Movement of loose, sandy detritus by shallow water flows: An experimental study

Abstract Interactions between flowing water and natural, loose, sandy detritus, containing small amounts of clay, silt and pebbles, were studies in flumes, using small discharges over a large range of slopes. These studies formed part of a general investigation of particle transport by water in rivers, overland flows, and rain-impacted, shallow flows. All flows strong enough to move the full size range of bed material were supercritical and channel systems always replaced the initial sheet flows after general sediment movement commenced. Even in water a millimetre deep, behaviourally distinct suspended, saltation, and contact loads existed. Suspended load was entrained throughout the flowswept area, wherever suitable particles were exposed. Saltation load, by contrast, was always entrained to full transportational capacity in the upstream few centimetres of the bed and, downstream, any new entrainment was balanced by deposition. Interaction between saltatory particles and the flow largely controlled channel formation, channel geometry, and the transportation of suspended and contact loads. As flow vigour increased, saltation underwent a series of changes in nature, the most marked being from movement of particles in individual trajectories to migration as ballistic dispersions (rheologic logic layers) which formed when interparticle collisions became common. These changes were reflected by both qualitatively observed features and transportation rates, so that, in terms of saltation, results could be divided into discrete “fields of interaction” on discharge-versus-slope plots. Transport rate increased rapidly with slope if saltatory grains moved in individual trajectories. However, at a bed-load particle concentration of about 0.25% by volume, the rheologic layer formed and, thereafter, transport rate increased less strongly with slope, closely following a stream-power law. Below slopes of 0.01 small rills transported little sand but, through slope range 0.01–0.04, transporting power increased greatly and became enormous by a slope of 0.3. Concentration of flow into channels gave small, overland discharges considerable power to transport pebbles. Although processes in the small channels, in general, closely paralleled those of rivers, repetitive sedimentary structures contrasted markedly with those of deeper, subcritical flows.