Rock fall | Permafrost creep | Periglacial debris flows

Rock fall

Individual falling pieces of rock usually reach runout distances which correspondiing to a 25-30° slope of the trajectory. Cumulative events build up scree slopes and fans of corresponding surface slope (Figure 1). Systematic grain-size sorting can often be observed with larger blocks move farther than fine material (sand, silt), because of reduced friction along partial paths in air. Quantitative models involve

1 - Rock fall in the year 1991 at Randa Mattertal, Valais, Switzerland. The rock fall occurred in two events.

• type of movement (rolling, saltation, rotation)
• surface characteristics (topography, vegetation, etc.)
• contact reactions (translation, indentation, lever effects)
• and block forms (mass point, sphere, squared stone, etc.)

One major unsolved problem in vegetation-free areas, as they are characteristically found in periglacial mountain belts, concerns the destruction of falling/jumping rock particles. Larger volumes of destabilized rocks are much less frequent but can reach far greater distances with much lower inclinations of the trajectory, especially on glaciers (Figure 2). The latter effect is due to:

• reduced friction over smooth snow/firn/ice surfaces
• production of meltwater influencing basal friction and viscosity of the sliding/flowing mass
• existence of moraines from previously more extended glacier stages inducing air jumps and/or channelling.

2 - Relation between average slope and the corresponding catchment area of 82 debris flows in the summer of 1987 in Switzerland.

Permafrost creep

Cumulative deformation results from the steady-state creep of perennially frozen talus or morainic material rich in ice. This movement leads to the formation of striking lavastream-like landforms traditionally called rock glaciers (Figure 3). Such features clearly indicate the present or former existence of ice-rich frozen ground. Here, reduced internal (particle-to-particle) friction and ice-induced cohesion enable large-scale stress transmission. The latter is impossible in non-cohesive debris which is unfrozen.

3 - Rock glacier in the Grisons, Eastern Swiss Alps. This rock glacier show typical signs of compressive flow in the lower part, indicated by large bent ridges and furrows.

4 - Surface velocities of rock glacier Murtèl over the period 1986-96, determined by computer aided photogrammetry (left), and trajectories/surface ages computed from this velocity field (right) (after Kääb et al. 1998).

A wealth of information about the distribution, internal structure and mechanical behaviour of creeping mountain permafrost is now being collected. Methods used include rock glacier and permafrost mapping, geophysical soundings, borehole logging, long-term observations of borehole temperature and deformation, as well as precision photogrammetry. Numerical models combine the principle of mass conservation with some reasonable creep law for the frozen material. Corresponding experiments confirm growing evidence in the field that the age of large rock glaciers, from several hundred meters to more than a kilometer long, must be expected to be in the range of many millennia (Figure 4). In connection with spatial permafrost modelling, relict rock glaciers from past colder time periods can be used for quantitative paleotemperature reconstructions.

Periglacial debris flows

Debris flows of highly variable size belong to the most effective processes of debris transfer and evacuation in periglacial belts of cold mountain areas. They usually form as a result of intense snowmelt, heavy precipitation or outbursts of water from glacierized areas. Quantitative estimates concerning characteristics of debris flows constitute an important part of hazard assessments in high-mountain areas.

5 - Debris flow starting zone in the Saastal/Gerental. The picture shows over steepened frozen permafrost zone in a former glacier covered glacier forefield.

Surface inclination of starting zones on slopes and in rock gullies is generally steeper than about 25°. The largest volumes mobilized result from retrogressive erosion by water emerging at the foot of (unfrozen) talus cones or moraines (Figure 5).

6 - Debris flow at the Piz Julier in the Upper Engadin Switzerland.

Significat parts of the eroded material are usually deposited on the debris flow fans which are characterized by a rough surface topography, often including large blocks deposited. Hence, many debris flows constitute a relatively small sediment input to the main stream in the valley (Figure 6).

The fact that debris flow trajectories have an overall slope greater than about 10° or 20% (path length is in most cases less than approximately 5 times the elevation difference) can be used to estimate maximum runout distances (Figure 7). Large flows are faster and reach greater distances than small ones. As a consequence, relatively flat debris-flow fans are reached by low-frequency/high-magnitude events and vice versa. The most spectacular events usually relate to glacier floods from water pocket ruptures and outbursts of moraine- or ice-dammed lakes. Even in such cases, maximum volume evacuated per unit channel length usually remains below s 500 to 700 m³. The reason for this is that formation of a breach within morainic material with highly variable grain sizes leads to a negative feedback, by the development of block pavements, which protects deeper layers from further erosion.

7 - Relation between average slope and the corresponding catchment area of 82 debris flows in the summer of 1987 in Switzerland

29 August 2011
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