Type de publication:Book Chapter
Source:Hazards related to permafrost and to permafrost degradation. PermaNET project, state-of-the-art report 6.2, ADRA - Association pour la diffusion de la recherch e alpine, p.pp. 1-27 ; www.permanet-alpinespace.eu (2011)
Rockglaciers are the most prominent features of alp ine permafrost. They are creeping accumulations of debris, moving usually at rates of cm/yr or dm/yr. The observed changes on rock glacier dynamics are all related to velocity changes, and show in most cases an increase in velocity. Depending on the importance of the velocity increas e, the five following types of reaction can be distinguished: * Moderate positive and negative velocity changes, re lated to changes of MAGST with a time lag of one to two years. * Acceleration of rock glacier displacement, with opening of crevasses on the rock glacier surface. * Rupture and dislocation of the lower part of rock g laciers: in several cases, the lower part of the rock glacier starts to move significantly more than the upper part. * Total collapse of the lower part of the rock glacier: the lower part of the rock glacier breaks down as a debris flow and is totally removed * Very strong acceleration of the rock glacier: the a cceleration speeds up to very high values. One case is known so far, were velocities reached values as high as 80 m/yr ! The expected effects of velocity changes can be eit her local and limited to the rockglacier surface, or affect the downslope area : * Veloctiy increase will induce increased damage to i nfrastructures built on rockglaciers. * Strong acceleration will increase the surface instability, and the formation of scarps can lead to local rockfall hazard on the rockglacier surface. * Velocity increase will induce increased rockfall ac tivity on the rockglacier front, as well as a progression of the front. * Partial or total rupture and collapse can occur in a few cases, and threaten potentially large areas downslope. * Where rockglacier fronts are overhanging steep slopes or torrential catchments, secondary processes mobilizing the released debris can induce an increased hazard downslope. The following recommendations can be made in order to reduce hazard due to rockglaciers : * Infrastructures on rockglaciers should be avoided. * The zones in front of active rockglaciers should be avoided. * If the front of an active rockglacier is overhanging a steep slope, a security zone should be observed downslope of the rockglacier, with conside ration of potential direct and indirect processes like rockfalls and debris flows. * Paths and trails crossing rockglaciers or passing in front of rockglaciers should be regularly checked for security.
Edytem Equipe Dynamique des Milieux de Montagne 2011-2015Environmental Sciences/Global ChangesBook sections