Assessing Path Tortuosity on Rill Flow Resistance

The departure of the thalweg from a straight path is accounted for by the tortuosity t, a parameter of rill morphology. Few research has been done on how t affects flow resistance for rills. The analyses of a plot investigation reported in this study are finalized calculating the tortuosity parameter and evaluating the reliability of a theoretical flow resistance equation. The Structure from Motion algorithm was used to generate a three-dimensional Digital Terrain Model. First, an approximation of the thalweg was obtained by photo-interpretation for the four investigated rills. A calculation routine was then applied to identify the cross sections perpendicular to this rill thalweg, with a fixed distance d, and determine the relative lowest points. The real rill thalweg resulted from the line connecting these points. Among the several investigated distances, the value of 0.075 m was selected for tortuosity calculation. The detected non-monotonic relationship of the Darcy-Weisbach friction factor f against t was attributed to the three additive components resulting from sediment transport, bed roughness, and localized energy losses caused by curves. In fact, compared to the third component, the first two have the opposite influence on f. The theoretical approach gave an accurate and t-dependent estimation of f.