Comparison of Bed Load Sampler and Tracer Data on Initiation of Motion

Abstract

Concerns that the Helley-Smith bed load sampler and painted tracer techniques measure different critical discharges for the initiation of motion for each particle size in coarse bed material rivers are investigated for two sites on the Roaring River in Colorado. For the sampler technique, the size of the largest particle trapped is paired with the discharge at the time of sampling. For the tracer technique, the maximum size of tracer moved is paired with the maximum discharge between emplacement and observation. Performance considerations suggest that the sampler technique may yield a larger critical discharge for a given particle size than will the tracer technique. The limited field data support this when the tracer measurements refer to the absolute maximum size moved. When the tracer measurements refer to the largest size moved for which all smaller sizes were moved, the data overlap the sampler data. Caution should therefore be displayed when using data from either technique to develop or test formulas for initiation of particle motion, as such formulas may show a small level of technique dependence.