Large-scale laboratory observations of wave breaking turbulence over an evolving beach

Abstract

Wave breaking turbulence over an evolving beach was observed in a large-scale laboratory flume, as part of the CROss-Shore Sediment Transport EXperiment (CROSSTEX). The data set included comprehensive measurements of water surface elevation, fluid velocity, and morphology for irregular waves under erosive and accretive wave conditions. For the both conditions, the beach reached a quasi-equilibrium state, defined as when the bar shape was stable. Wave breaking characteristics, such as wave heights, average rate of energy dissipation by bores, and surf similarity parameter, were investigated in response to morphodynamics of the bar. Turbulent kinetic energy (TKE) was estimated using the method by Shaw and Trowbridge (2001). As the beach evolved, a less amount of TKE was observed at the trough for the erosive case, while more TKE was observed at the trough for the accretive case. It was also found that the temporal variation of the time-averaged TKE were closely associated with the average rate of energy dissipation by bores. Comparing with the bar trough, the vertical distribution of nondimensionalized time-averaged TKE and turbulence dissipation rate at the bar crest showed a large increase near the bottom, probably due to a strong cross-shore RMS velocity. Finally, in the quasi-equilibrium state, time-averaged TKE, and turbulence dissipation rate at the bar trough were smaller than those inside the surf zone. Inside the surf zone, significant turbulence intensities were observed due to a second breaking on a shallow water depth. Copyright 2010 by the American Geophysical Union.