Plot investigation on rill flow resistance due to path tortuosity

The path tortuosity t is an indicator of rill morphology accounting for the deviation of the thalweg from a straight alignment. The effect of t on flow resistance has been little investigated for rills. This paper reports the results of a plot investigation aimed to establish the suitable accuracy of the rill thalweg measurement to determine the tortuosity parameter and to test the reliability of a theoretical flow resistance law. Four rills were incised in clay soil (CS) and clay-loam soil (LS) and shaped by a clear flow discharge. The three-dimensional Digital Terrain Models were created by the Structure from Motion technique. For rills on LS, an approximate thalweg was tracked by photo-interpretation, and a specific calculation routine was applied to identify the cross sections with a constant spacing d. The actual rill thalweg was obtained as the line joining the lowest points of these cross-sections. Among the different tested d values, d = 0.075 m was chosen to determine t. For both CS and LS, the Darcy-Weisbach friction factor f featured a non-monotonic relation with t, which was explained as the result of three additive components due to bed roughness, sediment transport, and localized energy losses due to curves. The effect of the former two components on f contrasts that of the third, resulting in a linearly decreasing f-t relationship and constant flow velocity for the three lowest tortuosity values, and an increased friction factor and reduced flow velocity for the highest tortuosity value. The flow resistance law was positively tested, and the predicted friction factor was dependent on t.