Permanent displacement of composite breakwaters subject to wave impact

This paper is concerned with the permanent displacement of composite breakw aters under wave impact. It demonstrates that permanent displacement is a m ore rational criterion to design composite breakwater as well as to assess its performance when compared to merely a factor of safety. The horizontal wave force and uplift force acting on the caisson are determined from the G oda formulae and expressed as a fraction of the effective weight of caisson using the wave coefficients. A yield wave coefficient is defined for the h orizontal wave force when the limiting condition against direct sliding is reached. For a storm that renders a wave coefficient larger than this yield value, a rigid-body motion is induced in the caisson. The equation of moti on is double-integrated to obtain the magnitude of permanent displacement. The proposed procedure is illustrated with design example and compared with 35 case histories among which sliding was observed for 24 of them.