New technique for measuring water depth in rill channels

Abstract

Water erosion is one of the most important soil degradation processes and rill erosion contribution to total soil loss is usually dominant as compared to interrill erosion. Rill erosion modelling requires that rill flow has to be adequately modelled. Flow depths in rills are typically of the order of millimeters to several centimeters and bed topography, characterized by steep slope values, significantly affects flow hydraulics. In this paper, a new technique for measuring the water depth inside a rill channel is proposed and the effects on flow resistance estimate are examined. This technique couples an accurate ground survey of the rill channel, obtained by close-range photogrammetry, with the survey of the water tracks inside the channel. The comparison between the water depth measurements carried out by a micro-hydrometer (local technique)and those obtained by the water tracks (volumetric technique)demonstrated that the data pairs are characterized by a wide scattering respect to the line of perfect-agreement. Furthermore, the hydraulic radius values obtained by the proposed volumetric technique are always greater than those corresponding to the local technique. The developed analysis showed that the volumetric technique is affected by the adopted cross-section spacing even if reliable results can be also obtained using a distance interval which is equal to ten times the best investigated reference condition, which corresponds to the minimum tested distance interval, having the same order of magnitude of the mean 3D-DTM resolution. Using of the volumetric measurement technique for the water depth improved the agreement between the measured Darcy-Weisbach friction factor values and those calculated by the theoretical flow resistance law based on a power-velocity profile. Finally, a constant scale factor (equal to 0.8257)was proposed to convert the Γ function of the power velocity profile obtained by local measurements into that corresponding to the volumetric technique.