High-resolution observations of the intensity of secondary circulation along a curved tidal channel

High horizontal resolution moving vessel acoustic Doppler current profiler (ADCP) observations of the spatial pattern of cross-stream velocities in a curved tidal channel show radially outward surface velocities up to 10 cm s^-1 which are maximum midchannel, consistent with helical secondary flow in a vertical plane normal to the depth-averaged velocity. The 30-m-cross- and 150-m-along-channel resolution observations are from a 2700-m-long section of a 350-m-wide horizontally and vertically well mixed tidal channel with a radius of curvature 1-5 km. The along-channel resolution allows the intensity of the curvature induced secondary flow to be estimated from the linear correlation between the observed cross-channel component of vertical shear and the shear estimated from the streamwise velocity and its varying curvature using an existing analytic model. The two shears are highly correlated and the regression line slope demonstrates that the observed curvature induced secondary flow is 30% more intense than that predicted by the model for a typical bottom drag coefficient. The secondary flow is 50% more intense than that predicted using the drag coefficient which best fits the streamwise velocity profile. Numerical solutions demonstrate that the intensity of the secondary flow is sensitive to small changes in the shape of the eddy viscosity profile; hence intensity may be sensitive to the way turbulence is modeled. Lagged correlation of the observations showed that the secondary flow adapts to changes in curvature and primary flow over a 300-m length scale, or 20 water depths, consistent with the existing model and laboratory studies.