Dynamics of development and variability of surface degradation in the subalpine and alpine zones (an example from the Velká Fatra Mts., Slovakia)

2.1 Material and Method

The object of research and analyses were degradative morphogenetic forms [4] (DMF) over the timber line at the end of Hornojelenská valley. DMF are perceived as a result of erosion-denudation system, which is caused by the whole complex of exogenous morphogenetic processes influencing soil mantle. DMF are used for surface degradation development and monitoring of the Western Carpathians since 1970´s [4].

The extent of DMF in time periods (1960, 2003, 2009, 2012) was determined from satellite and aerial images. The interpretation of these images can help get the information, which is hard to obtain by field measurements and survey, especially in inaccessible mountain terrain.

The following cartographic materials have been used as an input: aerial images from 1961 (resolution 2400 dpi, height of flight 750 m - Military Topographic Institute) and satellite images from 2003, 2009 and 2012 (raster resolution 30 cm Military Topographic Institute). These materials have been georeferenced for further processing in the ArcGIS 9.3 program which was used for all procedures applied in this study. ArcGIS 9.3 played a major role in producing geo-spatial data in digital form, as well as in storing digital data, thus accessibility of data modification and manipulation. GIS application was also used in producing thematic maps (location of the study area, map of slope inclination, slope orientation and geological units of the study area, map of spatial representation of the evaluated DMF categories and image of avalanche chutes within the study area – Figures 1, 2, 3 and 5) as well as those geospatial data derived from satellite images processing. The formations of DMF have been vectorised on a scale of 1:200 and classified into following categories:

Figure 4

Proportional (percentual) change of individual categories of DMF since 2003 to 2012 compared with 1961.

Figure 5

Imagingof the study area and avalanche chutes (numbered 1-7) in 1961 and 2012

1. Surface with the tendencies to be destroyed (or threatened) by snow abrasion (slope surface exposed to the influence of avalanches, but more or less overgrown by continuous turf cover with no obvious signs of destruction);

2. Surface in the initial stage of destruction (disrupted vegetation cover with bare bedrock up to 30% of the area);

3. Surface with partially bare bedrock (significantly degraded surface taking 31-70% of the area, turf fragments);

4. Surface with a total bare bedrock (continuous vegetation cover is missing almost completely, bare bedrock taking 71-100% of the area);

5. Erosive-avalanche chutes completely abraded by snowpack (7 significantly carved out erosive-avalanche chutes with total absence of soil);

6. Erosive-avalanche chutes with debris (erosive-avalanche chutes filled mainly with loose and partially toughened debris, occurring soil and vegetation cover);

7. Debris covered area (the area with piled up debris localised off erosive avalanche chutes).

We have also focused on the area, currently covered with wood vegetation and turf that are exposed to the permanent influence of polygenetic processes of surface destruction and are bound to avalanche kettle holes. Within the vegetation category, we have also evaluated the subalpine and alpine meadows, yet treeless in 1961.

A detailed analyses of the individual DMF categories may give a clearer vision of the current geomorphological processes. Their transformation into other categories provides data about the slowing or intensification of destructive processes. The transformations were observed in 1961-2003,2003-2009 and 2009-2012. Within each DMF category, we have evaluated the relative (percentual) change into a different category. For this purpose, the DMF layers from individual periods were converted into raster. In order to capture changes as precisely as possible, the cell size of 1 dm² was chosen.

Special attention was paid to characteristics of seven avalanche chutes. Our goal was to analyse their changes at the same time periods as the dynamics of development of individual DMF was examined. The subject of the research was to examine changes in their lengths, which can thus provide the data about the intensity of present geomorphological processes. Their greatest lengths, measured from the beginning of the avalanche chute to the end of the last destructed structure in their gravitational unit, were vectorised in ArcGIS 9.3.

To understand the dynamics of the development of destructive morphogenetic forms at the end of Hornojelenská valley, it is necessary to analyse data on the relief, exposure, geological bedrock and soil types in detail. Therefore, the first part of this paper mostly deals with the abiotic characteristics of the study area. The data about the relief was acquired by means of the digital elevation model analyses with a spatial resolution of 10 m. Equally, maps of slopes and slope exposures were acquired from the model as well. The data of geological bedrock were obtained from geological map in the scale 1:50 000 [9]. The characteristics of soil types were obtained from field survey [13].

The basis for assessing the dynamics of DMF was the aerial survey photo showing the condition of avalanche kettle in 1961. It provides data about the area of greatest DMF extension at the end of the Hornojelenská valley just before planting avalanche control woody vegetation and taking biological-technical measures. The current geomorphological processes can be seen on satellite images taken in 2003, 2009 and 2012. The photos from 2003 were chosen on purpose to capture the development of DMF approximately 40 years since the time avalanche control measures were put into practice. During that period, the planted woody vegetation took over the basic avalanche control function and its impact should become obvious regarding the intensity of soil degradation processes.

Development and dynamics of DMF categories

Destructed formations within the research area were generated out of the map data in the form of polygons using four time horizons (1961, 2003, 2009, 2012). Subsequently, they were classified according to destructive morphogenetic forms. Detailed information about the extent and relative representation of individual DMF in selected time horizons is presented in Table 1. A spatial representation of the evaluated categories according to individual years is illustrated in Figure 3.

The largest area of destructed surfaces was recorded on the aerial images from 1961. Based on the analysis of de–structed formations in 2003, their total area has decreased by 27%. The significant reduction in the area of DMF is referred to as the result of biological-technical avalanche control measures taken [12]. The results from other time horizons, compared with conditions in 1961, show a gradual expansion ofDMF to almost 90%. The gradual increase in degraded areas is predominantly being caused by climatic factors induced geomorphic processes [4, 12, 14]. Especially the presence of heavy and wet snow in the spring period [15], which disrupts a continuous vegetation cover [4], opens the way for the realisation of other destructive processes [16].

Woody vegetation above the avalanche kettle consists mostly of dwarf mountain pine and spruce. Although the woody vegetation is old enough to fulfil its avalanche control function [17, 18], it still cannot fully prevent the break-off and formation of avalanches in case of large snow-pack. To fully serve the avalanche control function, the forest stands above the avalanche break-off line do not yet have the corresponding structure to meet the natural forest structure of timber line. The plants of arborescent stature lack the sufficient thickness of [12], and thus decrease the avalanche control efficiency of vegetation. The results of the analysis dealing with surface degradation covering the last 10 years reveal that the DMF area tends to expand.

The factors that directly affect the morphogenesis of high mountains surface have already been monitored in the study area for several decades [4, 19]. Their indirect effects on geomorphological and sedimentary processes are less known [20]. To take a measurement of meteorological factors (such as the snowpack depth, amount and intensity of precipitation, air temperature, wind speed, etc.) as well as their manifestations depending on the geoecological characteristics of the monitored area (which include the attributes of snowpack in situ), is very complicated particularly due to extreme conditions and their considerable spatial and time variability [21]. Alpine geomorphological hazards such as avalanches, landslides and other demonstrations of slope instability may result from human activities as well as climatic variability factors, acting alone or in combination [22]. While the management of both landscape and human activities is important when considering the standpoint of the anthropic impact on the adverse effects of natural hazards, climatic factors play a significant role in the formation of geomorphological risks by affecting all the landscape elements [23].

Detailed analysis of Table 1 revealed the gradual expansion of DMF within the categories of the areas with completely and partially exposed surfaces, areas at the initial stage of soil and vegetation destruction, as well as the increase in the area of erosive-avalanche chutes with debris completely abraded by snowpack. Figure 4 shows their common characteristic - even though the area of these DMF categories in 2003 was smaller relative to the extent observed in 1961, they have been gradually enlarging. Regarding the areas with partially exposed surface and areas with erosive-avalanche chutes completely abraded by snowpack, the area has even enlarged when compared with the conditions monitored in 1961. Figure 4 illustrates the percentual changes of area of the evaluated categories during years 2003, 2009 and 2012, compared with year 1961. The percentual coverage of DMF categories in year 1961 was used as the standard. Figure 4 shows the fluctuation of the evaluated categories and their trend since 1961. The zero value on the “x” axis presents the state of the evaluated categories in 1961. The “y” axis shows the percentual change of the categories in years 2003, 2009 and 2012.

Erosive-avalanche chutes completely abraded by snow-pack. The first monitoring of aerial image from 1961 already suggested that the series of seven avalanche chutes would be classified as the areas with the most destroyed surfaces. When monitored in 2003, their area had decreased by 335 m². However, due to the influence of polygenetic destructive processes the area enlarged again in 2009 by 449 m². Compared with 1961, the present area is larger by more than 1,575 m². The enlargement of avalanche chutes can be observed predominantly in the direction of gravity, while the significant extension can be also seen in the upper parts - so called collecting areas which slope down the chutes in a funnel shape.

The enlargement of this DMF category might have been caused mainly by water-gravitational and snow-gravitational processes. During the course of the last 40 years, forest stands planted above the avalanche break-off line have taken over the avalanche control function [12] and the major part of snowpack in situ remains fixed in their area. Among the snow-gravitational processes, glide avalanches (e.g. release of the entire snow cover), which are formed after overcoming the friction between the snowpack and the bedrock, represent the main reason of the chutes´ destruction. The increase in weight of a snow slab and consequent initiation of avalanches in chutes can be caused by additional precipitation or the load of other material coming from unstable snowpack from the upper parts of the slope. The small areas of the chutes hold relatively large masses of snow which completely abrade their surface. Their presence in the study area after afforestation can be explained by the observations of climate trends in mountain areas during the period from 1981/82 to 2007/08 [14]. The observations revealed that the altitude range of 1,000-1,500 m a.s.l. records significant decrease of solid precipitation at the expense of mixed type. It results in a heavier and thicker snowpack which, after being broken off, is referred to as a glide avalanche [6]

The kettle at the end of the Hornojelenská valley is primarily perceived through the prism of avalanche events. Water-gravity processes seem to be significant for the surface development of this area (the period since the stabilisation of effective avalanche control vegetation). These processes become severe especially in the warm half of the year, when their destructive forces demonstrate themselves in the form of splash erosion processes and concentrated surface run-off (coming from rainfall and melting snow [24]). The phenomena bound to the running water activity in conditions of cold climate are actually polygenetic forms of surface destruction [25]. They are created simultaneously by water from snowmelt, rainfall, solifluction, ground needle ice and by wind eventually. Therefore, it is difficult to separate demonstrations of water erosion itself from other erosion and gravitational-erosion processes; and to subsequently determine the proportion of water erosion to the soil destruction objectively. However, the surface run–off is currently the dominant factor of surface destruction [4].

During the course of observed time horizons, both the area and length of chutes had been changing. Geomorphological processes caused formation of the seven major avalanche chutes at the end of the Hornojelenská valley, the longest of which is 366 m long (2012). Their detailed characteristics in individual years are presented in the Table 2. Avalanche chutes (Figure 5) have been numbered from bottom to top.

Table 2 shows that chute no. 3 is subject to the greatest influence due to the present geomorphological processes. Compared with 1961, its actual length has extended almost three times. Partially, it is a result of a local destructive process that led to the revealing of partially tough deluvial sediments at the bottom of the chute [26]. The vegetation masking/camouflage function plays a significant role in the chute’s length identification. Luxuriant vegetation can cover relatively narrow forms of surface destruction and prevent their precise identification from satellite images. Thus the quasi-extension of the chute was visible on the image from 2003. In addition, its bottom deepens as a result of geomorphological processes. Compared with an adjacent slope, the bottom is carved in its deepest point by up to 12 m. Considering the deforestation of the study area in the 15^th century (at the beginning of the 16^th century at the latest [27]), the average intensity of the chute’s vertical erosion is 20-24 mm/year during the 500-600 year long absence of the forest. Lower values of denudation in the same location (the average of 16.74 mm/year) are provided [4]. The higher speed of erosion furrows formation was observed in the Kopské saddle area in Belianske Tatry Mts., which is comparable to the geological conditions of the study area [19].

Erosive-avalanche chutes with debris. These are erosive grooves that are found on less steep slopes where the accumulation of material mostly occurs [25]. From the first monitoring, their area had been slightly increasing. However, in recent years (2009 and 2012), we can talk about their stagnation. Their further development strongly depends on following the climatic conditions [28]. In case of longer periods with no significant avalanche events and/or with no torrential rain precipitation, the condition of erosive-avalanche chutes with debris is expected to stay the same, or more precisely a slight decrease in their area can be expected. Another option is the increase of their area due to intense destructive processes in the header of avalanche chutes, and thus the chutes are silted with debris. The third possibility is a reduction/decrease in their area at the expense of erosive-avalanche chutes completely abraded by snow cover category, or more precisely their shift in the direction of the slope.

Area covered with debris. Similarly to the former category, there is a strong uncertainty of further development in case of this category. Table 1 shows that the increase of the area in 2003 as well as the decrease in subsequent years is evident. Nowadays (2012), the area is smaller by 218 m² compared with the beginning of the monitoring.

The reduction in size of the area with debris is explained by its overgrowth of luxuriant vegetation. Mainly, it is the result of successfully performed avalanche control measures which eliminate the influences of the major destructive factors - avalanches. Consequently, they form favourable conditions for the vegetation growth, especially the woody one. Compared with 1961, the presence of vegetation (debris overgrowing) indicates the presence of a higher proportion of nutrients and soil components in the debris, which were shifted from the header of avalanche chutes with active soil degradation processes. The frost heaving caused by the needle ice in combination with processes of snow patch and water-gravitational erosion results in disrupting the cohesion of the soil and its subsequent transport through the chutes into the debris location [25].

Surface threatened by snow abrasion. This category covers the slopes under the line of avalanche break-off point covered with turf, without visible signs of disturbance. Their area reaches the peak in 2003 within the observed period from 1961 to 2012. The area of this surface type is reduced at the expense of extensively destroyed formations which are classified into categories of surface destruction at different stages. Table 1 shows a significant increase of the area (an increase by 1.1683 ha in 2003) after the avalanche control measures. After the effects of the main modelling factor - avalanches moderate, and the surface becomes consequently destroyed by crawling snow, ice and water-induced erosion. The trend of the increasing density of the snow cover [29] and its more intensive melting at the end of a winter season [14], probably contributed to the gradual expansion of DMF with areal forms of surface destruction. Smooth parts among the chutes are affected not only by a destruction caused by avalanche abrasion or snow cover movements [30]. These modelling factors are followed by the destructive influence of water erosion processes in a spring-summer period and cryogenic processes in the regelation period [4, 31]. From the point of potential water erosion of soil, the observed area is ranked among the most exposed areas. The average intensity of potential soil erosion by running water is 7.2 mm/year. The measured elevations of bare soil due to cryogenic processes at this locality reached 94 mm, while the elevation under the intact turf covers only 8 mm [4]. These phenomena is closely related to the frost melting of the destroyed turf cover and the enlargement of areal DMF categorised into the areas in the initial stage of destruction, DMF with partially exposed surface and DMF with totally exposed surface.

The expanse of all three categories of areally destroyed morphogenetic forms was decreasing after 1961. However, it has been gradually increasing since 2003 (Table 1).The area of DMF with partially exposed surface category started to expand. Between 2003 and 2009, the area reached the expanse it used to have in 1961. At present, the area is larger by 0.32 ha. That is probably due to the fact that this category represents a transition between the surfaces at the initial stage of destruction and total vegetation removal. This category is the most sensitive to any changes influencing the surface destruction. Its area is affected by the processes stimulating vegetation development as well as the increasing intensity of soil degradation processes.

The significant importance of avalanche control measures and their influence on the vegetation development is obvious in Figure 4. The development of predominantly grass-herbaceous vegetation is significantly reflected in decreasing areas in the initial stage of destruction. Compared with 1961, their area decreased by 9,787 m² in 2003, by 8,459 m² at present. It is also reflected in the development of surfaces threatened by snow abrasion. However, no major destruction of the surface can be seen. Moderation of an avalanche eroding effect has not a negligible influence on the reduction of areas with totally exposed surface. The reduction in the area by 0.5 ha was recorded in 2003. However, the decrease of such areas is possible only with the soil residues remaining and/or at the slopes with the inclination angle up to 25°.

Variability assessment of individual DMF categories

The qualitative and quantitative variability of individual categories are shown in Tables 3, 4 and 5. The horizontal direction shows the percentage changes of individual categories in favour of others. The values in the vertical direction show how much the category increased at the expense of others (expressing the percentage increase of each category).