Temporal And Spatial Variations Of Surf-Zone Currents And Suspended-Sediment Concentration Elucidated From A Large-Scale Laboratory Study

Temporal and spatial variations of surf-zone currents and suspended-sediment concentrations were investigated at the U.S. Army Engineer Research and Development Center's Large-scale Sediment Transport Facility. A longshore-uniform fine sand beach, 35 m alongshore, 20 m across-shore, and 25 cm thick was created. Two unidirectional, long-crested irregular wave conditions, one resulted in spilling breakers and one in plunging breakers, were examined. Wave and current, and sediment concentration were measured at 20 and 16 Hz, respectively, at various longshore and cross-shore locations and throughout the water column. Longshore and cross-shore currents, and sediment concentration exhibit great and different temporal and spatial variations. Caution should be exercised when averaging these parameters over time and space. The temporal variations of longshore current were relatively small across most of the surf zone. Over 70% of the variations are within 60% of the mean value. The wave motion, with a peak at principal incident wave frequencies, dominated the temporal variations of cross-shore current. Temporal variations of sediment concentration under the irregular waves were episodic, characterized by occasional large values induced by suspension events or due to horizontal advection. The variance at the peak incident wave frequency was not significant except very near the bed. Time-averaged longshore-current profiles were logarithmic in shape below the wave trough. Depth-averaged longshore current matched well with the current measured at an elevation of 1/3 of the water depth from the bed. Time-averaged cross-shore current profiles were characterized by an onshore flow near the surface, and a balancing offshore flow near the bed. Sediment concentration decreased very rapidly upward through the water column across most of the surf zone except at the plunging breaker-line where relatively homogeneous concentration was measured throughout much of the water column above 4 cm from the bed. Depth-averaged sediment concentration over the range from 1 cm above the bed to the bottom of wave trough roughly equaled the concentration measured at an elevation from the bed equal to 20% of the water depth. A reasonable estimate of the time-averaged longshore sediment flux was obtained from the product of time- averaged profiles of longshore current and sediment concentration. Accurate estimates of cross-shore sediment flux, however, could not be obtained from the product of time-averaged current and concentration.