Using a broadband ADCP in a tidal channel. Part I: Mean flow and shear

This paper discusses the principles of measuring the mean velocity and its vertical shear in a turbulent Row using an acoustic Doppler current profile r (ADCP), and presents an analysis of data gathered in a tidal channel. The assumption of horizontal homogeneity of the first moments is fundamental t o the derivation of the mean velocity vector because the velocity is never homogeneous over the span of the beams in a turbulent Row. Two tests of thi s assumption are developed-a comparison of the mean error velocity against its standard deviation and against the mean speed. The fraction of the samp les that pass these tests increases with increasing spatial averaging and e xceeds 95% for distances longer than 55 beam separations. The statistical u ncertainty of the velocity and shear vector, averaged over 10 min and longe r, stems from turbulent fluctuations rather than Doppler noise. Estimation of the vertical velocity requires a correction for the bias in the measured tilt. The mean velocity and shear estimates from this natural tidal channel show more complex depth-time variations than found in idealized one-dimensional channel flow, which seldom occurs in nature. The ADCP measurements reveal t he secondary circulation, bursts of up- and downwelling, shear reversals, a nd transverse velocity shear.