Laboratory measurements of wave-induced near-bed velocity over a sloping natural sand beach

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Baryla, Andrew John
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Near-bed horizontal (cross-shore) and vertical velocity measurements over a sloping natural sand beach were acquired in the Hydraulics Research & Testing Facility (HRTF) wave flume. A probe was developed to measure the velocity field close to, but at a fixed distance from the dynamic sand bed. The data were acquired using a three-component acoustic Doppler velocimeter. Optical backscatterance sensors along with a wave staff provided simultaneous suspended sediment concentration and water level data. The near-bed velocity field is examined as close as 1.5 cm above a trough and crest of a ripple under three different types of wave forcing, namely, Stokes waves, Stokes groups, and irregular waves. Although both horizontal and vertical velocity measurements were made, attention is focused primarily on the vertical velocity. The results clearly indicate that the measured near-bed vertical velocity (which was outside the wave-bottom boundary layer) is distinctly non-zero and not well predicted by linear theory. Spectral and bispectral analysis indicate that the vertical velocity responds differently depending on the location over a ripple, and that ripple induced effects on the velocity field are present as high as 4 to 8 cm above the bed (for ripples with wavelengths on the order of 8 cm and amplitudes on the order of 2 cm). At greater heights above the bed the wave-induced motion predicted by linear theory begins to adequately compare with the measurements. Co-spectra etween the near-bed vertical velocity and suspended sediment concentration suggest that the vertical velocity may play a core central role in the transport of sediment than previously thought. These observations raise questions about experiments performed in oscillatory water tunnels, that do not reproduce the vertical velocity structure induced by waves.