Jet and hydraulic jump near-bed stresses below a horseshoe waterfall

Horseshoe waterfalls are a common but unexplained feature of bedrock rivers. As a step toward understanding their geomorphology, a six-component force/torque sensor was used to evaluate clear-water scour mechanisms in a 0.91-m broad-crested horseshoe waterfall built into a 2.75-m-wide flume to measure longitudinal, lateral, and temporal profiles of the near-bed spatially averaged stress vector. Jet impact caused moderate near-bed stress. Downstream of the impact point, the stress vector shifted into a much more forceful upthrust with peak stress beyond the boil crest. Instantaneous fluctuations in near-bed lift exceeded drag by 5-10 times and occurred over a wider range of frequencies, suggesting a larger role for particle entrainment. Predictions of bed stress significantly underestimated actual conditions. These results show that horseshoe waterfalls have an extended region of high scour along their centerline corresponding with the three-dimensional convergent dynamics associated with both jet impact and jump-enhanced lift fluctuations.