Wave—current interaction in the bottom boundary layer during storm and non-storm conditions: observations and model predictions

Abstract

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/ U 18 varied from below 0.2 to more than 7. Velocity profiles were highly logarithmic ( R 2 > 0.95) 60% of the time and 27 profiles collected at 2-h intervals had R 2 ⪖ 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/ U 18 ratio. Drag coefficients ( C D= τ c/ρU 100 2) ranged from about 2.5 × 10 −3−12 × 10 −3 in direct response to the wave-current variation; the use of a constant C D of3 × 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 (<1cm) 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.