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Michael Li, L. Wright and Carl Amos (1996)

Predicting ripple roughness and sand resuspension under combined flows in a shoreface environment

Marine Geology, 130(1):139-161.

Ripple measurements and flow and sediment dynamical data obtained from the shoreface of the Middle Atlantic Bight using instrumented tripods were analyzed to evaluate various predictors of ripple geometry and roughness. Ripple roughness controls on sand resuspension and suspended sediment concentration profiles under combined waves and currents were also evaluated. The limited observation of sand ripples in the field indicates that the Grant and Madsen (1982) method overestimates ripple roughness, while the Nielsen (1981) method tends to under-predict ripple roughness. A modified ripple predictor is thus proposed based on the Grant and Madsen method, and it is shown to give reasonable predictions under the present experiment conditions. This modified ripple prediction along with wave, current and suspended sediment concentration data recorded by the tripods were then brought into the combined-flow bottom boundary layer model of Grant and Madsen (1986) and the modified Rouse equation of Glenn and Grant (1987) to calculate sand resuspension coefficient γ0 and to predict suspended sediment concentration profiles. It was found that under low-energy fair-weather conditions, sand ripples are in the equilibrium range and ripple roughness increases with the bed shear stress. This causes strong vortex activity close to seabed and thus higher resuspension coefficient. Reference concentrations are moderate due to this high resuspension coefficient, even though bed shear stresses are low. Under moderate storm conditions, ripple break off occurs and ripple roughness will decrease with bed shear stress. This reduces the vortex activity and hence sand resuspension coefficient γ0. The combination of this moderately high bed shear stress and reduced but still moderate ripple roughness favours sand suspension and produces the highest reference concentration for the encountered experimental conditions. As bed shear stress is further increased, ripples are nearly washed out and sand resuspension coefficient is further decreased approaching the previously-suggested constant value of 1.3 × 10−4. This corresponds with the lowest reference concentration despite the high bed shear stress. Suspended sediment concentrations predicted by the modified Rouse equation using this time variable resuspension coefficient and properly calculated bottom boundary layer parameters are reasonable compared to the field measured concentration profiles.



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