Trajectories and Speeds of Wind-Driven Currents Wear the Coast

Abstract

Detailed observation of drogue movements within 800 m of a straight shoreline indicates the primary current generated by local winds to be directed within a few degrees of parallel to the shore nearly independently of wind direction. Subtle vertical structure in the onshore-offshore speed component is dependent on vertical stratification such that unstratified water produces a two-layered flow (onshore in the surface layer, offshore in the bottom layer), and moderately stratified water produces a three-layered flow (onshore in surface and bottom layers, offshore at intermediate depths). Theoretical conclusions from Jeffreys' constant eddy viscosity theory support the unstratified velocity profile and accurately predict the alongshore current speeds. Numerical solutions with a depth-dependent eddy viscosity indicate that the three-layered flow pattern is a direct result of the density gradient. Even in these shallow waters the inclusion of Coriolis effects in the theory is necessary for a complete understanding of the current observations. Simple theoretical calculations on the response characteristics of various sized surface-tracked drogues as a function of wind speed indicate that drogue size should be carefully selected in terms of expected magnitudes of wind and current speeds.