THE GENERATION OF LOW-FREQUENCY WAVES BY A SINGLE WAVE GROUP INCIDENT ON A BEACH

Abstract

The generation of a single low-frequency wave (LFW) pulse by a single group of waves incident on a beach is investigated by means of laboratory experiments and a numerical model. This simplified case allows the LFW to be measured in isolation, after the incident group has passed and before there is any reflection from the wavemaker. A beach consisting of two different slopes (1:100 and 1:20) was used, and runs were made with the water level on each slope. The results were simulated using a composite numerical model, with Boussinesq equations in the deeper water and nonlinear shallow water equations in the surf zone. For some calculations, a friction term was included. For the 1:20 slope, the outgoing LFW is well predicted even without the friction term. With a 1:100 slope, a friction factor of 0.01 gave a good result, in this case reducing the amplitude of the outgoing LFW by a factor of about 2 compared with the frictionless result. The nondimensional equations show that the friction term is insignificant if the beach slope is large compared with the friction factor. Runs of the surf zone part of the model show the outgoing LFW to be correlated with the swash motion. Its amplitude is largest if the duration of the wave group is similar to the swash period of the largest wave in the group. The model also showed a slightly stronger than linear dependence of LFW amplitude on incident wave amplitude.