Measuring rill flow velocity by dye tracer technique

Mean flow velocity, V, is one of the most important hydrodynamic variables for rill erosion phenomena. The measurement of flow velocity is fundamental to obtain predictive relationships for modeling water soil erosion and can be performed by the dye tracer technique. This method gives the surface flow velocity Vs, which needs to be multiplied by a correction factor αv to obtain V. This paper is aimed to evaluate αv for two different rill roughness conditions, using an experimental set up which simulates flows moving in fixed bed rills. In particular, two flumes covered by soils with different textures, named Sparacia and Aranceto, were used. For each roughness, ten slopes values ranging from 0.1% to 15% were investigated and, for a given slope, five discharges Q (0.20 – 0.87 L s‐1) were applied. For each (Q, s) pair 20 measurements of Vs were performed. The results showed that the correction factor is independent of the Reynolds number for both the roughness conditions. For the Sparacia soil, for the higher s values, αv can be estimated by a particular Froude number Fs and s, while is constant for the lower s values. For the Aranceto soil, no statistically significant relationships were detected. Finally, the correction factor increased with roughness, but only for a limited slope range concerning the lowest investigated s values.