Long wave generation by wave groups in the nearshore

This study investigates the forcing of leaky mode long wave by incident wave groups in the nearshore. Using both field and model data, two previously proposed generation models are evaluated: the bounded long wave of Longuet-Higgins and Stewart (1964), and the breakpoint-forced long wave of Symonds et al. (1982). New methods of parameterizing incident wave height modulations are proposed, including an amplitude time series and a clearly defined groupiness factor. Cross-correlations between observed amplitude and low frequency time series clearly document the release and shoreline reflection of a group-forced long wave. The shallow water amplification of the component identified as the bounded long wave is less than predicted by theory, but much larger than the shoaling of a free long wave. A time-variant numerical model is developed to further explain the observed cross-correlation signal, and to evaluate the relative importance of the two modes of long wave generation. The model simulates the forcing of leaky mode long waves by incident wave groups as they progress through the shoaling and breaking regions. New methods of modeling both short and long waves in the nearshore are proposed. Field observations are used to verify the model: the natural cross-correlation signal is exceedingly well predicted using model generated data. Simulated time series reveal that the modification of the cross-correlation signal toward shore is related to a fundamental change in bound wave dynamics, and not the addition of the breakpoint-forced wave as previously speculated. The relative importance of breakpoint-forced and bounded long waves is determined through a series of model tests with plane beaches and monochromatic wave groups. Consistent with field observations, the breakpoint-forced wave is secondary to the bounded long wave under almost all conditions, except at very low frequencies and with high beach slopes. Overall, model predictions indicate that the directly group-forced component accounts for approximately half the total long wave height found under field conditions. This is consistent with previous observations that both leaky and edge wave modes are energetic on natural beaches