Subduction and dispersion of a buoyant effluent plume in a stratified English Bay

St Austell Bay in Cornwall is unusual for UK coastal waters because it has very weak tidal currents and becomes thermally stratified in summer. Furthermore, water movement in the bay is very dependent on meteorological conditions, and the tidal regime plays only a secondary role in determining the fate of pollutants. The ensuing variability creates problems for environmental agencies because, unlike locations with strong tidal currents, it is not possible to invoke the repeatable nature of the tides when determining the optimum site for a marine outfall. One approach to this problem is to identify and study a worst case situation, which in St Austell Bay occurs during onshore sea breezes. Dye was introduced into an outfall for 13 h on a day when there were light winds in August 1993. The observations showed that in the calm conditions of the early morning, the discharge was subducted to a depth of 4 m by a local density front which caused it to move towards the shore. From late morning onwards surface heating masked the density front and, although the plume then rose to the surface, it was still advected towards the shore by the sea breeze. Its width increased linearly away from the outfall, apparently spread by wind-driven shear diffusion in the surface waters. The observations demonstrate that it is possible to explain the behaviour of the outfall plume in terms of the oceanography of the bay. Furthermore, the plume responded in a similar way to the movement of spot dye releases made two years earlier, which suggests that the circulation under sea breeze conditions may be deterministic. The work highlights the fact that marine outfall surveys in places with small tidal currents need to be carefully planned if they are to be of value.