Experimental observations of edge waves

Abstract

The main purpose of this paper is to provide some carefully documented experimental observations of the subharmonic generation of edge waves over a plane beach by waves normally incident on the beach from the distant ocean. In order to establish experimentally the details of the subharmonic instability mechanism, it is important first to determine the properties of the primary wave field whose stability is to be investigated. Thus, a detailed appraisal has been made of the wave field established in the tank and over the beach in the absence of edge waves. These data have been particularly useful in defining the amplitudes of the incident- and reflected-wave fields at the toe of the beach, the magnitudes of which are central to the triggering of the edge-wave instability. Details are presented of the edge-wave field over the beach, as well as marginal stability curves and initial growth rates of the instability, the latter two of which are compared with theoretical estimates obtained from extant theories of the instability mechanism. Some experiments are also described in which edge waves were established as modes forced by topographical imperfections at the beach.