AN EXPERIMENTAL STUDY OF LOW-FREQUENCY WAVES GENERATED BY RANDOM GRAVITY WAVES IN SHOALING WATER

A laboratory study of the nonlinearity of random waves (the primary waves) and low-frequency waves induced by the primary waves on inclined beaches is presented. The experiments were carried out in a wave flume that is 65 m long, 1.2 m wide with a water depth of 0.7 m. Three plane beaches with different slopes (1/20, 1/30 and 1/40) were used in separated experiments. Incident primary waves with a Person-Moskowitz (PM) spectrum were mechanically generated with significant wave height ranging from 0.047 to 0.125 m. The time series of surface elevations at different water depths along the beach were simultaneously recorded. Low-frequency waves are obtained from the measured wave data with a low-pass filter. The results show that the primary waves on the beach are highly nonlinear. The surface skewness and kurtosis, two statistical measures of wave nonlinearity, are functions of a nondimensional parameter H-S/d, where H-s is the local significant wave height and d is the water depth. The spectra of low-frequency waves in shoaling region are affected by beach slope and the energy of incident primary waves. The energy ratio between low-frequency waves and primary waves strongly. correlates to the local surface skewness. The growth (dissipation) rate of low-frequency waves on beaches is controlled by the Iribarren number, xi = beta/(H-0/L-0)(1/2), where beta is the beach slope, H-0 and L-0 are the significant wave height and wavelength of incident primary waves, respectively.