[1] We report an experimental investigation of the motion of bed load particles under steady and spatially uniform turbulent flow above a flat sediment bed of uniform grain size. Using a high-speed video imaging system, we recorded the trajectories of the moving particles and measured their velocity and the length and duration of their flights, as well as the surface density of the moving particles. Our observations show that entrained particles exhibit intermittent motion composed of the succession of periods of "flight" and periods of rest. During one flight, a particle may go through phases of reptation, during which it moves in nearly persistent contact with the rough bed, and phases of saltation, during which it travels sufficiently high above the bed to reach high velocities. The distributions of longitudinal and transverse particle velocities obey a decreasing exponential and a Gaussian law, respectively. Interestingly, these observations are similar to those previously reported for viscous flows. The experimental results presented here support the erosion-deposition model of Charru (2006) and allow the calibration of the involved coefficients. In particular, noting t*, the Shields number, and t* c , the threshold Shields number, we find that (1) the surface density of moving particles increases linearly with t* − t* c ; (2) the average particle velocity increases linearly with t* 1/2 − t* c 1/2, with a finite nonzero value at the threshold; (3) the flight duration scales with a characteristic settling time with no significant dependence on either t* or the settling Reynolds number; and (4) the flight length increases linearly with t* 1/2 − t* c 1/2. The results presented in this paper should provide a valuable physical framework to describe bed form development in turbulent flows.

We report the results of an experimental investigation of the flow induced by the collapse of a column of granular material ͑glass beads of diameter d͒ over a horizontal surface. Two different setups are used, namely, a rectangular channel and a semicircular tube, allowing us to compare two-dimensional and axisymmetric flows, with particular focus on the internal flow structure. In both geometries the flow dynamics and the deposit morphologies are observed to depend primarily on the initial aspect ratio of the granular column a = H i / L i , where H i is the height of the initial granular column and L i its length along the flow direction. Two distinct regimes are observed depending on a: an avalanche of the column flanks producing truncated deposits for small a and a column free fall leading to conical deposits for large a. In both geometries the characteristic time scale is the free fall of the granular column c = ͱ H i / g. The flow initiated by Coulomb-like failure never involves the whole granular heap but remains localized in a surface layer whose size and shape depend on a and vary in both space and time. Except in the vicinity of the pile foot where the flow is pluglike, velocity profiles measured at the side wall are identical to those commonly observed in steady granular surface flows: the velocity varies linearly with depth in the flowing layer and decreases exponentially with depth in the static layer. Moreover, the shear rate is constant, ␥ = 0.3 ͱ g / d, independent of the initial aspect ratio, the flow geometry, position along the heap, or time. Despite the rather complex flow dynamics, the scaled deposit height H f / L i and runout distance ⌬L / L i both exhibit simple power laws whose exponents depend on a and on the flow geometry. We show that the physical origin of these power laws can be understood on the basis of a dynamic balance between acceleration, pressure gradient, and friction forces at the foot of the granular pile. Two asymptotic behaviors can be distinguished: the flow is dominated by friction forces at small a and by pressure forces at large a. The effect of the flow geometry is determined primarily by mass conservation and becomes important only for large a.

The transient surface flow occurring when a cylindrical pile of dry granular material is suddenly allowed to spread on a horizontal plane is investigated experimentally as a function of the released mass M, the initial aspect ratio a of the granular cylinder pile, the properties of the underlying substrate ͑smooth or rough, rigid or erodible͒ and the bead size. Two different flow regimes leading to three different deposit morphologies are observed as a function of the initial aspect ratio a, whatever the substrate properties and the bead size. For aՇ3, the granular mass spreads through an avalanche on its flanks producing either truncated cone or conical deposits. For aտ3, the upper part of the column descends conserving its shape while the foot of the pile propagates radially outward. The obtained deposit looks like a ''Mexican hat'' and the slope angle at the foot of the deposit is observed to saturate at a value of the order of 5°. For a given ground and bead size, the flow dynamics and the deposit morphology are found to be independent of M and to vary only with the initial aspect ratio a. Further investigation indicates that the deposit morphology depends only slightly on the substrate properties and the bead size, except when a becomes large. In particular the same dynamical regimes and deposit morphologies are recovered for the same range of a, independent of the substrate properties or the bead size. Moreover the rescaled deposit radius, the rescaled spreading velocity, and the fraction of energy dissipated during the flow do not depend on M, the substrate properties, or the bead size, but vary only with a. We believe this to be the signature of the fact that the flow develops near the free surface of the granular pile so that the dynamics is essentially controlled by grain/grain interactions.

Cliff collapse is an active geomorphological process acting at the surface of the Earth and telluric planets. Recent laboratory studies have investigated the collapse of an initially cylindrical granular mass along a rough horizontal plane for different initial aspect ratios a = Hi/Ri, where Hi and Ri are the initial height and radius, respectively. A numerical simulation of these experiments is performed using a minimal depth‐integrated model based on a long‐wave approximation. A dimensional analysis of the equations shows that such a model exhibits the scaling laws observed experimentally. Generic solutions are independent of gravity and depend only on the initial aspect ratio a and an effective friction angle. In terms of dynamics, the numerical simulations are consistent with the experiments for a ≤ 1. The experimentally observed saturation of the final height of the deposit, when normalized with respect to the initial radius of the cylinder, is accurately reproduced numerically. Analysis of the results sheds light on the correlation between the area overrun by the granular mass and its initial potential energy. The extent of the deposit, the final height, and the arrest time of the front can be directly estimated from the “generic solution” of the model for terrestrial and extraterrestrial avalanches. The effective friction, a parameter classically used to describe the mobility of gravitational flows, is shown to depend on the initial aspect ratio a. This dependence should be taken into account when interpreting the high mobility of large volume events.