Langmuir circulation and the dispersion of oil spills in shallow seas

Aspects of Langmuir circulation (Lc) which relate to the dispersion of floa ting material are reviewed. These include convergence, dispersion by advect ion (particularly of a plume of floating oil when wind and current are in d ifferent directions) and the spread and dispersion by cell instability or b reakdown first described by Csanady. There are, however, processes which co mpete with Lc to diffuse floating material. In shallow tidally mixed seas, where the environmental impact of an oil spill may be greatest, cross-wind dispersion caused by Lc will dominate over that produced by bottom turbulen ce if the ratio of the wind speed, W, to current, U,is sufficiently large. Observations and rough estimates suggest a transition near W/U = 15. A simp le model is devised to estimate cross-wind dispersion in shallow unstratifi ed waters when turbulence generated at a flat seabed dominates that produce d by Lc, but when the effects of Lc are still evident in aligning filaments of oil, as may commonly be the case in moderate winds in coastal or contin ental shelf waters. (C) 2001 Elseaer Science Ltd. All rights reserved.