NDMA guidelines on Landslides and Snow Avalanches
Landslide refers to downward and outward movement of slope materials such as rocks, soil etc. due to gravity. Primary reason of Landslides includes gravity and accumulation of soft soil, debris and rocks on a steep slope. The other factors that increase the risk include erosion, weakening of rocks and soils due to heavy rains, deforestation, earthquakes, volcanic eruption etc.
Landslide prone areas in India
As per the Geological Survey of India, around 15% of Indian landmass is prone to landslides. The landslide prone areas listed by the National Disaster Management Authority include Himalayan states, Arakan-Yoma belt in the north east, Meghalaya plateau, Western Ghats and Nilgiri hills. Various kinds of Landslides have been experienced in 22 states and parts of Union Territories of Puducherry and Andaman and Nicobar Islands.
The Himalayan regions and north east hilly tracts are most vulnerable because of tectonic reasons and on account of their being young fold mountains, created by the collision of the Indian landmass with the Eurasian plate. Himalayan mountains have rugged topography, high seismicity and high rainfall. Each of these reasons contributes to the region’s high vulnerability to landslides.
In comparison to the Himalayan states, Western Ghats and Nilgiri Hills are less prone. The major reason for Landslide vulnerability in Western Ghats is the deposits of overburden materials such as loose soil, tumbling stones, debris etc. on the steep hill slopes. The landslides and flash floods in this area are triggered by intense rainfall. Nilgiri hills, which are located at the convergence zone of the Eastern and Western Ghats are prone to landslides mainly due to overburden deposited on the slopes.
The Geological Survey of India (GSI) was declared as the nodal agency for landslides. The responsibilities as the nodal agency include coordinating all the activities related to landslide hazard mitigation, and monitoring the occurrence of landslides in the country.
Snow Avalanche
Snow Avalanche is a rapid, down slope movement of large detached mass of snow, ice and associated debris such as rock fragments, soil and vegetation.Small avalanches, or sluffs, occur in large numbers whereas large avalanches that may encompass slopes a kilometre or more in length with millions of tons of snow, occur infrequently bust cause lot of damage.
There are two basic types of avalanches viz. loose snow avalanches and slab avalanches.Loose snow avalanches form in snow masses with little internal cohesion among the individual snow crystals. Such an avalanche originates at a point and grows wider as it sweeps up more snow in its descent.Slab avalanches originate in snow with sufficient internal cohesion to enable a snow layer, or layers, to react mechanically as a single entity. A slab release may take place across an entire mountainside, with the fracture racing from slope to slope to adjacent or even distant slide paths.
Causes and effects of Snow Avalanches
Avalanches occur due to combination of fixed (prime factors) and variable factors (exciting factors). The prime factors include topographic factors and vegetative factors. The variable factors include weather conditions and the weight of the snow cover.
The force generated by a medium to large avalanches can damage or destroy man-made structures. The debris from even small avalanches is enough to block a highway or rail-road and in some cases passing vehicles can be swept away and destroyed. Generally, use of avalanche areas in the summer does not constitute any hazard.
Avalanche prone areas in India
In India, Himalayan region is well known for occurrence of snow avalanches particularly the Western Himalayan region (snowy regions of Jammu and Kashmir, Himachal Pradesh and Uttarakhand).
NDMA guidelines on Landslides and Snow Avalanches
The main objectives of the NDMA guidelines are to institutionalise the landslide hazard mitigation efforts, to make society aware of the various aspects of landslide hazard in the country and to prepare the society to take suitable action to reduce both risks and costs associated with this hazard. The recommendations include:
- Continuously updating the inventory of landslide incidences affecting the country.
- Landslide hazard zonation mapping in macro and meso scales after consultation with the Border Roads Organization, state governments and local communities.
- Pilot projects to be taken up in different regions of the country to carry out detailed studies and monitoring of select landslides to assess their stability status and estimate risk.
- Setting pace setter examples for stabilization of slides and also setting up early warning systems depending on the risk evaluation and cost-benefit ratio.
- Completion of site specific studies of major landslides and plan treatment measures, and encourage state governments to continue these measures.
- Institutional mechanisms have to be set up for generating awareness and preparedness about landslide hazard among various stakeholders.
- Training and capacity building measures to be taken up for professionals and organizations working in the field of landslide management.
- New codes and guidelines to be developed on landslide studies and existing ones have to be revised.
- An autonomous national centre for landslide research, studies and management has to be established.
Implementation of above action points would increase efficacy in management of landslides and avalanches in the country. The above measures should be duly backed by requisite operational, legal, institutional, and financial support.
