The action of a surface current used as a breakwater

Abstract

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.