Rock Avalanches: Basic Characteristics and Classification Criteria

Rock avalanches represent the specific type of flow-like landslides—dry granular flows—that pose major threat to population in mountainous regions and in the adjacent plains. Being extremely mobile, they can affect areas up to dozens of square kilometers, extending sometimes for more than 10 km from the feet of the collapsing slopes. The internal structure of their deposits is characterized by intensive fragmentation of inner parts overlain by much coarser carapace. Such internal structure is typical of the vast majority of large-scale rock slope failures, both long runout and forming compact blockages in narrow river valleys. Therefore, all of them should be classified as rock avalanches, rather than as rock slides. Three additional classification criteria closely related to rock avalanche mobility and allowing more strict definition of a particular rock avalanche are discussed, i.e. the confinement conditions, debris distribution along the rock avalanche path, and directivity of debris motion. Besides providing information on debris motion mechanism(s), these characteristics predetermine the assessment of the exposure of elements at risk that might be affected by rock avalanche. It is demonstrated that transformation from the block slide to granular flow depends somehow on the morphology of the transition-deposition zone and on the mechanical properties of the basal surface, but is independent from the type and mechanical properties of the host rocks.