Generation and propagation of dust cloud during high energy rock-avalanches

F.V. De Blasio, G. Dattola and G.B. CrostaDept. of Earth and Environmental Sciences, University of Milano-Bicocca, Milan, Italy

Rock avalanches are initially intact rock masses that collapse catastrophically and that during sliding are subjected to a severe fragmentation processes reducing progressively the clasts diameter. The potential energy is so dissipated by friction and fragmentation, in addition to other energy sinks. During the motion of a rock avalanche, particles of tens of micrometers size are generated from crushing, grinding, or chipped off the rock and released to the air generating a suspension hereafter called dust cloud.  

The dust cloud moves away from the rock avalanche sliding path, partly thrust by the energy of impact of the avalanche against obstacles, and partly inheriting the speed of the rocky mass. Moreover, having density slightly higher than air, the cloud is responding to downward thrust exerted by the gravity field. Thus, the cloud velocities may be variable depending on the geometry of collapse and on the initial rock avalanche speed. At high cloud speed, hazards include severe abrasion  and air blast. Also after the high velocity phase the cloud may be hazardous, reducing visibility for hours until dust particles are completely settled. If this process takes place for example in proximity of facilities and transportation lines, problems may arise to traffic flow.

For this reason the prediction of the cloud formation and further motion is an important, albeit poorly developed subject. We are developing a simple physical model which describes cloud formation and motion. Firstly, the cloud is assumed to form by high-energy chipping of the rocks. To calculate the cloud movement, the shape is split up in a set of deformable sub element. By initially imposing the strongly limiting condition of incompressibility, namely, that cloud density does not change, the equations of motion for a deformable cloud can be written. The equations are then solved numerically. Several situations are considered, including (i) a change in the slope inclination, (ii) the presence of an obstacle, (iii) initial high cloud speed inherited by the travelling rock avalanche, in comparison with zero initial speed. So far, the model is capable to reproduce the cloud motion and the increase in the pressure when it strikes an obstacle.

Case studies considered in conjunction with this theoretical work include the recent events of the Pousset and Gallivaggio rock avalanches both in Northern Italy, where rock dust could be recovered from different locations along the cloud path, promptly after the event.