Wave-current interaction in the bottom boundary layer during storm and non-storm 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 (Ub 1 10) ranged from 2.3 to 22 cm s-1 and the steady currents (Ur) ranged from 1.8 to 28.1 cm s-1 at 0.18 m above the bottom; the ratio Ub 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 {slanted equal to or greater-than} 0.994 which allowed reliable estimates of the current shear velocity (U*c) and roughness length (zoc). Mean U*c values had magnitudes of 0.3-2.4 cm s-1 and zoc, which ranged from 0.04 to 3.5 cm, was strongly correlated to the Ub U18 ratio. Drag coefficients (CD = ??c/??U1002) ranged from about 2.5 ?? 10-3-12 ?? 10-3 in direct response to the wave-current variation; the use of a constant CD of 3 ?? 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 zoc 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 zoc when the waves are characterized by either H 1 3 or H 1 10.