ABSTRACT
Extremely Energetic Rockfalls (EERs for brevity) are here defined as rockfalls for which the combination of both large volume (at least some thousands of m3) and free fall height in the range of hundreds of metres, results in a total released energy larger than 1/50 of kilotons. Documented examples include several events with different size in the Alps (Civetta, 2013, ca 50,000 m3; Dru, 2005, 2012, 265,000, 59,200 m3; valFiscalina—Cima Una, 40,000 m3; and Thurwieser 2004, ca 3 Mm3; Cengalo, 2011, 1.5 Mm3 in Switzerland), in the Apennines (Gran Sasso, 2006, 30,000 m3), Rocky Mountains (Yosemite, Happy Isles, 38,000 m3), and Himalaya. EERs may become more frequent as a consequence of permafrost thawing at higher altitudes and on steep and sharp mountain peaks.
In contrast to low energy rockfalls where block fragmentation is limited, in EERs the impact after free fall causes an immediate and efficient release of energy much like an explosion. The severe disintegration of the rock and air blast are capable of snapping trees many hundreds of metres ahead of the fall point. Pulverized rock at high speed can abrade tree logs, and the resulting suspension flow may travel much further the impact zone, blanketing vast surrounding areas.
Using published accounts of some of these events, we introduce some basic physical considerations of the involved phenomena based on analogies with explosions and explosive fragmentation. Of the initial energy, one part is used up in the rock disintegration, and the rest is shared between the shock wave and air blast. Simple and practical model results expressed by analytical formulas may be useful in estimating the entity of devastation in future events.
