Laboratory experiments are conducted to study the rheological behaviour of high-concentration granular–liquid mixtures. Steady uniform free-surface flows are obtained using a recirculating flume. Cases in which a loose deposit forms underneath the flow are contrasted with runs for which basal shear occurs along the flume bottom. The granular motions are observed through the channel sidewall, and analysed with recently developed Voronoï imaging methods. Depth profiles of mean velocity, solid concentration, and granular temperature are obtained, and complemented by stress estimates based on force balance considerations. These measurements are used to probe variations in rheological behaviour over depth, and to clarify the role of the granular temperature. The flows are found to evolve a stratified structure. Distinct sub-layers are characterized by either frictional or collisional behaviour, and transitions between one and the other occur at values of the Stokes number which suggest that viscous effects intervene. The observed frictional behaviour is consistent with shear cell tests conducted at very low shear rates. On the other hand, the collisional data corroborate both the Bagnold description and the more recent kinetic theories of granular flows, provided that one accounts for the inertia of the interstitial liquid.

This paper presents a theoretical approach to the problem of designing the opening in a slit-check dam. The approach is based on the conservation of the mass of water and sediments and on the energy balance under steady conditions. It leads to a relationship among opening width, sediment characteristics, mountain river geometry, and water and sediment discharge. The final relationship can be simplified to make it suitable for practical applications. Also, the problem of unsteadiness of both water and sediment is considered, as well as the possibility of treating the unsteady flow as a sequence of steady states. The results of the theory were checked in a laboratory investigation using a scale model. Different opening widths were tested under conditions of steady and unsteady supplies of water and sediment. The mean grain size of the sediment, as well as the rates of sediment and water discharge, were changed in the experiments. The results of the experiments confirm the theory quite well.

The paper addresses the dynamic impact of channel flows of water and mixtures of water and sediments on a test wall. Two types of impact may occur: the first characterized by the formation of a complete reflected wave moving upstream; the second type consisting in the vertical deviation of the flow along the wall with a jet-like behaviour. The paper deepens this second type of impact. During the impact of the flow against the wall, the pressure distribution along the wall and the impact force were measured. In order to explain the phenomenon, a rational approach based on the conservation equations for mass, momentum and energy was adopted; from this approach some simplified solutions were derived. We discuss the possible effects of the impact of single gravel particles and the influence of the dimensionless groups that govern the phenomenon. A good agreement between the experimental data and the results of the theoretical approach was obtained.

The paper documents a detailed set of measurements from laboratory experiments involving open–channel flows of granular–liquid mixtures. Polyvinyl Chloride pellets mixed with clear water are used, and represent simplified analogues of the mixtures of rocks and muddy water encountered in stony inertial debris flows. The flows are examined in steady uniform conditions, achieved by setting up a closed loop with an inclined channel and a high–speed conveyor belt. Such idealized conditions make it possible to estimate stresses within the flowing mixture, and to accumulate statistics of local granular configurations and motions. These are extracted from video sequences imaged through the sidewall, using algorithms based on the Voronoï diagram. Estimates of granular concentration and granular temperature are derived from local grain patterns and from a detailed analysis of Lagrangian and Eulerian velocity correlations. Depth profiles of kinematic and dynamic quantities are then obtained for various ratios of solid to liquid discharges. These data were earlier used to probe depth variations of rheological behaviour in granular–liquid flows. To make them available for other purposes, they are assembled here into a comprehensive dataset, and provided in digital form in the electronic supplement to this special issue.