MEASURING AND PREDICTING SUSPENDED SEDIMENT CONCENTRATION PROFILES IN THE SURF ZONE UNDER SPILLING AND PLUNGING BREAKERS

Surf zone provides one of the most active areas for sediment suspension, especially that of sand-sized grains. Measuring and predicting this active sand suspension is crucial to the quantification of coastal morphodynamics. The relatively fast fall speed of sand grains is countered by the intense turbulence generated by wave breaking. Surf-zone sediment suspension is influenced by many factors, including: sediment properties, bottom shear stress, turbulence intensity, water depth, beach slope, wave-induced orbital velocity, and surf-zone currents. In this study, sediment suspension was investigated in the 3-D Large-scale Sediment Transport Facility at the U.S. Army Engineer Research and Development Center. Two irregular, long-crested unidirectional wave conditions, one that resulted in spilling breakers and one in plunging breakers, were generated. Sediment-concentration profiles were measured at several locations across the surf zone along with measurements of wave, current, and bottom profiles. The objectives of this study are to 1) examine the characteristics of time-averaged suspended sediment concentration profiles; 2) evaluate some of the existing predictive models; and 3) toward developing an improved concentration-profile model.

Sediment concentration decreases rapidly, over nearly four orders of magnitude, upward through the water column across most of the surf zone. An exception occurred at the breaker-line for the plunging case, where relatively homogeneous concentration was measured throughout the water column above 4 cm from the bed. Nearly identical sediment concentrations were measured within 3 cm from the bed near the breaker-line for both the spilling and plunging cases, despite the significant differences in bed conditions, near-bottom velocities, and turbulence intensity. While high in the water column, approximately one order of magnitude greater sediment concentration was measured at the plunging breaker line than at the spilling breaker line. Across most of the surf-bore dominated mid-surf zone, suspended-sediment concentrations were rather similar for both the plunging and spilling cases. A new model incorporating the turbulent energy generated by the different forms of wave breaking was developed and tested based on the accurate laboratory data.