Influence of infiltration on suspended sediment under waves

Abstract

The effect of percolation through a permeable bed on sediment suspension under regular waves was examined in a laboratory wave tank (28 m×1 m×1 m), using acoustic backscatter sensors to make rapid (3 Hz) suspended sand profile measurements (0.005 m vertical resolution). Waves of 1.7 s period and heights ranging from 0.14 to 0.185 m were used over sand with a D 50 of 255 μm. Infiltration velocities of 0–5.0×10 −4 m s −1 were used. With percolation through an initially flat bed, ripple development was suppressed, particularly at lower wave heights; ripples took longer to form and were more three-dimensional. Suspension was also suppressed. The total suspended load was correlated with Shields number (at the 1% significance level) when the Shields number was modified to take account of both the infiltration [Nielsen, P., 1997. Coastal groundwater dynamics. Proceedings of Coastal Dynamics, American Society of Civil Engineers, pp. 546–555] and ripple steepness [Coastal Eng. (1986) 23]. The ripple steepness was found to be the most important factor relating to the reduction in the total suspended loads. The influence of infiltration on time-averaged concentration profiles over equilibrium ripples was investigated by switching the percolation on and off for 5-min periods. The total suspended loads were reduced by up to 50% with percolation on. These results suggest that fluctuation of the water table and drainage within a beach will affect sediment transport and ripple dynamics, and that for sand of 0.25 mm, percolation will tend to reduce suspension and transport.