Site specificity of bed load measurement using an acoustic Doppler current profiler

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Abstract

[1] Concurrent measurements of bed load transport velocity (v) from the bottom tracking feature of an acoustic Doppler current profiler (aDcp) and bed load transport rate (gb) from conventional pressure difference samplers are presented. Data sets were collected from both gravel bed and sand bed reaches of the Fraser River, covering a bed material range of 0.25–25 mm. Strong relations, in which v explained >70% of the variability of measured gb, were observed in a gravel bed and a sand bed reach. Differences in correlation between v and gb among the contrasting environments are attributed to variations in both the bed load particle size and aDcp operating parameters. Similar values of v were associated with lower mass transport rates in a sand bed than in a gravel bed reach. A nondimensional data collapse, accounting for differences in bed load particle size, explained 42% of the observed variance of the combined data set. Longer averaging times were required in the gravel bed reach, likely due to the stochastic nature of bed load entrainment in gravels and the resulting heterogeneous bed load velocity field. Bed load transport was modeled using both shear stress models and a kinematic model that utilizes the estimated bed load velocity. The standard shear stress models provided poor matches to the measured bed load transport rates: the sand bed data were overpredicted, and the gravel bed predictions correlated poorly with the measured predictions. Use of the kinematic model yields an estimate for the product of bed load concentration and bed load layer depth. This work highlights the potential of acoustic techniques for estimating bed load.

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