom layers. The model results suggested that the short period variability in the observed currents was related to interfacial oscillations generated by the strong tidal flow above the sill.