Flow velocity is one of the most important hydrodynamic variables for both channelized (rill and gullies) and interrill erosive phenomena. The dye tracer technique to measure surface flow velocity
Vs is based on the measurement of the travel time of a tracer needed to cover a known distance. The measured
Vs must be corrected to obtain the mean flow velocity
V using a factor
αv =
V/
Vs which is generally empirically deduced. The
Vs measurement can be influenced by the method applied to time the travel of the dye-tracer and
αv can vary in different flow conditions. Experiments were performed by a fixed bed small flume simulating a rill channel for two roughness conditions (sieved soil, gravel). The comparison between a chronometer-based (CB) and video-based (VB) technique to measure
Vs was carried out. For each slope-discharge combination, 20 measurements of
Vs, characterized by a sample mean
Vm, were carried out. For both techniques, the frequency distributions of
Vs/
Vm resulted independent of slope and discharge. For a given technique, all measurements resulted normally distributed, with a mean equal to one, and featured by a low variability. Therefore,
Vm was considered representative of surface flow velocity. Regardless of roughness, the
Vm values obtained by the two techniques were very close and characterized by a good measurement precision. The developed analysis on
αv highlighted that it is not correlated with Reynolds number for turbulent flow regime. Moreover,
αv is correlated neither with the Froude number nor with channel slope. However, the analysis of the empirical frequency distributions of the correction factor demonstrated a slope effect. For each technique (CB, VB)-roughness (soil, gravel) combination, a constant correction factor was statistically representative even if resulted in less accurate
V estimations compared to those yielded by the slope-specific correction factor.
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