Flow experiments with gravel and blocks at small scale to investigate parameters and mechanisms involved in rock avalanches

Laboratory experiments play an important role in improving the modelling of rock avalanches since they contribute to a better understanding of the mechanisms that characterise propagation and to identifying parameters influencing velocity and deposit characteristics. Tests analysed in this paper consist of unconstrained flows of aquarium gravel and small blocks down an inclined board, which ends with a horizontal part where the mass flow comes to rest. The varied parameters are fall height, volume, material, slope inclination and friction angle. When blocks, i.e. small bricks of 1.5 cm x 3.1 cm x 0.8 cm, are used, two different starting arrangements are considered: poured in randomly into the release container or piled orderly one on top of the other. Runout, width and length of the final deposit are measured manually while mass front velocity and deposit morphology are derived with specific processing from films and images. The fringe projection method, an optical technique, allows retrieving the thickness of the final deposit and then to compute the position of its centre of mass. Therefore, the analysis of the experimental results is no longer limited to data such as runout and apparent friction angle (Fahrböschung), but it also allows to consider the distance travelled by the mass centre. By analysing the velocity of the mass front as it enters the accumulation zone, it is possible to observe an initial uniform decelerated motion under the effect of friction followed by a transitory part, where the impulse given by the rear part of the mass affects this motion and causes a relative acceleration. This provides experimental evidence to theories by Heim (1932), Van Gassen and Cruden (1989) and Legros (2002) which state that a transfer of momentum occurs between the rear approaching part and the front part slowing down ahead, inducing the mass to spread and consequently the front to travel further. The greater the volume of the mass, the greater the duration of the interaction between the rear and front parts and the longer is the runout. Conversely, no change in the travel distance of the centre of mass is observed. The experimental results suggest that the concept of a straight energy-line based on a simple frictional model is not adequate to evaluate the distance travelled by the centre of mass. As stated by Legros (2006), only models that take into account a velocity dependent term of energy dissipation, such as the Voellmy model, should be used. Moreover, the energy line seems to depend on the geometry of the set-up. The tests with bricks piled orderly at the start seem to provide experimental evidence for the phenomenon described as the spreading of a coherent mass by Davies and McSaveney (1999). First, the mass behaves as a compact body (the bricks remain packed together) and energy dissipation takes place mainly through friction at the base. Then, it shatters and energy is mainly dissipated through friction/collisions between the bricks. Having “spared” a part of the energy in the first part of the sliding, the mass enters the accumulation zone with a higher velocity and consequently travels further than a mass sliding as a loose material from the start. This mechanism of propagation could partly explain the large travel distance of rock avalanches.