Spatiotemporal Characteristics of Near-Bed Pressure Gradients on a Barred Beach

Abstract

Observations of the cross-shore variations of pressure gradients measured on a fixed barred beach in a large-scale laboratory wave flume are presented in this paper. The wave-induced components (0.1-1.0 Hz) of the free surface elevation, near-bottom vertical and cross-shore velocities, and pressure gradient are used in the analysis. The cross-shore variation of the pressure gradient showed the maximum value in the area of wave breaking over the bar, and the pressure gradients were considered to be influenced by the fluctuation of the water surface elevation, which are shown to be a rough proxy for the water surface slope. The cross correlation related to the vertical velocity showed no correlation in the wave breaking area. Surprisingly, although several measuring points were located in the wave breaking area or in the inner surf zone, the spatial variation of the pressure gradient and water surface elevation had a correlation coefficient greater than 0.8 with time lags approximately 0.1 of the wave period from offshore to onshore in both regular and irregular wave cases. This indicates that the near-bottom pressure gradient can be estimated using the water surface elevation over a complex barred bathymetry and through the inner surf zone.