The challenge of integrated natural hazards for continued sustainable development in the Himalayas - Dr Malay Mukul, IIT Bombay

CURRENT SCIENCE, VOL. 127, NO. 9, 10 NOVEMBER 2024 1005
CURRENT SCIENCE
Volume 127 Number 9 10 November 2024
EDITORIAL

During the last decade, developmental activities have in-creased progressively in the Himalayas in response to the increase in population and the need for better infrastruc-ture. Defense requirements along the Indo-Tibetan border have also necessitated the improvement of road and rail connectivity in the Himalayas. Tourism has also flourished as more and more people travel to the Himalayas from the plains. Dams have been built on the Himalayan rivers to generate hydroelectricity to meet the increasing demand. All these essential developmental activities have strived to be sustainable. Nevertheless, there is a growing realization that natural processes can throw an unexpected spanner in the works of the most carefully thought-out development projects. Landslides, glacial lake outburst floods (GLOFs) and earthquakes can strike the Himalayas at any time and place, and understanding the Earth’s processes that trigger these natural hazards is crucial to preventing loss of life and property during these events. The Earth’s processes typically operate over geological time scales but have the potential to strike anytime they cross the tipping point. The focus has to shift from disaster management to disaster prevention to avert increasing loss of life and property.
Climate change has resulted in concentrated rainfall over short intervals from the earlier norm of distributed rainfall in time and space. This results in flash floods and slope failure that can combine to form devastating mud- and debris flows. The warming climate has also accelerated the melting of Himalayan glaciers, resulting in glacial retreat and the formation of large glacial lakes at their terminus. These lakes are surrounded by glacial moraines that can easily breach due to their low mechanical strength by excessive rain, slumping of moraine material into the lake, or even earthquakes. Any such event will result in an extreme debris-laden flash flood in the river valleys downstream destroying everything in its way. Two such events are fresh in our memory. First is the 4 October 2023, North Bengal–Sikkim extreme GLOF event that killed more than 100 people, changed the entire morphology of the Teesta River valley and destroyed a dam. Second is the 30 July 2024, rainfall-induced flash flood in Wayanad, Kerala. Although not in the Himalayas, the villages of Punchirimattam, Chooralmala and Mundakkai were destroyed killing more than 420 people during this event.
Landslides also accompany these events adding to the destruction inflicted by the flash flood. In the Himalayas, fluvial terraces and past landslide debris in the valley are soft targets for erosion by the flash floods because they consist of unconsolidated and incohesive material. River terraces are popular sites for the construction of buildings and camp sites as they offer flat top surfaces in sloping mountainous terrains. In extreme-event flash floods, these are the first to be destroyed, as evident from multiple videos on social media where riverside constructions topple and assimilate into the river. Toe erosion, or the erosion of the base of existing and previously stabilized landslides, dur-ing GLOF events can re-activate them.
Landslides exist as a separate, independent hazard in the Himalayas too. Steep slopes, weak rocks and high rainfall combined with human activities lead to multiple Himalayan landslides. Landslides can dam rivers and create lakes that can eventually burst through the dammed river and cause a flash flood downstream. Like the river terraces, the landslide debris spreads out in the valley and provides flat surfaces for construction. Therefore, they are frequently used to develop residential and commercial real estate or other essential infrastructure similar to the river terraces. Landslides or avalanches into glacial lakes filled to the brim due to retreating and melting glaciers also cause overflow of the glacial lakes and flash floods downstream. The 4 October 2023, North Bengal–Sikkim GLOF event was attributed to the landslide of a large chunk of lateral moraine into the South Lhonak Lake. Therefore, land-slides and GLOF flash flood events can also be integrated hazards and be the cause and the effect of the other.
The other looming hazard in the Himalayas is the seismic hazard. Decades of work have revealed the presence of a major fault under the Himalayas, the Main Himalayan fault, which extends from the Tibetan Plateau to the Gan-getic plains. The fault slopes north and transitions from a near-surface brittle deformation regime to a ductile regime at depths greater than ~15 km. As evident from the Global Positioning System (GPS) measurements, the Himalaya is being continuously transported from north to south along this fault. In the ductile regime, this motion does not cause earthquakes. At the brittle-ductile transition, this motion is arrested because of friction along the fault typically absent in the ductile regime. This causes strain accumulation in the region and is characterized by micro- to moderate-earthquakes and maximum vertical uplift. Bends in the Main Himalayan fault near the brittle-ductile transition also cause strain accumulation. This region is located in the central part of the Himalayas and is likely where large and great future earthquakes will originate. For example, the magnitude 7.3, 2015 Kathmandu earthquake originated in this region. Large and great earthquakes are also known to trigger multiple landslides. Studies also reveal that many Himalayan glacial lakes are full due to climate-change-induced enhanced melting of glaciers. Therefore, there is a high probability that future large and great earthquakes will also trigger coeval landslides and GLOF events.
Therefore, the worst-case natural hazard scenario in the Himalayas will involve coeval earthquakes, landslides and GLOF events. This is what we need to be aware of and prepare for. Himalayan natural hazard models must inte-grate seismic, landslides and GLOF hazards for any reali-stic forecast of disasters and disaster preparedness. The first step is the integration of all known information and scientific insights. For example, we know that large earth-quakes will originate in the central part of the Himalayas and affect the Higher Himalayan lakes initiating GLOF events. We can also identify the most vulnerable landslide-prone regions should these events occur. The disaster-pre-paration strategies should include the possibility of all three hazards occurring together in the Himalayas. Also, a purely statistical approach to hazard estimation may not be sufficient and realistic. For instance, the basis of seismic zonation in India is the frequency of large earthquakes. Zone 5, with the maximum frequency of large earthquakes, is recognized as the most hazardous. However, recent studies make Zone 4 more hazardous as strain has accumu-lated here since the last major seismic event but not been released. Similarly, the frequency of landslides separates more landslide-prone areas from others. However, no strategy exists to forecast the initial landslide occurrence in a region where no previous landslides have occurred. New studies and strategies that account for the Earth’s processes in addition to statistics must be evolved to understand and prevent loss of life and property in future natural disasters.
Sustainable development under such a scenario must recognize and understand the collective hazard to the fullest extent possible. A wealth of scientific data and insights relevant to these hazards are available. For instance, although earthquakes cannot be predicted in time, we know that the Himalayan earthquakes will originate in the central part of the Himalayas. This insight can help save lives if the buildings and infrastructure in that part of the Himalayas are designed or reinforced to withstand large earthquakes. Similarly, all Himalayan hydroelectric pro-jects must factor in the high seismic hazard and GLOF sce-narios. The Himalayas are also riddled with fault zones that concentrate weak rocks in them. These fault zones are also likely to nucleate landslides. Therefore, identifying such fault zones can potentially locate future landslides. Using existing scientific knowledge and insights to miti-gate or prevent future disasters is the key to sustainable development in the Himalayas. The most tragic disasters are those where red flags existed but no timely action was taken to prevent the disaster. The Himalayan seismic haz-ard is a potential example of such a situation. Studies over two decades have red-flagged several parts of the Himala-yas as areas where strain has accumulated over centuries without any release through large earthquakes. The next large earthquake in these areas will cause large-scale loss of life and property. The time to minimize or prevent this is now. We should not wait for the event to happen and merely disaster manage its aftermath.
Finally, the lessons of disasters that have already occurred must not be forgotten. For example, the extreme events in Wayanad and North Bengal–Sikkim have explicitly esta-blished the boundaries of the river basin. The river deposits associated with these events will be vegetated and provide flat surfaces where it might be tempting to build in the future. The river basin must be left alone and everything inside moved to higher and safer grounds. If these lessons are ignored, the disasters will keep repeating. The magnitude of the disasters will keep increasing as the population in-creases and more lives and infrastructure come in harm’s way. Our survival may ultimately depend on how well we recognize the multiple red flags in nature and get out of harm’s way when there is still time. Maintaining maxi-mum sync and harmony with nature during developmental activities is the only sustainable way forward. We must embrace this or be prepared to suffer the consequences.

Malay Mukul
Indian Institute of Technology-Bombay,
Mumbai 400 076, India
e-mail: malaymukul@iitb.ac.in

Published by
Praful Rao
SaveTheHills
savethehills@gmail.com
9475033744