Dispersion in flow from a continuous source at sea

Diffusion experiments, using a continuous release of fluorescent dye from an anchored source, were carried out in the Holy Loch and two coastal areas of the Irish Sea. In each case the dye distribution was sampled by a vessel making a series of crossings of the plume. The data were analysed in terms of σ_y, the standard deviation of the dye distribution at right angles to the axis of the plume, as a function of the diffusion time t₁ and the corresponding eddy diffusion coefficient K_y. In general the value of K_y increased with t up to a diffusion time of at least 2 h. In many cases the relation between K_y and t was approximately linear, corresponding to spreading with a constant diffusion velocity B such that K_y = B²t. The observed median values of B for the Holy Loch, Red Wharf Bay and the area of the Irish Sea off the Cumberland coast were 0·4, 1·1 and 1·4 cm s⁻¹ respectively. The vertical diffusion was also analysed, but in less detail, and other features of the plumes, including their variability, are discussed. In general the rates of diffusion in the Holy Loch are similar to those reported by other workers for the Great Lakes and for coastal sea areas with weak currents and high stability. In the two Irish Sea areas the diffusion rates are similar to one another but considerably greater than in the Holy Loch.