Spectral analysis applied to the study of time series from the beach environment

Abstract

Spectral and cross-spectral analysis have been applied to two time series of beach data, published by ESSA (Harrison et al., 1968). The first series consisted of seven variables recorded at four-hourly intervals (with hourly value interpolated) and the second of four variables, recorded at high and low tide. In the first set, peaks in the spectrum at the semidiurnal tidal period were prominent for beach slope and breaker height, and in the second set for longshore current velocity, beach slope and sand size. Cross-spectral analysis gave coherence values for all possible pairs of variables; four notably high values occurred in the first set and three in the second. The results of the analysis are summarized in Tables I, II, III and IV which demonstrate the importance of the semidiurnal and diurnal tidal cycle in both process variables and the beach slope response variable. A strong coherence between beach slope, sand size and longshore current velocity shows that they are all affected by the tidal cycle.