Variations of wave parameter statistics as influenced by water depth in coastal and inner shelf areas

Abstract

Wave height, pressure and orbital velocity statistics, as influenced by the factor of water depth, are analyzed on the basis of five data sets collected in situ from two intertidal sites with a mean water depth of 0.8–2.7 m and three inner shelf sites (water depths 14.6–27.6 m). Acoustic Doppler Velocimeters (ADVs) were used to measure instantaneous wave pressure and current velocity at 0.20–0.35 m above sea bed. The zero-crossing wave analysis method was applied to analyze the wave data to determine the wave pressure amplitude P and orbital velocity amplitude U of individual waves, together with the frequency of occurrence distributions of P and U, which are compared with the three commonly-used, classic Rayleigh, modified Rayleigh and Weibull distributions. It is found that the distribution of P is almost identical to that of U for each of the five study sites. The distributions of P and U for the intertidal flat sites fit the Weibull distribution better than the classic or modified Rayleigh ones, indicating a shallowness effect which is associated with finite-banded wind-wave spectra and possible wave breaking or near-bed turbulence. On the other hand, the distributions of P and U measured at the three inner shelf sites agree almost equally with the three distributions. With increasing water depth, the distributions of P and U are shown to reduce from the two-parameter Weibull distribution at the shallow intertidal sites to the classic Rayleigh at the deep-water sites. For coastal engineering applications, an empirical formula is proposed to estimate more accurately the significant wave height in intermediate coastal waters based on the surface water elevation data.