Testing the Effect of the Rill Channel Slope on the Correction Factor of Surface Velocity

The knowledge of flow velocity is necessary for studying rill hydraulics and the related soil erosion processes. 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 correction factor αv = V/ Vs which is generally empirically deduced and can vary in different flow conditions. Experiments were performed by a fixed-bed sloping flume simulating a rill channel. Two different bed arrangements (smooth flume, and flume covered by sieved soil) were tested. The measurements were carried out for four slope values (9, 11, 13, and 15%) and different discharges. For each slope-discharge combination, 20 measurements of Vs were carried out using a Methylene blue solution. The travel time of the leading edge of the dye cloud was measured by a chronometer. For both the arrangements, the frequency distributions of Vs/Vm resulted independent of slope and discharge. All measurements resulted normally distributed, with a mean equal to one, and featured a low variability, demonstrating that Vm can be considered representative of surface flow velocity. The results highlighted that αv is not correlated with the Reynolds number, the Froude number, and the channel slope.