Assessing dye-tracer technique for rill flow velocity measurements

Abstract

Rill erosion is considered one of the most important processes affecting soil because of the large amount of soil loss. The rill network acts as sediment source and is able to transport both rill flow-detached particles and those delivered from the interrill areas. Small flow depth in a rill and steep slope values of its bed affect significantly flow hydraulics. When rill flow velocity is measured using a dye-tracing method, the mean velocity is calculated by multiplying the measured surface velocity of the leading edge of the tracer plume by a correction factor. The main uncertainty of the dye-tracing technique stands in the relationship between mean and surface flow velocity. In this paper, this relationship was firstly tested using the measured data pairs available from literature and then the influence of the adopted relationship on the estimate of the Darcy Weisbach friction factor was examined. The developed analysis showed that the applied estimate criteria of the correction factor do not affect the estimate performances of the theoretical flow resistance equation. Finally a new flow resistance equation for rill flows which can be directly calibrated by surface velocity measurements was deduced. The proposed procedure for estimating the friction factor was calibrated by rill data available from literature and was positively tested by the rill velocity measurements carried out in this investigation.