Observations of momentum advection due to secondary circulation in a curved tidal channel

Abstract

ABSTRACT Bottom friction causes a vertical shear in the water column resulting in secondary circulation causing a helical flow pattern around curved open channels. Secondary circulation has cross-stream and vertical components. Current models for secondary circulation include ‘non-linear’ advection terms in the momentum equations. Advection due to secondary circulation deforms the vertical profile of the primary flow which would normally be logarithmic in shape. A consequence of this is that the vertical shear is reduced which in turn reduces the driving force of secondary flow. Moving vessel Acoustic Doppler Current Profiler (ADCP) measurements from a curved tidal channel of the Otago Harbour show the radially outward movement of the cross-channel velocity maxima as flow progresses around the bend. This outward movement is due to advection of momentum by secondary circulation and is contrasted between ebb and flood tide. This radially outwards movement is approximately 40 m or 10% of the channel width. These measurements are of a much larger flow regime, , than previous works so this increase in scale makes these observations robust to the effects of measurement noise.