SLOPE STABILITY EFFECT OF FORESTS IN GARHWAL HIMALAYAS, INDIA
The Himalayan mountain belt extending 2500 km in length and 250 km in the width stretches in arcute shape, (Pal and Sah, 1987), all along the northern border of the India. Due to increasing demand of its natural wealth for the betterment and economic development of the country in general and for the growing need of its own habitants, the biotic pressure on land and other resources has increased. Garhwal Himalayas is bristling with the problems of surface erosion and slope instability. All in all, the immature and complex geology, seismic activities, heavy rainfall including cloud bursts and flash floods, indiscriminate construction and forestry activities combine to create slope instability problems. Geographically the area falls in newly created state of Uttaranchal (previously know as hilly region of state of Uttar Pradesh), India; and lies between 20º 26' North to 31º 28' North and, 77º 49' East to 80º 06' East. Garhwal Himalayas has a remarkable heterogeneity in all its natural and cultural environments. The terrain is evidenced by a variety of land-forms viz. Snow peaks, high mountains, steep escarpments, deep gorges, hanging valleys, glaciers, lakes, numerous quashing streams and swift cascading rivers, lush green meadows and vast forested tracts. Broadly speaking the entire region is largely a rugged and mountainous country.
Kharakwal, divided Garhwal into three major physiographic units, viz. Siwalik or Outer Himalayas, Himachal or Lesser Himalayas and Himadri or Greater Himalayas. The study area is endowed with a wide range variety of climatic and latitudinal conditions and the flora and fauna of the region show a consequential variability. The flora is broadly divisible in to sub tropical, temperate and alpine type, which roughly corresponding to Outer Himalayas, Lesser Himalayas and Greater Himalayas respectively. The Outer Himalayan in the study area is characterised by thick forest this zone is largely dominated by Dalbergia sisu and Shorea robusta .The lesser and the greater Himalayas in study area is represented by Pinus roxburghil, Quericus incan, Q. dilatata, Q Semecarpipla, Pinus excelsa , Albies pindow, Betula utilis and Rohododendron.
The nature and characteristics of soil at different locations in Garhwal are different. Leaving area under cultivation, the soil covers generally thin. The valley structures consist of both alluvial and diluvial materials. Cultivated areas with moderate slopes have a thicker soil cover consisting of relatively fine soil. These cultivated areas are generally on river terraces. It has been observed that the precipitous slopes are generally without soil cover. Whatever soil is found on these cliffs, exists in cracks, joints and along the foliation planes. Coarser soil is found to be scarce on the valley slope. Fine soil is found to be abundant on moderate slopes, while coarse soil in abundant on sleep slopes.
The High level of sediment discharge in to forests streams and rivers as well as resulting degradation in the aquatic habitat have widely been blamed on intensive and extensive forest management activities. Additionally, reduction in site productivity of forestlands can be related to accelerated erosion processes, both surface erosion and mass wasting. The burden of these environmental impacts that has been placed on forestry activities may be over stated in many cases but, depending on the particular practice and region, may be partially or totally justified. For example, in 1970, during monsoon rain, the Alaknanda river (major tributary of Ganga, in Himalayas) in Garhwal Himalayas, rose 60 feet, flooding hundreds of square miles. The waters swept away hundreds of homes, including one entire village, as well as 30 passenger-laden buses and five major bridges. Almost 200 people died. When the waters reached the plains, over 100 miles downstream, they dropped silt that clogged nearly 100 miles of canal, halting irrigation and power production in the northern part of the state for six months before the residue could be cleared. It was clear that this disaster was not strictly natural. The chief cause was the commercial lumbering that had expanded so dramatically after India's war with China. When mountain slopes were cleared of trees, rains washed away the topsoil, leaving the soil and rocks underneath to crumble and fall, in landslides. Much of the soil from the mountain slopes was deposited in the rivers below, raising the water level and forming temporary dams that could burst under pressure. At the same time, the bare slopes allowed much more rain to run off directly into the river. The end result was devastating floods. According to an estimate there are on an average 2 landslides every km² in Garhwal Himalayas and we tend to add one more every 5-sq. kilometre. The mean rate of land loss is to the tune of 120 m / km²/ year and annual loss of land is about 2500 tonnes for every km² of area.
Landscape-level models are increasingly being used to evaluate effects of alternative management practices on key geomorphic and biological processes. Where forest practices are of interest landscape models need to be sensitive to the wide range of potential influences and feedback between vegetation and geomorphic processes such as landslides, debris flows, and channel evolution. Landslide modelling requires detailed knowledge of the landscape, e.g., topography, vegetation, and soil. Despite advances in computing power, detailed process modelling of large areas at fine discretization remains intractable.
The nature and characterctics of soil at different locations in study area are different. Leaving area under cultivation, the soil covers generally thin. The valley structures consist of both alluvial and diluvial materials. Cultivated areas with moderate slopes have a thicker soil cover consisting of relatively fine soil. These cultivated areas are generally on river terraces. It has been observed that the precipitous slopes are generally without soil cover. Whatever soil is found on these cliffs, exists in cracks, joints and along the foliation planes. Coarser soil is found to be scarce on the valley slope. Fine soil is found to be abundant on moderate slopes, while coarse soil in abundant on sleep slopes.
I studied the problem of slope instability, landslides in particular, as induced by forestry activities. Following Caine and Mool, (1982), I adopt a very generalised definition of the term “landslides”. Most of the landslides that were encountered, are complex features. Broadly speaking, anything other than an established first order stream that has produced a conical pile of debris is called a “landslide”. These features may be rockfall, true slides, earthfloods, slumps, or collapse features associated with the road-cut or road protection work. I carried out a study where the slope stability of forest were determined from the incidence of slope failure by investigating the forest conditions, where the failure occurred frequently after heavy rains according to classification based on forest density (sparse forest, thick forest etc.), and forest constituents (shrubs, herbs etc.). It is in accordance to the theory that trees control erosion on slope, to stabilise landslide scars, and to absorb debris flow impacts. Roots due to their tensile strength and frictional or adhesive properties reinforce the soil, increase soil shear strength. Particularly, the trees roots may anchor into firm strata providing support with the upslope soil mantle through buttressing and arching.
The slope stability effect of forest were generally analysed from the incidence of slope failure by investigating the forest conditions in the study area where the failure occurred, according to classification based on type and density of associated forests. It as been found that from above point of view the most important factors responsible for landslide occurrences are alarming deforestation on hill slopes, overgrazing on slopes, obstructed or blocked drainage by construction and forestry activities. It is found that on most of slide faces the vegetal cover is absent. Only on those slides, which are fossil slide, or in process of stabilising sparse herbs and shrubs are found. It confirms that trees control erosion on slope, to stabilise landslide scars, and to absorb debris flow impacts. This confirms the theory that roots due to their tensile strength and frictional or adhesive properties reinforce the soil, increase soil shear strength. Particularly, the trees roots may anchor into firm strata providing support with the upslope soil mantle through buttressing and arching. This is high time that deforestation, and overgrazing should be immediately checked in Garhwal Himalayas. Moreover, afforestation should be increased in the valley, making use of grasses and plants of high root density.
Institute of Forest Ecology, LDF, Mendel University,
Zemìdìlská 3, 613 00 Brno, and Czech republic