Measurements of Surf Beat and Set-Down Beneath Wave Groups

A laboratory study was conducted to measure the amplitudes of long waves in shallow water as induced by wave grouping. In a 55 m long wave channel with a plane beach at the end, two primary waves of nearly equal frequency were generated. Due to a sophisticated control of the wave paddle — including second order wave generation as well as active wave absorption at the paddle face — the wave action at the difference frequency was limited to an incident forced wave, propagating at the group velocity, and a reflected free wave generated in the surf zone. For the incident forced — or bound — wave, also known as set-down, the experimental results show good agreement with the existing theory. Furthermore, the experiments confirm qualitatively a theoretical model by Symonds et al. (1982) explaining two-dimensional surf beat as a result of the time-varying breakpoint of the incident primary waves.