Early theoretical investigation of the effect of wind stress on coastal warers were performed by Ekman (1905) and later by Jeffries (1923). It was recognized that the well-known Ekman spiral type of solution obtained for the open ocean, in which surface current is inclined to the wind at 45° towards the high pressure side, must be modified by the presence of a coastline which prevents any resultant drift developing across vertical sections parallel to the coast. For example, according to the deep coastal water solutions obtained by Jeffries, hurricane force winds blowing alongshore produce a surface drift which is inclined at an angle of 18° to the wind- a smaller inclination than that produced in the open ocean. However, for hurricanes meeting the coast at right angles, the surface drift and wind are again inclined at 45°. Since the presence of the coastline does not affect the depth averaged drift. For more commonly encountered wind speeds (of, say, ~10 ms−1), the mathematics indicate a surface response which follows the direction of the coastline for a wide range of wind vectors. This result is of particular importance for the present chapter. The theory also indicates that the local response time under changing wind conditions has a predicted dependence on the square of the distance from the shoreline. For distances from the shoreline of relevance here (2–40 km), a steady state can develop within time scales of =1 hour, which is much shorter than typical response times for the open ocean.

Computer modeling of the dynamics of selected oceanic regions, such as the Irish Sea (e.g. Proctor, 1981), has greatly improved out insight into wind-driven surface currents, but relatively few detailed attempts to compare theory or modeling predictions with experiment have been made, mainly because true surface currents are difficult to measure. An interesting study along these lines was made by Murray (1975), who focused on a comparison between predictions for near-shore wind-driven currents and flows determined by tracking drogues (using a theodolite technique) to within a distance of 800 m of a straight coast this work indicated that currents generated by local winds are directed parallel to the shoreline to within a few degrees, nearly independent of wind direction or speed, in agreement with the results of analytic theory. However, current speed was found to be strongly controlled by the wind angle to the shoreline and, to a lesser degree, by wind speed.

The drogue tracking technique employed by Murray (1975) is best suited to operations in restriction areas of water close to coast. It is difficult, however, to get a truly synoptic view of currents in more extensive regions using drogues, since strong biasing of the data in time and/or space is normally produced. Ideally, a constant throughput of drogues, evenly distributed across the region under study, is required, but this approach poses severe logistic constraints in all but the narrowest strips of coastal water. Current meters give useful data at selected points, but are less reliable in providing information near the surface.

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