Sturzstrom and High-Velocity Rock Avalanches

A sturzstrom is an exceptionally rapid, long-runout movement of a large mass of rock from a steep mountain slope. It is commonly treated as a high-velocity rock avalanche in which the collapsing rock becomes intensely fragmented and flows downslope as a granular mass.

Movement and exceptional mobility

The movement begins with gravitational failure of a rock slope and changes rapidly from sliding or falling into avalanche-like flow. Its defining feature is exceptional runout: the debris may travel across valley floors and gentler terrain much farther than an ordinary frictional slide would be expected to travel.

• Mobility is commonly expressed by the ratio of vertical fall height to horizontal travel distance, called the H/L ratio; a lower ratio indicates greater mobility.
• Fragmentation, interactions among particles, basal processes, entrainment and sometimes trapped air may contribute to mobility, but no single explanation applies universally.

Causes and geomorphic signatures

Sturzstroms occur mainly in high-relief terrain where steep, jointed or weakened rock slopes can collapse. Triggering or preparatory factors include earthquakes, intense rainfall, weathering, freeze-thaw action, river or glacier undercutting, and loss of glacial support.

• Deposits are typically broad, thick and composed of highly fragmented, angular rock debris.
• The surface may be irregular or hummocky, while the deposit can spread far beyond the foot of the source slope.

Hazard significance

Because of their speed, volume and long reach, sturzstroms can bury settlements and infrastructure with little warning. In narrow mountain valleys, the debris may form a landslide dam, creating risks from upstream inundation and subsequent dam-breach flooding.

• Hazard zonation must consider possible runout paths, not merely the unstable source slope.
• Secondary effects can include river diversion, sediment pulses and cascading flood hazards.