This paper describes detailed measurements and analysis of the time-varying distribution of void fractions in three different breaking waves under laboratory conditions. The measurements were made with highly sensitive optical fibre phase detection probes and document the rapid spatial and temporal evolutions of both the bubble plume generated beneath the free surface and the splashes above. Integral properties of the measured void fraction fields reveal a remarkable degree of similarity between characteristics of the two-phase flow in different breaker types as they evolve with time. Depending on the breaker type, the energy expended in entraining air and generating splash accounts for a minimum of between 6.5 and 14% of the total energy dissipated during wave breaking.
The spatial and temporal variation of energy dissipation rates in breaking waves controls the mean circulation of the surf zone. As this circulation plays an important role in the morphodynamics of beaches, it is vital to develop better understanding of the energy dissipation processes in breaking and broken waves. In this paper, we present the first direct field measurements of roller geometry extracted from a LiDAR data set of broken waves to obtain new insights into wave energy dissipation in the inner surf zone. We use a roller model to show that most existing roller area formulations in the literature lead to considerable overestimation of the wave energy dissipation, which is found to be close to, but smaller than, the energy dissipation in a hydraulic jump of the same height. The role of the roller density is also investigated, and we propose that it should be incorporated into modified roller area formulations until better knowledge of the roller area and its link with the mean roller density is acquired. Finally, using previously published results from deepwater wave breaking studies, we propose a scaling law for energy dissipation in the inner surf zone, which achieves satisfactory results at both the time‐averaged and wave‐by‐wave scales.
Following the rapid and destructive impacts of storm erosion, beach recovery is a key natural process of restoration, returning eroded sediment to the subaerial beach and rebuilding coastal morphology. While the effects of storm erosion have commonly been investigated, detailed studies into poststorm recovery are currently lacking. This study investigates wave‐driven recovery processes of the berm and beachface on a microtidal, swash‐aligned sandy beach. Following complete removal of the berm by a significant storm event, the entire 76‐day rebuilding of a swash berm is analyzed at the timescale of every semidiurnal tidal cycle, utilizing high‐resolution (5 Hz) swash and subaerial beach profile measurements from a continuously scanning fixed lidar. Tide‐by‐tide rates of subaerial volume change during berm recovery were most frequently observed between 1 and 2 m3/m/day, including losses and gains an order of magnitude larger than the more gradual rate of net gain (0.7 m3/m/day) observed for the entire recovery period. Patterns of berm crest formation and vertical growth were found to be primarily governed by the neap‐spring tide variations in total water levels. Tide‐by‐tide beachface and berm volume changes were used to classify four principal behavioral modes of subaerial profile variability during recovery. Using decision tree classification, modes were differentiated according to nearshore dimensionless fall velocity, swash exceedance of the berm crest, and ocean water levels. The findings provide novel behavioral and parametric insight into the tide‐by‐tide rebuilding of the beachface and berm by swash throughout a complete poststorm recovery period.
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