(Dr. R K Chadha)
Hyderabad: In the dynamically active Himalayan Mountain belt landslides along slopes can happen suddenly during earthquakes and heavy rains or debris can creep more slowly under the influence of gravity, water saturation, and anthropogenic activities. And, Joshimath is experiencing the latter phenomenon, as the township is located on a consolidated deposit of debris formed by weathering of the glacial mountaintop where the famous ski resort of Auli is located.
This debris of silty-sandy deposits embedded with gneissic and schistose rock fragments has created an accumulated feature called a Talus slope over a period that is sinking today. Such Talus slopes will fail, naturally, or can be triggered by anthropogenic activities and any structure on it will collapse slowly due to basement failure, as debris below them will creep. There are a few external and internal factors that can lead to slope failures in mountains causing subsidence of debris land. These are,i) erosion of the base of the slope through river cutting, ii) deformation of slope foundation due to seepage of water and increase in the water table with no proper drainage increasing pore pressure, iii) piping or tunneling, and iv) induced vibrations, both anthropogenic and natural.A cursory analysis of available data shows that Joshimath’s subsidence is happening due to a combination of all the above factors and has hastened the creeping movement in the last two years. Firstly, in February 2021 a rock and ice avalanche in the Higher Himalaya of Chamoli district brought huge volumes of debris-laden water by Dhauliganga leading to very fast erosion in the valley causing instability in the base slope of Joshimath township. The base erosion was further aggravated downstream of Alaknanda river when Joshimath recorded 190 mm of rainfall in 24 hours during a cloud burst in October 2021. Secondly, the water that entered the tunnel under construction during the avalanche disaster and later during heavy rainfall could have led to an increase in the water table with no proper drainage. This condition could lead to increased pore pressure in the rocks that reduce the shear strength and loss of cohesion. Thirdly, induced vibrations caused by blasting during construction activity either for roads or piping and increased and constant vehicular movement in a water-saturated debris slope could lead to failure.
So, the Joshimath tragedy is a combination of natural forces where man-made activities hastened the process. The question is what should be done now?
Instead of pointing fingers let us look at short and long-term measures to alleviate the sufferings of the people living under great fear in Joshimath. The situation looks to have gone beyond any technological remedial measures for the structures. It if for the authorities to provide hope and succor to the people living under constant fear in partially damaged houses; they should be immediately evacuated.
A rapid survey of the affected areas should be taken up by experts as a follow-up action to map cracked buildings, ground cracks, and subsidence areas to demarcate a boundary that is likely to slip in near future. It is not that the entire township will slip at a time, though it may cause further create instability of the slope. It is the right decision of the government to suspend all activities about construction at the NTPC site. A 24×7 vigil is required by authorities along with a continuous discussion with an expert on the developing situation to safeguard the interests of people.
As a long-term measure medium or large hydroelectric power plants should be avoided in the Higher Himalayas that can disturb the delicate balance of the environment. It is advisable to be guided by our earlier experience of dams like Bhakra and Pong constructed in the foothills of Himachal Himalaya. Even the world’s highest NurekDam in Tadjikistan is located in the foothills near the border of Afghanistan.
Developmental activities in certain environmentally sensitive regions need to be carefully planned. Tunneling is a major part of any development. For example, the construction of the 9.8 km long tunnel connecting two stations on the Banihal-Katra railway link in Jammu and Kashmir is a great engineering feat.
Similarly, the 9 km Atal tunnel built under the Rohtang Pass in the Himachal Himalaya on the Leh-Manali is another example of proper selection and planning. Somehow, the NTPC project in Joshimath seems to have been poorly planned without many studies of the surrounding tectonics and the environment. One has to keep in mind that the Himalayas is one of the most seismically active regions of the world and any little doubt needs to be addressed when any engineering activities are taken up. After all, the Silent Valley project in Kerala was abandoned in 1983 in the interest and safety of the people, so can the NTPC project in Chamoli be abandoned? The answer is simple, Yes.
