Slope processes and phenomena

Slope processes and phenomena are caused by the action of gravity forces and lead to a variety of deformations - collapse, talus, landsliding, solifluction, stone streams, rapid flows and the like. The most distinct manifestation of gravitational forces occurs on steep slopes, with failure and displacement of rock waste and stone blocks resulting from the weathering processes. The rate of material displacement can vary, being high on certain sites.

The history of the sedimentary material within the permafrost regions is associated not only with cryohydric and temperature weathering of hard rocks, but also with the permafrost processes proper which lead to separation of the frozen soils. Such permafrost processes include frost cracking, thermal abrasion, thawing out of the texture generating ice and recurring ice wedges, etc. Among the most common depositional forms of the gravitational processes themselves are scree creep and accumulation of rudaceous materials at the foot of slopes. Thickness of these formations reaches several tens of metres. They are widely developed, for example, in the mountain regions of the North-East of Russia; they contain vast masses of congelation ice resulting from water infiltration and freezing. This type of slope process plays an insignificant role on flat and lowland territories of the permafrost regions.

Displacement of weathering products on landslide slopes is not of importance in the permafrost regions. This is explained by the presence of the perennially frozen ground and the comparatively shallow seasonally thawing layers of the ground. Landslides arising locally are mainly of small thickness and they are connected with the process of soil thawing or with sliding of ground masses along the tilting ice layers. Sliding of the deposits of the seasonally thawing layer is predetermined mainly by the emerging postcryogenic structure and texture. At the locations of thawed ice schlieren, failure-prone zones are formed, while soil aggregates are preserved as structural separations. As a result, hydraulic conductivity of the seasonally thawing clayey silts (for instance) increases by a hundred times. Moreover, during the warmest years, when depth of the seasonally thawing layer increases by 10-20%, the most ice-rich (lower) part of this layer becomes actively permeable. This gives rise to supra- permafrost water even in heavy clay rich soils and the possibility of flowing of whole blocks of ground.

Rock streams are slope formations widely developed in the area of permafrost, composed of scree-block-gravel materials of hard and semihard rocks. Their frequency among slope formations is rather great as is the role played by them in the displacement of weathering products along slopes. Rock streams are confined to slopes with steepness ranging between 3 and 40°. Their dimensions, shape and arrangement are heterogeneous. They arise on vast rocky slopes, can form as rock rivers and applanation terraces, fill narrow hollows, compose large-area block fields, etc. (Fig. 5.12a). The processes that lead to the formation of rudaceous materials, their displacement and accumulation within the limits of catchment areas are also heterogeneous. The main and permanent mechanisms of rock streams are thermogenic and cryogenic creep.

Thermogenic creep is conditioned by periodical (diurnal and seasonal) variations of temperature leading to the cyclic expansion and reduction of the sizes of rock waste and pulse-like displacement of rudaceous material downslope.

Cryogenic creep is associated with uplifting of rock waste in the direction

Fig. 5.12. Rock stream on Jurassic rocks (a) and golets ice in the base of rudaceous cover of rock stream (b) (photos A.I. Tyurin).

Fig. 5.12. Rock stream on Jurassic rocks (a) and golets ice in the base of rudaceous cover of rock stream (b) (photos A.I. Tyurin).

perpendicular to the slope (on account of ice lenses and layers arising in the body of the rock stream) and with the further sinking of debris together with infilling materials, resulting from the thawing of the ice. Multiple repetition of the cyclic process leads to a gradual movement of rock streams down-slope. Among the accompanying processes are: plastic-viscous flow of a fine grain matrix and creep of rock debris of the seasonally thawing layer along the overwet or ice-rich bottom. The above takes place when intense heaving of rock debris occurs in the seasonally thawing layer as well as a moving down and spreading out of fine soil. As a result, at the base of the layer there is more fine material, while in its upper portion there remains rudaceous material which is very permeable. In the spring, meltwater penetrates to the bottom of the coarse debris and freezes there, thus forming the so-called 'golets' ice (see Fig. 5.12b). During the warmest or rainy years when depth of seasonal thawing increases greatly, golets ice thaws, fine earth underlying the rudaceous rocks gets supersaturated and plastic-viscous flow begins, moving the whole overlying sequence of rudaceous material.

The rudaceous material in rock streams is conveyed at a rate of centimetres a year. Within the limits of a rock stream this rate varies both in time and in different parts. Often, rock streams are distinguished as a specific genetic type of slope formation of the mountain regions in Siberia -mountain or rock stream creep.

The processes of viscous and visco-plastic displacement of granular materials within the limits of the freezing regions occur both on slopes with a vegetation mat and over the predominantly smooth surfaces of accumulation. Among these the most common is the process of solifluction or viscoplastic (slow) flow of loose deposits on the slopes under the influence of their own weight, the component of which is directed down the slope thus causing plastic deformations of the ground.

Solifluction usually develops in fine silty clays and silty sands, often with a high content of rudaceous materials. The rate of solifluction is a function of slope steepness, depth of soil thawing, composition of deposits, durability of ground cover, terrain type, etc. Viscoplastic displacement of soils down-slope manifests itself in the laminated deposits in which the layers of ice-rich silty clays and silty sands alternate with peat and humus interlayers (Fig. 5.13). The thickness of the deposits that form in a syngenetic way is greatest in the lower and least in the upper parts of slopes. Usually, two types of slow solifluction are distinguished, namely, covering and differential.

Cover solifluction is movement of ground that occurs more or less uniformly and rather slowly. In this case there is no appearance of flow on the slopes. This type of solifluction has typical rates of 2-10 cm yr"1 and manifests itself on slopes of up to 15° steepness. A specific feature of cover solifluction is that material movement occurs without substantial change of the inner structure of the ground. Moisture content of deposits does not exceed the liquid limit. In the upper part of the solifluction layer there is a layer that preserves its shape, usually making up 25-30% of total thickness

Fig. 5.13. Structure of solifluction terrace. North-eastern piedmont of Chukotka (according to L.A. Zhigarev): 1,2- light and heavy sandy silty materials, respectively, with fragments of porphyrite; 3 - heavy clay silty-sand with fragments of porphyrite; 4 - vegetation-peaty layer and buried peat interlayers; 5 - poorly-sorted sand; 6-7 - fine- and coarse-fragment eluvium of porphyrites; 8 - boundary of the perennially frozen soils.

Fig. 5.13. Structure of solifluction terrace. North-eastern piedmont of Chukotka (according to L.A. Zhigarev): 1,2- light and heavy sandy silty materials, respectively, with fragments of porphyrite; 3 - heavy clay silty-sand with fragments of porphyrite; 4 - vegetation-peaty layer and buried peat interlayers; 5 - poorly-sorted sand; 6-7 - fine- and coarse-fragment eluvium of porphyrites; 8 - boundary of the perennially frozen soils.

of the ground flow, but at low velocities it can be as much as 90%. Slow solifluction is likely to be accompanied by sorting of material. The most apparent manifestations of this are alternating stripes of rudaceous material and fine earth.

Differential solifluction as distinct from the cover type shows itself on the surface in the typical landforms of micro- and mesorelief: solifluction lobes, terraces, stripes, etc. Such forms arise owing to differential rates of displacement both on the whole slope and within one solifluction flow. Different rates are largely conditioned by the ice content of soils varying in different parts of the slopes.

Within the limits of the freezing regions along with the slow solifluction considered above there is the so-called rapid solifluction or viscous flow of thawing soils on the slopes. This type of slope transfer, often called mudflows, develops on slopes of 15-25° steepness in highly supersaturated soils of the seasonally thawing layer. It is accompanied by disturbance of the internal soil structure as well as by breakage of the ground cover. The moisture content of deposits exceeds the yield point. Viscous flow develops during periods of intense melt or in cases of heavy precipitation. The layer of preserved shape is absent.

Mudflows occur frequently on cut slopes as perennially frozen ice-rich soils become exposed by slumping and thaw (Fig. 5.14). The rate of soil displacement in mudflows is rather great, reaching sometimes several metres per minute.

Fig. 5.14. Solifluction slumps on the slope towards the Yuribcy river of the Kazantsev plain, the Gydan peninsula (photo G.I. Dubikov).
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