Abstract
During a study of sediment dynamics at an offshore field site adjacent to Middlekerke Bank, Belgium, high-frequency measurements of turbulence and vertical profiles of the time-averaged suspended sediment concentration, C̄, were obtained in the bottom 1.2 m of the water column above a rippled bed in a water depth of approximately 20 m using the autonomous multisensor instrument STABLE. During the experiment, a combination of large waves and strong currents resulted in the resuspension and transport of bottom sediments. Values for the physical roughness of the sea bed, k s , have been derived. Estimates of the bed shear stress attributable to currents in the presence of waves, τ ̄ c(tke) , and the peak wave-only bed shear stress, τ ̂ w , have been obtained using the turbulent kinetic energy ( tke) method and linear wave theory, respectively, and have been combined to obtain peak, τ ̂ wc , and time-averaged, τ ̄ wc , wave-current ( w– c) bed shear stress values for gain- and ripple-scale roughness using existing models. A new semi-empirical expression giving accurate prediction of measured vertical C̄ profiles for a wide range of w– c conditions has been derived. Using τ ̂ w , τ ̄ wc and k s values as input parameters to the expression, estimates of the dynamic in situ grain settling velocity, wave mixing coefficient and total diffusive bed shear stress that agree well with previous measurements and with theory have been obtained. Results indicate it may now be possible to predict vertical C̄ profiles, and hence suspended sediment transport rates, with knowledge of flow turbulence, wave orbital motion and C̄ measured accurately at only one location near the bed.
Published Version
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