Experimental studies of the refraction of uniform wave trains and transient wave groups near a straight caustic

An experimental study of gravity wave behavior near a straight caustic was carried out in a wave tank that was 4.26 meters (14 feet) square and 30.48 centimeters (1 foot) deep. A uniform wave train was generated in shallow water of constant depth and propagated toward a portion of the tank where the depth increased linearly to a value equivalent to deep water. The straight line at which the depth started to increase was at 50° to the wave crests at the generation source. The waves propagated up to this line, then refracted, totally reflected, and turned back toward the shallow water. The results of the experiment show that the experimental model not only successfully produces a caustic but also verifies the theoretical results near the caustic derived by Chao (1971) within the framework of a linear theory. The same bottom model was used to examine the refraction of transient wave groups. The transients were recorded at several places in the tank in such a way as to compare two alternative hypotheses: one that the waves refracted according to refraction equations involving their spectral phase speeds, and the other that group velocities were involved. The measurements show that the wave groups were refracted according to the phase speeds of the spectral components.