Measurement of bed load velocity using an acoustic Doppler current profiler

A new technique has been developed to measure the apparent velocity of bed load (v(a)) using an acoustic Doppler current profiler. The technique involves estimating the bias in bottom tracking due to a moving bottom. Mean v(a) measured at sampling stations in the gravel-bed Fraser River correlated well (r(2) = 0.93, n = 9) with mean bed load transport rates measured using conventional samplers. Mean v(a) was also correlated (r(2) = 0.44, n = 19) with boundary shear stress estimated by a log-la,,v fit to the mean velocity profile. Estimates of v(a) from individual 5 s ensemble averages were extremely variable: the coefficient of variation for a sampling station ranged from 1.0 to 6.4, and 25 min of sampling were required to achieve stable estimates of the mean and coefficient of variation (within 5% error). Variance was due to both real temporal variability of transport and measurement error. The mechanisms that produce this variability are discussed and preliminarily quantified.