Surface Spills in a Wave Environment

An increasing awareness of environmental issues, coupled with a number of recent events, has focused considerable attention on the likely consequences of a large oil spill. If the impact of these accidental releases is to be minimised, the risk assessment and contingency planning must be based upon a sound understanding of the dispersion processes. This is required to determine both the lateral spreading of the oil slick and the vertical mixing within the water column. Although a number of oil slick propagation models are available there are no existing solutions that adequately take into account the additional mixing associated with the wave climate. Indeed, it is widely believed that this uncertainty was responsible for the inability of the models applied to the Braer spillage to predict the extent of the oil contamination. This paper summarises work undertaken within the Department of Civil & Environmental Engineering at Imperial College. A new experimental facility has been developed to allow the formation of a gravity-driven surface spill within a wave flume. This facility allows regular waves to be generated and superimposed onto the surface flow. Concentration and velocity profiles are measured in the presence and the absence of waves using instrumentation developed especially for this research project. In addition, photographic evidence of the wave-induced mixing was recorded. The results of this experimental work clearly show that waves significantly increase the vertical mixing within the gravity-driven surface flow. A physical explanation for this has been provided. By incorporating this understanding into an existing Gaussian patch model, it is possible to indicate likely changes in the lateral and vertical spreading of a hypothetical surface spill subject to significant wave action. Comparison between these results and the wave-induced far-field effects recently identified by Buick and Durrani allows the practical significance of the present results to be assessed.