A Field Study of Waves in the Surf Zone

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Abstract
As a method of measuring waves in the surf zone, a remote sensing photographic technique was developed which utilizes synchronized 16 mm memo-motion cameras directed at poles installed in the nearshore zone. Results using this method are reported for a field observation carried out on a gently sloping beach. Wave data were obtained at forty points in the surf zone extending from near the swash zone to outside the breaking point. Principal results from analysis of the field data are: 1) The skewness and kurtosis of the sea water surface elevation exhibited systematic changes in the nearshore zone and were found to be useful parameters characterizing wave non-linearity. 2) The wave height distribution as a function of time in the surf zone was found to depend on the defining procedure used, either the zero-up or zero-down crossing method. For example, the wave height distribution defined by the zero- down crossing method became bi-modal in the neighborhood of the breaking point. However, distributions for statistically representative waves were found to be essentially independent of the defining procedure. 3) The ratio of significant wave height to water depth was almost constant in the surf zone (except in the vicinity of a depression in the sea bottom) and equal to 0.73, which is the maximum value for the solitary wave. 4) The power spectrum of the water surface elevation in shallow water was found to be different from that observed in deep water. The phase velocity estimated by a cross spectral analysis agreed well with that predicted by the solitary wave theory.

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