Hydraulic conditions in experimental rill confluences and scour in erodible soils

Abstract

Few studies have examined processes active within rill networks or the effect of confluence geometry on rill sediment flux, and it is not clear if observations from river confluences also apply at rill scale. Laboratory experiments were carried out in artificial rectangular rill channels with fixed and erodible beds to identify the effects of confluence geometry on hydraulic conditions and scour patterns. Symmetrical and asymmetrical confluences with angles from 19° to 90° were used. Hydraulic changes in the confluence zone in fixed bed experiments included decreased flow velocity, increased shear velocity, transition from supercritical to subcritical, and from transitional to turbulent flow. Bed scour in the confluence zone resulted, generally increasing with confluence angle, but a more important influence was junction symmetry or asymmetry. Symmetrical confluences produced a symmetrical scour pattern mirroring the original channels, but in asymmetrical confluences scour was more complex, evolving to a symmetrical pattern with confluence angles higher than the original system. Results are consistent with channel evolution to an optimally branched system controlled by minimum power criteria, but validation requires testing with more precise velocity measurement.