Avalanches are masses of snow moving rapidly down a mountain slope or cliff and are responsible for a large number of injuries each year, particularly on building and civil engineering sites in mountainous areas. There are two main types of avalanche: slab avalanches and avalanches of loose snow.
Various factors intervene in the release of avalanches. A heavy fall of snow may produce a layer of snow that moves as an immediate avalanche within 3 days of its deposition. The structure of the snow layer is one of the prime factors in avalanche causation; wind and temperature are other decisive factors. Avalanches usually occur during periods of high wind which cause overloading and wind slabs. There is usually a rise in temperature, a factor that normally accompanies heavy precipitations; this temperature rise reduces the cohesion between snow strata but at the same time promotes compaction and consolidation. Temperature rise therefore has a dangerous effect at first, which does not persist except when there is significant melting. On slopes with an incline of over 140% there is no danger of avalanche since the snow cannot accumulate over the long term. The lower limit of dangerous incline is quite low (30%), although avalanche defence construction is not built on slopes of less than 70% incline. Local conditions such as relatively smooth ground surfaces or the presence of convex slopes will increase the avalanche hazard.
Immediate avalanches account for 60-80% of all avalanches and, for example, the Alpine avalanche disasters of 1951 were the consequence of exceptionally heavy snowfalls. In France it is accepted that, at an altitude of 1,500 m, following snowfalls of 50 mm water equivalent (about 50 cm of snow), there is a serious avalanche hazard. When a level of 100 mm water equivalent has been exceeded, the danger becomes widespread and avalanches are numerous.