Acoustic Doppler current profiler measurements of the velocity field of an ebb tidal jet

Abstract

A broadband acoustic Doppler current profiler was used to measure the velocity field of the ebb tidal jet formed at the entrance to the Otago Harbour, South Island, New Zealand. Four separate surveys were conducted where various sets of transects were run continuously over a 13 hour period, producing time series of velocity profiles with 1 hour sampling over a grid with an across-jet resolution of 20 m and an along-jet resolution of 150 m. The velocity field of the ebb tidal jet is phase locked to the tidal response of the harbor, making it possible to combine the data from separate tidal cycle surveys to produce synoptic maps of the depth-averaged velocity field. The spatial structure of the ebb tidal jet at the time of peak flow exhibits many features of a classical plane jet. There is a strong asymmetry between the ebb and flood flow structures; the ebb flow extends beyond 2 km from the entrance, while the flood flow is limited to within 600 m of the coast. The flow along the jet axis becomes pulse-like in time with distance from the entrance. The pulse that develops propagates way from the entrance at a speed slower than the advective speed of the ebb jet. The formation of a propagating eddy observed along the eastern side of the jet suggests that this momentum pulse may be associated with the pair of separation eddies generated by the accelerating ebb tidal flow. Copyright 2001 by the American Geophysical Union.