Q) “Landslides are seen in most of the North, North Eastern and South Western states of India.” Illustrate. Also explain the causes and suggest measures of mitigation.
India has the highest mountain chain on earth, the Himalayas, which are formed due to collision of Indian and Eurasian plate, the northward movement of the Indian plate towards China causes continuous stress on the rocks rendering them friable, weak and prone to landslides and earthquakes.
The slow-motion of the Indian crust, about 5 cm/year accumulates stress to which natural disasters are attributed. Some landslides make unique and unparalleled catastrophes. Landslides and avalanches are among the major hydro-geological hazards that affect large parts of India besides the Himalayas, the Northeastern hill ranges, the Western Ghats, the Nilgiris, the Eastern Ghats and the Vindhyans.
Landslides in North, North Eastern and south western states in India:
- covering about 15 % of the landmass The Himalayas alone count for landslides of every fame, name and description- big and small, quick and creeping, ancient and new.
- The North-eastern region is badly affected by landslide problems of a bewildering variety. Landslides in the Darjeeling district of West Bengal as also those in Sikkim, Mizoram, Tripura, Meghalaya, Assam, Nagaland and Arunachal Pradesh pose chronic problems, causing recurring economic losses worth billions of rupees.
- A different variety of landslides, characterized by a lateritic cap, pose a constant threat to the Western Ghats in the South, along the steep slopes overlooking the Konkan coast besides Nilgiris, which is highly landslide prone.
Landslides are frequent in Himalayas because:
- Heavy snowfall in winter and melting in summer.
- This induces debris flow, which is carried in large quantity by numerous streams and rivers.
- Himalayas are made of sedimentary rocks which can easily be eroded. These aides landslides.
- Drifting of Indian plate causes frequent earthquakes and resultant instability in the region.
- Man-made activities like grazing, construction and cultivation abet soil erosion and risks of landslides.
- Himalayas not yet reached its isocratic equilibrium which destabilizes the slopes. It leads to landslides.
- Diurnal changes of temperature are much more in northern India than in southern slopes. This weakens the rocks and aids mass wasting.
But landslides are less frequent in Western Ghats when compared to Himalayas
- Western Ghats are eroded, denuded, aged, mature and worn out by exogenic forces. Moreover, they are far less in height. Hence landslide occurrence is rare.
- Less occurrence of earthquakes because they’re on more stable part of Indian plate.
- While steep slope on western side with high rainfall creates idea condition for landslide but gentle eastern slope with low rainfall and rivers in senile stage, counters the condition.
- W Ghats are older-block mountain therefore they are more stable now. Moving of Indian plates doesn’t affect them.
- Small & swift flowing streams of western side and big matured rivers on eastern side (like Krishna, Godavari, etc) cannot carry large amount of debris.
- Western Ghats have lesser man-made interference as of now than what Himalayas have faced.
- Himalayan landslides can be attributed to the fact that Himalayas lies at the convergence zone of two lithospheric plates, i.e., Indian plate in the south and Eurasian plate in the north. Thus geologically, it is considered very active.
- Himalayan regions are prone to frequent earthquakes leading to loosening of soils that further leads to landslides.
- Himalayas haven’t yet reached its isostatic equilibrium and hence under constant influence of earthquake which can trigger landslides.
- Himalayas are mainly composed of sedimentary rocks which can get disturbed under slightest of stress and slide down.
- Himalayas are loftier than other mountain ranges and have greater slope due to which soil once loosened, triggers a full blown land or mud slide.
- Himalayas have numerous rivers and streams which can carry large amount of debris with them. Sometimes they loosen up big boulders and parts of mountains and bring them down with themselves.
- Himalayas have large amount of snow which can become unstable due to heat from sun and trigger landslides (avalanches).
- Construction activities like road and tunnel construction not only loosens the soil, but they also require blasting due to which the mountains become really fragile and prone to landslides.
- Global warming has led to quicker melting of snow and more percolation of water within the underlying surface of hill.
- Inappropriate agricultural practices like shifting or jhum cultivation, which require clearing of forests for agriculture, makes top soil prone to movement by removing the trees which bind them.
- Himalayas being source of many rivers has lead to construction of multipurpose dam projects which has affected the already fragile Himalayas.
- Himalayan region is centre of huge diversity when it comes to trees & this diversity has led to discriminate chopping of trees leading to soil erosion which in turn leads to Landslides.
- Illegal mining & Industrial activities too have contributed a lot when it comes to reasons of landslides in the region.
Landslide mitigation can be done
- By restricting or even removing the population from landslides prone areas.
- By restricting certain types of land use where slopes are vulnerable.
- By installing early warning systems based on the monitoring of ground conditions such as strain in rocks and soils, slope displacement, and groundwater levels.
- Landslides pose a recurrent hazard to human life and livelihood in most parts of the world, especially in some regions that have experienced rapid population and economic growth.
- Hazards are mitigated mainly through precautionary means—for instance, by restricting or even removing populations from areas with a history of landslides, by restricting certain types of land use where slope stability is in question, and by installing early warning systems based on the monitoring of ground conditions such as strain in rocks and soils, slope displacement, and groundwater levels.
- There are also various direct methods of preventing landslides; these include modifying slope geometry, using chemical agents to reinforce slope material, installing structures such as piles and retaining walls, grouting rock joints and fissures, diverting debris pathways, and rerouting surface and underwater drainage. Such direct methods are constrained by cost, landslide magnitude and frequency, and the size of human settlements at risk.
Vulnerability to landslides can be evaluated only if we know the exposure to landslide hazard and our preparedness to face that hazard. Vulnerability will be close to nil in the case of well managed and protected slopes.