Two-Dimensional Surf Beat

Abstract

Surface elevation and velocity measurements in shallow water on beaches show large fluctuations known as surf beat in the long period range from approximately 20s to 2000s (Munk, 1949; Tucker, 1950). A laboratory study was conducted to determine if two shoaling primary waves of nearly equal frequency would generate two-dimensional surf beat at their difference frequency. The experiments, carried out in the Scripps Institution of Oceanography Hydraulics Laboratory 30 m glass walled wave channel, show that the beat frequency motion in the channel consists of the sum of a forced progressive wave and two free standing waves. The progressive wave is forced by the local nonlinear interaction of the primary waves and grows sharply in shallow water. One of the free standing waves is generated directly by the wavemaker. The data is consistent with the hypotheses that the second, much larger, free wave is generated in shallow water as the reflection of the long, forced progressive wave, leading to the observed standing wave surf beat pattern.

Measurements of Surf Beat and Set-Down Beneath Wave Groups

“…Secondly, disturbances occur when reflected long waves are re-reflected from the paddle. Flick et al (1980) show that for critical frequencies this may even result in resonance, which makes model testing on surf beat phenomena virtually impossible. In the following sections methods to prevent both types of disturbances are discussed.…”

Swash on Steep and Shallow Beaches

“…In contrast to swash at wind wave frequencies, there axe no comprehensive laboratory experiments which provide suggestions about the nondimensional parameters controlling the magnitude of swash oscillations at surf beat frequencies. Laboratory experiments have been hampered by both the generation of spurious free long waves (Bowers, 1977) and multiple reflections between the beach and wave generator [Flick et al, 1981], Thus, although laboratory measurements with nonmonochromatic incident waves do exist, the swash motions at surf beat frequencies are contaminated to an unknown degree. Work presented at the 19th ICCE (Kostense and Vis) describes the first variable depth experiments apparently free of both paddle generated free long waves and long waves re-reflected from the wavemaker.…”