Indian Himalayan Region (IHR)

The Himalayan states include 10 hill states- Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, Arunachal Pradesh, Nagaland, Manipur, Mizoram, Tripura, Meghalaya, and two partial hill states, namely Assam and West Bengal.

The Indian Himalayan Region (IHR), with geographical coverage of over 5.3 lakh kilometre square, extends over 2,500 kilometres in length between the Indus and the Brahmaputra river systems. The IHR physiographically, starting from the foothills in the south (Siwaliks), extends up to Tibetan plateau in the north (Trans-Himalaya). Three major geographical entities, the Himadri (Greater Himalaya), Himanchal (Lesser Himalaya) and the Siwaliks (Outer Himalaya), extending almost uninterrupted throughout its length, are separated by major geological fault lines.

The name ‘Himalaya’ has been derived from two Sanskrit words, Hima (snow) and Alaya (abode), ‘the abode of snow’. The massive Himalayan arc extends over 2,500 km between Nanga Parbat (8,126 m), in the west and Namcha Barwa (7,755 m), in the east. Physiographically, starting from the Siwalik Hills in the south, the Himalayan mountain range extends to the Tibetan plateau in the north. The broad divisions are the Siwaliks, the lesser Himalaya, the greater Himalaya and the trans-Himalaya; extending almost uninterrupted throughout its length, separated by major geological fault lines.

The Himalaya originated as a result of subduction of the Tethyan oceanic crust that lay between India and Tibet followed by continent-continent collision. A petrotectonic assemblage of the subduction complex consisting of the Shergol ophiolitic melange, the Nidar ophiolite, the Nindam flysch Formation and blue-schist rocks were obducted over the Indian margin during initial collision of the Indian continent with Tibet in the middle Eocene. The collision was followed by continued convergence of India against Tibet resulting in crustal shortening both in the Indian and the Tibetan margins. This crustal shortening produced regional metamorphism and polyphase deformation of the Higher Himalaya zone, the Tso Morari Crystallines and the Pangong Tso group. Continued convergence produced major folds, nappes and thrusts of the Higher, Tethys and Lesser Himalayas on the Indian plate and the Indus, Shyok and Karakoram zones of the Trans-Himalaya. The major structures include the Main Karakoram Thrust, Zanskar Thrust, Tso Morari dome, Zanskar synform, Sum dome, Kashmir-Chamba nappe, Panjal Thrust and MBT. The compressional phase was followed by an uplift phase in the early Miocene which formed a foredeep to the south in front of the uplifted cordillera. The Siwalik group sediments were deposited in this foredeep and were later folded and faulted. The moisture for snowfall in this part of the range is delivered primarily by the summer monsoon. The mountains form a natural barrier that blocks monsoonal moisture from reaching the Tibetan Plateau to the north. This makes the plains south of the mountains green with vegetation, while much of the Plateau is brown and comparatively barren. This range contains some of the world’s tallest peaks. Mount Everest, for example, is just one of nine peaks in the area that stand taller than 8,000 meters (26,247 feet) in elevation. The higher regions of the Himalaya are snowbound throughout the year and in spite of their proximity to the tropics, the perpetual snow level seldom falls below 5,500 m. The Himalayan ranges encompass numerous glaciers (9,575); notable among them is the Siachen, which is the largest glacier in the world except the polar region. The Himalayan glaciers are the source for several large perennial rivers, which in turn further define and shape the mountain configuration and drain into major river systems of the continent. The IHR, including the Himalaya proper and the north-eastern hill states, lies between latitudes 21°57' N and 37°5' N latitudes and longitudes 72°40' E and 97°25' E, covering an area of 5,33,000 km2. It stretches over 2,500 km from Jammu and Kashmir in the west to Arunachal Pradesh in the east, covering partially/fully 12 states of India, but its width varies from 150 km to 600 km at different places.

The region is vast, rugged and varied. The higher ranges remaining perpetually snowbound, overlooked by more than 13 peaks surpassing 7000 m in elevation. The IHR plays a vital strategic role in safeguarding the entire northern boundary of the nation. Apart from the national security standpoint, the Himalayan region is also important for its high forest cover. More than 65% of its geographical area is under forests, representing one-third of the total forest cover and nearly half (46%) of the very good forest cover of the country. More than 9,000 Himalayan glaciers and high-altitude lakes form a unique reservoir storing about 12,000 km3 of freshwater. This reservoir releases its wealth to the rest of the country, mostly in a manner that sustains the lives of millions, deep into the plains.

In the Indian Himalayan region (IHR), landslide-driven hazards have intensified over the past several decades primarily caused by the occurrence of heavy and extreme rainfall. However, little attention has been given to determining the cause of events triggered during pre- and post-Indian Summer Monsoon (ISM) seasons. One extreme rainfall landslide event occurred in Sadal village, Udhampur district, Jammu and Kashmir Himalaya, during September 2014. Toward the receding phase of the ISM (i.e., in the month of September 2014), an unusual rainfall event of ~488.2mm rainfall in 24h took place in Jammu and Kashmir Himalaya in contrast to the normal rainfall occurrence. Geological investigations suggest that a planar weakness in the affected region is caused by bedding planes that consist of an alternate sequence of hard, compact sandstone and weak claystone. During this extreme rainfall event, the Sadal village was completely buried under the rock slides, as failure occurred along the planar weakness that dips toward the valley slope. Rainfall data analysis from the Tropical Rainfall Measuring Mission (TRMM) for the preceding years homogeneous time series (July–September) indicates that the years 2005, 2009, 2011, 2012, and 2014 (i.e., closely spaced and clustering heavy rainfall events) received heavy rainfalls during the withdrawal of the ISM; whereas the heaviest rainfall was received in the years 2003 and 2013 at the onset of the ISM in the study region. This suggests that no characteristic cyclicity exists for extreme rainfall events. However, either toward the onset of the ISM or its retreat, the extreme rainfall facilitates landslides, rockfall, and slope failures in northwestern Himalaya.