WAVE CURRENT INTERACTION IN THE BOTTOM BOUNDARY-LAYER DURING STORM AND NONSTORM CONDITIONS - OBSERVATIONS AND MODEL PREDICTIONS

Bottom boundary layer measurements of current velocity profiles and bed response under combined wave and current conditions were obtained at a water depth of 145 m on the shelf off central California during December 1988. High quality logarithmic current profiles, excellent time-series bottom photographs, and a large variation in the relative strengths of the wave-induced oscillatory currents and the quasi-steady low frequency currents provided a dataset that is ideal for examining the effects of wave-current interaction near a rough boundary. During one period of 3 days that included a brief storm event, the wave-induced bottom currents (U(b1/10)) ranged from 2.3 to 22 cm s-1 and the steady currents (U(r)) ranged from 1.8 to 28.1 cm s-1 at 0.18 m above the bottom; the ratio U(b)/U18 varied from below 0.2 to more than 7. Velocity profiles were highly logarithmic (R2 > 0.95) 60% of the time and 27 profiles collected at 2-h intervals had R2 greater-than-or-equal-to 0.994 which allowed reliable estimates of the current shear velocity (U*c) and roughness length (z(oc)). Mean U*c values had magnitudes of 0.3-2.4 cm s-1 and z(oc), which ranged from 0.04 to 3.5 cm, was strongly correlated to the U(b)/U18 ratio. Drag coefficients (C(D) = tau(c)/rhoU100(2)) ranged from about 2.5 x 10(-3)-12 x 10(-3) in direct response to the wave-current variation; the use of a constant C(D) of 3 x 10(-3) for steady flow over a rough bed would have underpredicted the shear stress by up to four times during the storm event. The large z(oc) and U*c values cannot be explained by changes in the carefully-observed, small (< 1 cm) physical bed roughness elements that covered the mud-rich study site. A side-scan sonar site survey also eliminated the possibility of flow disturbance by larger upstream topography. The observations clearly demonstrate the importance of wave-current interaction near a rough boundary. Comparison of the observations with results of the combined flow models of Grant and Madsen and Glenn show the models provide good predictions of U*c and z(oc) when the waves are characterized by either H-1/3 or H-1/10.