The action of a surface current used as a breakwater

The conditions under which an outward-flowing surface current can prevent the passage of waves coming in from the sea are investigated mathematically. Two types of current are considered: (a) a current with uniform velocity extending to a depth h; (b) a current with velocity decreasing uniformly and vanishing at depth h. They have very similar effects. The mean velocity required to stop waves of given frequency is rather greater in case (a) than in case (b). The water current produced by a curtain of air bubbles from a perforated tube on the sea bottom is investigated theoretically on the assumption that the bubbles are very small. Evans (1955) has measured the surface currents produced in a tank by a bubble curtain and finds them smaller than predicted. The discrepancy is partly due to the fact that the bubbles were not very small.