Particle saltation trajectories in supercritical open channel flows: Roughness effect

Bed load sediment transport governed by particle saltation in supercritical open channel flows causes bedrock incision in high-gradient mountain streams and hydro-abrasion at hydraulic structures. Hence, a better understanding of turbulent flow characteristics and particle dynamics is of prime importance for the prediction of river and landscape evolution, and for a sustainable design of hydraulic structures. To this end, we experimentally investigated single-particle dynamics in supercritical open channel flows over fixed planar and abraded beds covering smooth, transitionally rough, and hydraulically rough regimes. The experiments were performed at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zürich in a wide range of hydraulic conditions with various channel width-to-depth aspect ratios (b/ho) and Froude numbers (F). We investigated the motions of five particle groups differing in shape, hardness, and diameter. By means of a high-speed camera, the particle velocities and trajectory parameters—such as hop height and hop length—were determined using particle tracking velocimetry. The results reveal that at least 120 individual particles should be used in the tests to determine sound statistics of particle trajectories. In the present test conditions, the particles are dominantly transported in saltation. The effects of particle properties such as shape and diameter on the particle trajectories are negligible, while the rolling probability of the particles for a given flow condition increases with increasing diameter. Bed roughness was found to be the key parameter having a significant effect on the saltation trajectories and horizontal energy transfer. Hop height, hop length, and particle velocities, as well as horizontal energy transfer, increase with increasing bed roughness. Based on the saltation trajectory data, non-dimensional equations are developed for each hydraulic regime and used to enhance the saltation–abrasion model of Sklar and Dietrich. The present findings contribute to a better understanding of particle motion characteristics under different hydraulic and roughness conditions, and their effect on the mechanics and prediction of bedrock incision and hydro-abrasion in three-dimensional supercritical open channel flows occurring in steeply sloped rivers and at hydraulic structures.ISSN:0197-9337ISSN:1096-983