Extremely Energetic Rockfalls

Extremely energetic rockfalls (EERs) are defined here as rockfalls for which a combination of both large volume and free fall height of hundreds of meters results in energy larger than about 80 GJ released in a short time. Examples include several events worldwide. In contrast to low energy rockfalls where block disintegration is limited, in EERs the impact after free fall causes immediate release of energy much like an explosion. The resulting air blast can snap trees hundreds of meters ahead of the fall area. Pulverized rock at high speed can abrade vegetation in a process of sandblasting, and particles suspended by the blast and the subsequent debris cloud may travel farther than the impact zone, blanketing vast areas. Using published accounts and new data, we introduce physically based models formulated on analogies with explosions and explosive fragmentation to describe EERs. Results indicate that a portion of the initial potential energy of the block is spent in rock disintegration at impact (typically 0.2%–18%), while other sources of energy loss (air drag, seismic, sound, and ground deformation) are negligible; consequently, more than 80% of the potential energy is converted to kinetic energy of the fragmented block (ballistic projection, shock wave, sand blast, and dust cloud). We also propose simple estimates for the flow of the dust cloud associated with an EER and its long settling time. The areal extent of the affected zone is estimated from the energy balance and an empirical power law relationship.