Entrained air in bore-driven swash on an impermeable rough slope

Abstract

The characteristics of entrained air present in the swash flow are studied through a series of laboratory experiments. A single, well-defined, highly repeatable, large-scale bore was generated by using a dam-break mechanism. The resulting plunging breaker generated a fully turbulent bore that travelled up and down a 1:10 impermeable coarse sand slope. For the bore-driven swash event, first the hydrodynamic properties were determined through a combined PIV-LIF system, yielding the ensemble-averaged flow depths, bed-parallel and bed normal velocities and turbulence statistics. Secondly, fibre optical probes were used to obtain the ensemble-averaged void fractions, bubble sizes and bubble velocities which were processed to investigate quantitatively the bubble clouds on the slope. In addition, detailed observations of the bubble clouds were obtained using a Bubble Image Velocimetry system. The results yielded for the first time a comprehensive and very accurate dataset of the hydrodynamics of and the entrained air in the highly unsteady and non-uniform swash flow. Analysis of the results firstly gave new insights into the hydrodynamics of the flow. Fluctuations in the free surface are present throughout the swash event and these directly affect the bed-normal velocities. In addition, flow near the initial shoreline is affected by the change in direction of the flow from the surf-zone into the swash-zone and in the backwash acceleration is reduced due to the water present in the surf-zone delaying the time when the flow becomes super-critical. Observations of the BIV images revealed that near the shoreline most of the entrained air is present in two compact bubble clouds that are transported with the flow into the swash-zone. The maximum void fraction of the first bubble cloud is approximately 0.20 and that of the second bubble cloud is 0.18 and up to 6mm and 4.7mm of the flow depth are attributable to air for the two bubble clouds respectively. Further shoreward these values decrease rapidly as the majority of air entrained in the flow at the initial shoreline location has escaped the flow due to buoyancy. The locally entrained air in the first bubble cloud results in large air cavities, with individual bubble diameters up to 20mm and ensemble-averaged velocities up to 4m/s, which develop rapidly into multiple smaller bubbles. The bubbles in the second cloud are of more homogeneous size with ensemble-averaged diameter 1.6mm, but have velocities that are smaller than that of the mean flow indicating that they are not only controlled by the mean flow but are also significantly affected by turbulence. The energy dissipation attributable to the entrained air is around 1% of the total energy dissipation when considering the whole swash event or 3% if only the duration of the swash event is used when significant amounts of air are present. In comparison with breaking waves in the surf-zone and deepwater, the effect of the entrained air in the swash zone may be small. The maximum void fractions and bubble size distributions are similar, but the volume of entrained air and energy dissipation attributable to entrained air are much smaller because a significant amount of air entrained during the wave breaking process has escaped the flow before it reaches the swash-zone.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology