Field observations and predictions of bed shear stresses and vertical suspended sediment concentration profiles in wave-current conditions

During a study of sediment dynamics at an offshore field site adjacent to M iddlekerke Bank, Belgium, high-frequency measurements of turbulence and ver tical profiles of the time-averaged suspended sediment concentration, (C) o ver bar, were obtained in the bottom 1.2 m of the water column above a ripp led bed in a water depth of approximately 20 m using the autonomous multise nsor instrument STABLE. During the experiment, a combination of large waves and strong currents resulted in the resuspension and transport of bottom s ediments. 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, <(tau)over bar>(c(tke)), and the peak wave-only bed she ar stress, <(tau)over cap>(w), have been obtained using the turbulent kinet ic energy (tke) method and linear wave theory, respectively, and have been combined to obtain peak, <(tau)over cap>(wc), and time-averaged, <(tau)over bar>(wc), wave-current (w-c) bed shear stress values for gain- and ripple- scale roughness using existing models. A new semi-empirical expression givi ng accurate prediction of measured vertical C profiles for a wide range of w-c conditions has been derived. Using <(tau)over cap>(w), <(tau)over bar>( 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 (C) over bar profiles, and hence suspended sediment transp ort rates, with knowledge of flow turbulence, wave orbital motion and C mea sured accurately at only one location near the bed. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.