SUBDUCTION AND DISPERSION OF A BUOYANT EFFLUENT PLUME IN A STRATIFIEDENGLISH BAY

St Austell Bay in Cornwall is unusual for UK coastal waters because it has very weak tidal currents and becomes thermally stratified in summ er. Furthermore, water movement in the bay is very dependent on meteor ological 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 st rong tidal currents, it is not possible to invoke the repeatable natur e 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 si tuation, which in St Austell Bay occurs during onshore sea breezes. Dy e was introduced into an outfall for 13 h on a day when there were lig ht winds in August 1993, The observations showed that in the calm cond itions 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 shor e, From late morning onwards surface heating masked the density front and, although the plume then rose to the surface, it was still advecte d towards the shore by the sea breeze. Its width increased linearly aw ay from the outfall, apparently spread by wind-driven shear diffusion in the surface waters. The observations demonstrate that it is possibl e to explain the behaviour of the outfall plume in terms of the oceano graphy of the bay, Furthermore, the plume responded in a similar way t o the movement of spot dye releases made two years earlier, which sugg ests that the circulation under sea breeze conditions may be determini stic. The work highlights the fact that marine outfall surveys in plac es with small tidal currents need to be carefully planned if they are to be of value. (C) 1997 Elsevier Science Ltd.