Sep 26, 2023
Sep 26, 2023
Landslides and mountains are almost synonymous. This is one geo-hazard that is possible to predict, yet it paralyses the road communication network or buries entire township and kills en-mass. Generally, the landslides are sudden. However, suddenness is not the rule. Even slow moving landslides or soil creeps are equally disastrous. How fatal are the geo-hazards becomes clear from the fact that more than three million people were killed all over the world in the years between 1960-90 from geo-hazards and 75% of these belonged to developing countries. Development is a process that does interfere with the environment. Ideal development should be in harmony with nature. But geo-political considerations many times lead to rapid development and generate natural hazards like landslides.
A devastating landslide in Himachal Pradesh - Image courtesy GSI
Root cause of landslide is change in the angle of repose. Lose sediments, rocks, weathered material etc remain stationary and stable on a slope. If due to natural reasons like an earthquake, excessive rainfall, toe erosion by the river, snow avalanche or anthropogenic reasons like road construction or any other such developmental work the material on the slopes is disturbed it moves. Gravity pulls it down and often water within acts as a lubricant. The entire mass slides down or wasted. The process of landslide is often referred as mass wasting.
Why does a land slide, how does it happen are some of the questions that must be making the readers inquisitive. Instead of going in to technicalities we will go through some of the notorious slides that have affected Garhwal in Uttaranchal, the youngest state in the youngest but loftiest mountains of the world.
On 6th September 1893, heavy rains caused landslides in Birehi Ganga, a tributary of Alaknanda, at a place called Gohna. Piles of material rolled into the river from both the sides. On the eastern side an entire spur of a hill, 11900 feet high, rolled down in to the river. Consequently, a dam was formed on the spot.
T.H. Holland of the Geological Survey of India undertook the survey of this slide on 2nd March 1894. He recorded that a 1000 feet high dam was formed across the river at Gohna, 160miles north of Haridwar. The blockade gradually submerged the area and a four-mile long and one mile wide lake was formed. Holland noted that water in the lake was rising @ six inches per day. The observations of Holland were so precise that not only he calculated the rate of submergence of the land, but also predicted the period by which the water of the artificial lake will overflow. As per that on the night of 25/26 August 1884, the top 380 feet of the dam was washed away. The breach in the dam released 10,000 cubic feet water in merely four and half hours causing devastating floods downstream. The magnitude of the floods can be realized from the fact that downstream at Chamoli the water level rose by 160 feet and at Haridwar it rose by 13 feet. By 1936 the lake had shrunk to two miles and was only a square mile in 1959. G. Pant, an expert on landslides from the GSI observed in 1967 that the lake was less than a mile and by 1997 it was completely silted.
The higher reaches of the rivers in Garhwal have sacred pilgrim places. The pilgrimage to Gangotri Glacier and temple from where the Bhagirathi River begins its journey to become the might Ganga later was obstructed by a landslide in the late fifties. An avalanche along the Lod gad one of the tributary of the Bhagirathi River led to the damming of the stream. Later when the natural dam was breached the pilgrim path to Gangotri and a steel girder bridge at Dabrani were damaged severely.
Some other catastrophic slides include, Dhauliganga in 1956, Rishi Ganga in1967, Patal Ganga in 1970, Bhagirathi blockade of 5-6 August 1978, yet another blockade of Bhagirathi in July 1992 and Madhyamaheshwar in 1998.
Each time the story was almost same. A heavy precipitation acted as a lubricant on the loosely jointed rocks in these areas and caused mass movement. The rock mass tumbled down into the rivers and blocked them. Invariably a lake was formed behind the dammed river. Either the impounded water overflowed the dam or the loose barrier gave way. Net result was flash floods and heavy destruction down stream. The Bhagirathi, when it was blocked in 1978, a 30m high dam was formed across. A breach in the dam caused major flash floods in Uttarkashi and areas downstream. The 1992 blockade of Bhagirathi led to the closure of the Uttarkashi-Gangotri road for more than 25 days. Indian engineers need all the kudos for clearing this blockade by systematic and controlled blasting.
The question arises why such incidences are more in Garhwal as compared to the adjoining areas? The steep hill slopes with loosely jointed rocks are mainly responsible for these disasters. During monsoon, rainwater enters the joints in the rocks and elevated pressure triggers landslide movement. At Madhyamaeshwar it was noticed that the roots of the Cedar trees on the mountain slopes had opened huge fissures in the rocks, providing an easy access for water. Freezing and thawing of water lodged in the open joints and fissures of rocks is also responsible for landslides in high altitudes.
In order to avert a disaster it is imperative that the Himalayan terrain is carefully studied using the Geographical Information System (GIS) and all possible hazard zones identified. The GSI is already doing the job. What is required is a proper utilization of the information. For example, in California, The United States Geological Survey works jointly with Eldorado National Forest to monitor particular landslide. Instruments are installed to measure changes in local conditions including Rainfall, Groundwater data from pressure sensors, Movement data from displacement sensors and Ground vibrations from geophones. The data are collected every ten minutes and displayed on graphs. In the likely-hood of a landslide the residents of the area are forewarned and loss of lives averted.
The proverb, 'A stitch in time saves nine' fits the bill for landslides. Like weather bulletins, it is possible to issue bulletins for possible landslides in the areas prone to such wasting process. The government and the society have to take proactive steps, expertise is available with requisite manpower with Geological Survey of India. It is just a matter of availing it.
More by : V. K. Joshi (Bijji)
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