Darjeeling hill areas is unique from environmental Eco-perception. The relief varies from 100 Mts. above sea level to the mighty Kanchanjungha. There are different climatic zones with distinctive attributes and there are endangered animals like red panda etc along with memory orchids and medicinal plants are available in this hilly region.
The Darjeeling hill area is formed of comparatively recent rock structure that has a direct bearing on landslides. The causes of the landslides vary from one locality to another. Heavy monsoon precipitation is however a very common cause of these disasters. More over soils of Darjeeling hill areas are extremely varied, depending on elevation , degree of slope, negetative cover and obviously geolithology . The natural system of erosion in the hill gets more complicated when man interferes.
As the mountains serve as the source of resources for the population residing in the hills as well as in the plains, the form of environmental degradation is quite extensive other particularly is applied to the extraction of timber and other forest produces, mining and agriculture are taken into account. As human population expands in the hills, forests are being depleted for the extension of agricultural lands, introduction of new settlements, roadways etc. The growing changes coming in the wake of urbanization and industrialization leave deep impression on the hill ecosystem; disrupting normal functioning.
Due to unprecedented growth of population during the last few decades in the Darjeeling hill areas, nature has started reacting sharply to the accumulated human guilt. Landslide hazards, especially during rainy season have become a common factor to the people of the hill.
The Hill areas of Darjeeling District are located within the Lesser and Sub – Himalayan belts of the Eastern Himalayas. The area is bounded by the Sikkim Himalaya in the north, the Bhutan Himalaya in the east and Nepal Himalaya in the west. The southern foothill belt is demarcated by a highly dissipated platform of terrace deposits extending along the east west axis. The inner belt is defined by a ridgeline stretching from the Darjeeling Hill to the west and Kalimpong Hill to the east, overlooking the southerly flowing Tista valley in between. Prominent rivulets contributing to the Rammam – Rangit basin, dissipate the northern slope of Darjeeling Hills.
Tista, Great Rangit, Mechi, Balason, Mahananda, Lish, Gish, Chel,Ramman, Murti and Jaldhaka.
Important Roads with length in km
- Siliguri-Mirik -Ghum Connector—-60 kms.
- Sevak-Tista bridge –Kalimpong—-60 kms.
- Tista Bazar to Darjeeling———-25 kms.
- Sukna- Pankhabari-Kurseong—-25 kms.
- Siliguri Darjeeling NH 55———-76 kms.
The Darjeeling Hill area represents a unique geo- environmental perception. The area of study is primarily composed of erosional landforms produced by southerly flowing streams, which have exposed a full cross section of different tectonic units. The form units are, however approximately the same throughout the hill area, having more or less uniform lithology, structure, climate, soil and vegetative covers. According to Mallet (1875), Audent (1935) the tectonic units are found to be in the reverse order of stratigraphic superimposition, and is represented by Siwalik and Gondwana systems. Towards the inner Himalayas, the thrusted sheets of Daling and Darjeeling group of crystalline rocks succeed these. The contact between different groups of rocks is represented by thrusts, dipping at high angles towards north.
A brief description of various formations of the Darjeeling Himalaya is given here under
Raised Terraces: A recent to sub recent formation form a fringe along the hills, especially at the confluences of the rivers. These terraces are composed of gravels, pebbles and boulders mixed with sand and clay. The formation is semi-consolidated, stratified along with the evidences of upheaval at places. This type of high-level terraces is also called the Terai. A 40 m high terrace is found in the Tista valley at Kalijhora.
Siwalik: The Siwalik system in the Darjeeling hill areas is comprised of mudstones, sandstones, shale and conglomerates along with the bands of shale and lignite. In the Hill Cart Road and along the Tista River a few stretches of good exposers of Siwalik are found. The general strike of these rocks is NNE-SSW to NW-SE with dips varying between 30º to 60º.
Damuda Series: Just after Siwalik, coarse-grained hard sandstone, quartzites, carbonaceous shale and slates belong to Damuda series are found. The Damuda series of Darjeeling hill areas is equivalent to the Gondwanas of Indian peninsular region. The maximum width of the Damuda is about 2.5 km along the Tista valley. The maximum thickness is about 1000m. The general strike of the bed is from ENE to WSW, with a varying dip of 40º to 90º. In this belt coal seams of about 3 mt. are found near Tindharia region, Lish and Gish Rivers.
Daling Series: This series is comprised of chlorite shales, phyllites and schist associated with quartzite, which rest over Damuda series. Well-developed form of Damuda series is found along the Tista River and the stretches along the Tindharia –Paglajhora on the Hill Cart Road. The rocks are occasionally traversed by quartz and feldspar veins. The most important feature of this series is increasing metamorphism upwards, where slates form the lowest bed.
Darjeeling Gneiss: In the higher reaches of the Darjeeling hill areas, the Dalings gradually grade into the more metamorphosed rocks, which is known as Darjeeling Gneiss. The dips of the rocks are irregular and vary in between 40º – 70º. Darjeeling gneisses are highly foliated due to metamorphism. There are two prominent sets of joints in the Darjeeling gneiss, one running roughly NW-SE and the other NNW-SSE. The general direction of the hill spurs is in accordance with the joint directions.
The soils of Darjeeling Hill area have developed depending upon the underlying geological structure. But, in general the soils have been developed by both fluvial action and lithological disintegration. The soils that have developed in the Kalimpong area are predominantly reddish in color. Occasional dark soils are found due to extensive existence of phyllitic and schists. Soils in the highlands stretching from the west to the east of the district along most of the interfluvial areas are mainly mixed sandy loam and loamy, while those on the southern slopes of Mirik and Kurseong are mainly clayey loam and reddish in color. Sandy soils are mainly found in the east of the river Tista.
All the soils are definitely acidic in nature with the tendency to increase slightly in depth in most cases indicating the lacking of bases from surface and accumulation in the lower horizons. The weathering of lateritic type is the substantial mechanism in the transformation of the substratum. The variable thickness of the regolith and soils depend on the rate of weathering and gradient of the longitudinal slope profiles and intensity / gravity of mass movements. The basic soil types are yellow soils, red brown soils and brown forest soils. Red and yellow soils have developed on gneiss while brown on schists and shales. Coarse pale yellow to red brown soils are found on the Siwaliks while clayey dark soils are developed on Daling series.
The character of the bedrock is reflected only in the grain size composition of the soil. On the Darjeeling gneiss, very coarse-grained (50% -80%) particles are found. In Damuda and Daling series percentage of sandy and coarse particles in the soils are high. On the Siwaliks, silty – clay fraction is higher. The chemical content of the soil over Darjeeling gneiss is characterized by a high proportion of potassium derived from feldspar and muscovite mica. This soil is poor in lime, magnesium, iron oxides, phosphorous and nitrogen. Therefore lime is used in the tea plantation areas.
The amount of rainfall plays a very important role in causing instability of slopes. A very high intensity of rainfall within a short span of time is not uncommon in Darjeeling hill areas. It is found in the old records; that this natural phenomenon has occurred about 42 times during the period from 1891 to 1975 (Chatterjee 1982).
The isohyets, maps prepared on the basis of average annual rainfall during last 25 years in 3 subdivisions in Darjeeling hill areas, shows that the value increases from west to east, a maximum concentration of landslides fall between 210cm and 410cm of Isohyets.
Besides seasonality, another climatic feature in the Darjeeling hills is created by orographic factor; causing the vertical zonation of temperature and decline of precipitation. Thus the mountain front is exposed to heavy rainfall, especially the middle parts of the southern hills. The mean annual temperature fluctuate from 24ºc in the plains and drops below 12ºc on the ridge. During summer month the temperature reaches 16ºc-17ºc on the ridge and during winter drops at 5ºc-6ºc.
There is no distinct relation between total rainfall and altitude. The southern slopes of the ridges get much higher (4000-5000mm) precipitation than the leeward sides (2000-2500mm). The next main ridge with Tiger Hill gets 3000mm while to the north the Great Rangit valley receives about 2000mm of rainfall. The annual total rainfall in Darjeeling town fluctuates between 1870-3690mm.
In respect of landslide hazards, the duration of rainfall is very important. Long duration along with heavy down pour may cause deeper infiltration and overland flow, which ultimately may result into the occurrence of landslides on weaker slopes. The records show some of the long continued down pours. Amongst them the most remembered ones are in 1787,1789, 1827 (493 mm in one day) and in June 1950 (965 mm). The last such rainfall recorded during 1968 (2nd and 5th Oct – about 1780 mm). Thereafter, 358 mm in Oct 1973, 382 mm in June 1983, 457 mm in September 1986 and 350mm in 1990 were recorded.
Forests in Darjeeling Hill Areas
The principal economy of Darjeeling Hill Area depends on tea production, horticulture, agriculture and forestry. The major portions of the forests are today found at elevations of 2000 mts and above. The area located in between 1000-2000mts is cleared either for tea plantation or cultivation.
The four major forest types according to altitudinal variation found in Darjeeling Hill Areas are:
Tropical moist deciduous forest (300-1000mts)
Tropical evergreen lower montane forest (1000-2000mts.)
Tropical evergreen upper montane forest (2000-3000mts.)
Temperate forest (3000-3500mts.)
Sub temperate forest (above 3500mts.)
About 30% of the forest covers found in the lower hills are deciduous. Evergreen forest constitutes only about 6% of the total forest coverage.
Shora robusta remains the most prominent species of Tropical moist deciduous forest along with heavy under growth. In the slopes on southern portion of the Tista and the Great Rangit valley and in the Goke forests, this type is found. These species cannot thrive in areas of lower precipitation.
Tropical lower montane evergreen forests are found on steep higher slopes, where drainage condition is good; Dhupi (Cryptomaria Japonica) is a known variety. The impact of man on this variety is very conspicuous.
Tropical upper montane evergreen forests are found in the areas where high humidity along with dense fogs and less sunlight is available. Undergrowth is dense and contains Nettles, Raspberries, Ferms and bamboos. On the steep ridges, Rhododendrons and bamboos are abundant.
Present Status of Forests
Prior to 1863 very little attention was paid to the conservation and afforestation programmes. Darjeeling district had 11,000 hectare of Reserve Forest up to 1879. But, after independence due to rapid urbanization, the upper belt of the forest was taken for commercial use. Much of the natural forests in the Senchal, Ghum-Simana and Takdah ranges have been converted. Some patches of natural forests are still found in Reshop, Bara Senchal, Lopchu, Rongbong and Durbin Gram Panchayats. On the difficult terrains, still a few natural forest patches are found.
The analysis of the deforested areas indicates that landslides are most common in these tracts. Therefore, the areas with intense deforestation may be superimposed on the landslides affected area map in support of this hypothesis.
From the map it appears that the followings are the intense deforested areas, which are again landslide-affected area.
- The forest area within the tea garden areas.
- The forest area along the principal thoroughfares including NH55.
- The forest area at the fringe of the urban centers / settlement.
The hill areas of Darjeeling District is divided into 2 forest divisions, viz,– Darjeeling, Kurseong. The growing pressure of population during the last two decades has left clear marks on the forest resources of the region. Marked decline in forest cover were observed in Takdah-Ghoom-Simana- area of Darjeeling Sadar, Sukhna, Pankhabari regions of Kurseong division.
Land- Use Pattern in the Hill Areas
The land use practices play the most important role in determining the stability factors in respect of landslide hazards. The land use map of Darjeeling Hill Areas explains that there are agricultural activities, tea and medicinal plant plantations, construction works along with forests, rivers, jhoras etc.
The main problem in respect of land use in the Darjeeling Hill Areas is related to high density of population. There is very limited scope for extension of agricultural land to cope up with increasing pressure of population. As a result pressure on forested and other restricted areas is gradually increasing.
Another problem related to land use and consequent landslide is that in Darjeeling Hill Areas, roads have never been examined with its carrying capacity respect with geology etc. Along with new road construction the vehicular movements have increased to a great extent with the rapid growth of trade and commerce. Heavy traffic movements along with heavy rainfall are responsible for most of the landslide occurrences especially on the roads. In recent years, it has been observed that there is a constant increase in the vehicular traffic, especially heavy vehicles like trucks and buses. The record reflects that at present, the number of registered vehicles in the hill subdivisions are more than 6500.
According to the District Gazetteer of Darjeeling district (1980) the road lengths (in km) in the district are as follows;
- National Highway———— 100km
- State Highway————— 80km
- Major district road ——— 37km
- Ordinary district road—— 516km
(the figure includes Siliguri subdivision)
During the last 50 years, the length or pattern of the roads in the hill subdivision did not change significantly, though there has been at least 5 to 7 times increase in the number of vehicles, especially goods vehicles like trucks etc. It has been observed that even during night times, the traffic movement on the roads continues. As a result, due to constant lateral vibrations, the weaker geological structure has become unstable. Beside the above-mentioned roads, there are many roads, which are maintained by the Forest Department. Mention should be made about some of them like: (1) the Cart Road from Sukhiapokhri to Maneybhanjan and Batasia, (2) Cart Road from Simkona to Lalkuthi in Darjeeling Forest Division (3) Sukna-Sevok Road in Kurseong Forest Division (4) the Rassium – Labha Cart Road, South Boundary Cart Road, Central Cart Road.
Land Slide in Darjeeling Hill Areas
The Blockwise landslide affected areas explain itself the comparative intensity of landslides in the blocks. The rate of vulnerability is also high in Kurseong and some parts of Bijanbari and Gorubathan blocks. Moreover, these areas are cultivated with root crops like potato, ginger, cardamom and onions. These root crops are harvested just after monsoon in the months of September – October. This particular practice changes the cohesiveness of the soil and makes it vulnerable to erosion.
The slope instability factors along the main thoroughfares due to heavy vehicular movements is another big problem that causes frequent land slides along the roads, especially during rainy season.
The problem of quarry operation and their effect on environment is also another threatening problem and was studied thoroughly by Prof. S.R Basu. The illegally operated quarries along the Lish and Gish basin and Tindharia region not only disturbs the slope stability but also overburdens the rivers and their tributaries with excessive amount of load that ultimately leads to massive siltation along the river beds and the adjoining plains. Thus they destroy the ecological equilibrium of the area. Other than coal mining, stone quarrying from the slope especially under the road is another way of human intervention that causes occasional slope failure. The recent landslide near Giddapahar may be attributed to the effect of stone quarrying under the main thoroughfare.
The Critical Area Zonation map has been prepared by examining the geology, soil, and climatic factors along with land use pattern. It appears from the map that the Grid no. E4 under Rangli-Rangliot block are the most landslide prone areas, where human intervention is maximum.
Possible causes of Landslide hazards in the Darjeeling Hill Areas
- The trends of evolution or rising of young mountains is the basic reasons for frequent landslide hazards in the Himalayan region. This includes unstable geological structure, tectonic disturbances, parallel subsidence of Himalayan fore deep of slopes.
- Soil erosion and its conservation play an important role in the hill areas. Because of the presence of very thin soil cover plays an important role in the socio economic development of the hills and its people. All India Soil and Land use Survey under the Ministry of Agriculture, Govt. of India had carried out studies in some specified watershed areas. Otherwise, no systematic soil mapping has been carried out in the region. As such, there is no database, of how much soil cover has been destroyed.
- The soils of Darjeeling hill areas have developed mainly Darjeeling gneiss, schists and Phyllites. Due to heavy deforestation and excessive cultivation of root crops like ginger, potatoes, onions, cardamoms etc. the extent of soil erosion has increased considerably in the recent times. It is a fact that the entire Darjeeling hill areas do not get any soil deposition. Deposition of soil is only found in the river valleys. Thus, the prevention of soil erosion and conservation of soil is very necessary in the hills.
- By examining the land use pattern and changing characteristics since the last 150 years, it may be commented that the forest cover is in a precarious condition due to the rapid increase in cultivated land (with the exception of tea gardens), expansion of settlements, construction of roads. The rapid depletion of forest cover is noticeable in the tea plantation area. In most of the tea gardens in the hills, any type of shade tree or trees along the fringe line of the garden for the protection of the soil is more or less insignificant.
- Rapid expansion of settlements and towns especially along the roads is one of the important causes of frequent landslide hazards in the hills. Multi storied buildings without proper planning along the roads and on the steeper slope increase the load on the already deteriorated slopes.
- In the rural and inaccessible high hills. Demand for fuel is another important factor, which may be treated as an important cause for slope failure. Unscientific mining of low energetic coal seams and illegal felling of trees to meet the demand of firewood is practically unavoidable in the hills.
- During the last 2 decades there has been an unprecedented growth of population in the hill areas, especially in the towns. The explosion has been followed by the rapid increase in vehicular movements. The continuous horizontal vibration along the roads gradually destabilizes the already unstable slopes and geological formations.
- Lastly the demand of water for domestic and commercial purposes has also increased. The forest clearance, dissection of the upper portions of the slopes are reflected in the decrease in ground water level and consequent drying up of the streams during most part of the year.
- Examining the above mentioned analysis, the future of the Darjeeling hill areas does not look very bright. Systematic and scientific utilization/management of the natural resources is required.