The occurrence and mechanism of catastrophic mass flows in the mountain cryosphere

Abstract Catastrophic mass flows (CMF) in the mountain cryosphere are an important geomorphic process that may pose significant hazard to communities and infrastructure in cold mountains; they occur in association with snow, glaciers, and permafrost in environments that are particularly sensitive to changes in thermal regimes and heavy rainfall. CMF form a broad range of cryospheric hazards and include mass movements of glacial ice, rock avalanches, ice-rock avalanches, debris flows, and outburst-generated flows. In some high mountains, avalanches of glacial ice, rock, and ice-rock mixtures may be earthquake-triggered. Broadly, CMF in the mountain cryosphere are characterized by sudden onset, high mean velocity (≥ 5 m/s), and high mobility (i.e., long runout in relation to volume) and generally involve a mixture of earth materials, water, snow, and ice. In some cases, CMF runout may exceed 100 km from source. CMF commonly undergo dramatic process transformation during movement in response to melting of entrained ice and snow, entrainment of additional materials along their path, river-damming effects, and incorporation or displacement of water in the periglacial environment; process complexity, involving long and complex process chains with instantaneous and/or delayed cascades, thus represents a challenge to quantitative hazard assessment. CMF initiate in uninhabited or sparsely populated areas of the mountain cryosphere and frequently descend into denser populated areas where they impact on mountain communities and infrastructure. CMF have been responsible for several notable mountain disasters since 1940 resulting in the deaths of over 15,000 people worldwide. Our focus on an examination of process complexity illuminates an assessment of CMF hazard in ice-affected mountain regions and forms the basis for the development of mitigation strategies based on detection, warning, engineering techniques in source and runout areas, and land-use controls. The precise relationship between the magnitude/frequency of CMF and change in the mountain cryosphere since ca. 1900 remains uncertain.