Temporal and spatial variations of surf-zone currents and suspended sediment concentration

Abstract

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 (LSTF). A longshore-uniform fine-sand beach, 35 m alongshore, 20 m cross-shore, and 25 cm thick was placed in the facility for these experiments. Two unidirectional, long-crested irregular wave conditions were examined, one resulted in predominantly spilling breakers and one in plunging breakers. Waves and currents, and sediment concentrations were measured at 20 and 16 Hz, respectively, at various longshore and cross-shore locations and throughout the water column. Both currents and sediment concentrations exhibit great temporal and spatial variations in the surf zone. The variation patterns, however, of the longshore current, cross-shore current, and sediment concentration are substantially different. Caution should be exercised when averaging these parameters over time and space. For the two wave cases examined, the temporal variations of longshore current, including those at principal incident-wave frequencies, were relatively small across most of the surf zone. Over 70% of the variations are within approximately ±60% of the mean value. The wave motion, with a strong peak at principal incident-wave frequencies, dominated the temporal variations of cross-shore current. Temporal variations of suspended sediment concentration under the irregular waves were episodic, characterized by occasional large values induced by suspension events or due to horizontal advection. The variance of the concentration at the peak incident-wave frequency was not significant except very near the bed. Time-averaged longshore-current profiles over the predominantly rippled sand bed were logarithmic in shape below the wave trough. Depth-averaged longshore current (excluding the portion of water column above wave trough) 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 mass flux near the surface, and a balancing offshore flow below the wave-trough level (undertow). 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 still-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 time-averaged cross-shore sediment flux, however, could not be obtained from the product of time-averaged current and concentration.