Particle saltation trajectories in supercritical open channel flows over a smooth fixed bed

Abstract

High sediment transport dominated by bed load particle motion in supercritical flows cause (I) bedrock incision in high-gradient mountain streams, and (II) hydro-abrasion at hydraulic structures such as Sediment Bypass Tunnels (SBTs), and Sediment Flushing Channels (SFCs). Therefore, a better understanding of high-speed flow characteristics and bed load particle dynamics is of prime importance for the sustainable design of hydraulic structures, as well as studies of river and landscape evolution. To this end, we experimentally investigated single particle dynamics in supercritical open channel flow over a fixed planar bed. The experiments were conducted in a laboratory flume at VAW simulating a straight section of a SBT. Flow depth at the flume inlet was varied twice with ho = 0.045 and 0.10 m corresponding to the aspect ratios (flume width to water depth) of 4.44 and 2, respectively. 3D flow conditions occurred at such aspect ratios (narrow open channel flow) corresponding to hydraulic conditions of most SBTs. The approach flow Froude numbers were Fo = 2, 2.5, 3, 3.5, 4, 4.5 and 5. Motions of two particle types differing in shape and hardness (limestone and quartz) with a constant diameter, D = 7 mm were investigated at each hydraulic condition. By means of high-speed camera recordings with up to 250 frames/sec, the particle trajectories and velocities were determined. The results showed that the transport mode of particles was dominantly saltation, and the effect of particle shape on particle dynamics was negligible. The saltation characteristics such as hop lengths and particle velocities were in agreement with the literature data.