Observations of Long Period Waves in the Nearshore Zone

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Abstract
Field observation of waves in the nearshore zone were carried out in connection with several subjects, for example, shoaling and breaking deformation of waves, on-offshore and longshore sand transport studies, diffusion studies, and so on. A 16 mm memo-motion camera system, capacitance-type wave gages and electromagnetic current meters were used for the measurement of the waves and current. In the present paper, we focused our attention on an analysis of the resultant wave data for long period waves ranging from 0.08 Hz to 0.005 Hz in eight cases of seven observations. The results may be summarized as follows: 1) It was found more reasonable to interpret the observed long period waves as two-dimensional standing waves rather than three-dimensional edge waves. 2) It appears that the generation of the long period waves was not due to variations in the incident waves or radiation stresses. 3) Field data for the nodes and anti-nodes of the standing waves agreed fairly well with the solution given by Lamb for waves on a sloping beach, as modified for a composite beach consisting of two slopes. 4) A critical frequency was found which describes whether or not waves break on a slope. This parameter is also proportional to the angle of the beach face to the 3/4-power which is predicted by Miche's criterion of nonbreaking waves.

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