Breaker turbulence and sediment suspension in the surf zone

摘要

Field measurements of fluid velocities and suspended sediment concentrations were used to investigate the relative role of coherent vortices (related to wave breaking) in suspended sediment dynamics. The measurements were obtained from a barred (intermediate-type) beach and the instrument rig was exposed to a range of wave types during the experiment; including shoaling waves and both spilling and plunging breakers. The time scale of the observed breaker vortices was < < 5 Hz and thus in the frequency range of wave orbital motion. Coherent breaker vortices were separated from wave orbital motions and incoherent higher-frequency turbulence using a threshold magnitude for the vertical velocity component. Plunging breakers displayed the largest root-mean-square (RMS) and maximum vertical velocities; the latter reaching approximately 1 m s~(-1). These measurements were made at an elevation of 0.15 m above the bed. Given the observed scale of the breaker vortices and the phase match between suspended sediment concentrations and fluid velocities, the turbulence injected into the water column by the plunging breakers played a significant role in suspending bed sediment. On the other hand the role of coherent vortex structures in suspending sediment under spilling breakers and shoaling waves is more limited. RMS vertical velocities under the spilling breakers and shoaling waves in this experiment were respectively a factor 2 and 4 smaller than for plunging breakers. Different phase relationships were observed between the horizontal fluid velocity and vortex-induced sediment suspension for each of the wave types, which may contribute to an explanation of net offshore and onshore sediment transport observed during episodes of beach erosion and recovery, respectively.