While the entire Himalayan region is fragile, the zones that receive snow and rain are the most unstable
This article is part of a special edition on the Himalayas, published in February 1-15, 2023, issue of Down To Earth magazine
The last few decades have recorded rapid and magnified natural catastrophes across the world, with scientists linking such extreme events with global warming. The Himalayas have recorded such calamities like earthquakes, avalanches, cloud-bursts, intense rainfall landslide lake outburst floods (LLOFs are breaching of lakes formed by obstructions due to landslides), and glacial lake outburst floods (GLOFs are a sudden release of a significant amount of water retained in a glacial lake) in their most furious forms; ground studies indicate a warming climate as the cause.
The warming climate has the maximum impact in the areas intermediate between glaciated and non-glaciated terrain—these are the areas that receive snowfall as well as rainfall. It is pertinent to mention that most of the disastrous catastrophes, including the Kedarnath floods of 2013, the Chamoli flash floods of 2021 and the subsidence in Joshimath, have occurred in such intermediary climatic zones.
ESRI imagery shown with a zone of intermediary topography and climate zone in parts of Himachal Pradesh and Uttarakhand affected by major natural calamity
Such intermediary climatic zones, with picturesque glacial landscape and easy access offers a natural choice for tourism and subsequent developments. With the sudden change in topographic altitudes and availability of perennial sources of water, such places also offer the best suitable sites for the development of river valley projects. But while the entire Himalayan region is fragile due to its immature topography under continued active erosional and weathering processes, the intermediary zones may be regarded as the most fragile and unstable belt on account of the interplay of glacial and fluvial (dominated by rivers and streams) processes.
Like Joshimath, most human settlements in the Himalayas are located on north-to-easterly directed valley slopes, which see less solar insolation and are called sun-shadow slopes. As a result, these areas have seen prolonged glacial activity that resulted in gentler, deeply weathered slope covered with thick fluvioglacial debris (erosion and deposition of sediments caused by glacial meltwater). The debris comprises large boulders set in a mixture of clay and pebble-sized rock fragments. Scars of ancient landslides and some amount of creep (slow, downslope movement of soil and earth materials) are common on such slopes primarily due to the permanent wetness of the ground, however, the areas turn to a “sinking zone” or debris slide due to excessive ground saturation clubbed with unscientific surficial loading, slope modification and toe removal or erosion. Several areas of Himachal Pradesh, like Kothi near Manali, Prashar Lake Rari village near Bajaura in Beas valley and Reckong Peo, Urni in Kinnaur district are on such slopes.
The north-directed (Sun-shadow) valley slope of Joshimath is marked with a gentler slope with thick slope debris, and vegetation (to the left side of the river valley) and south-directed (Sun-facing), highly dissected and barren, valley slope to the right of the river valley
On the other hand, the south-to-westerly directed slopes on the Himalayas, which see sufficient solar insolation, are steeper, dissected, rocky, and marred with rock-fall and rockslide. Such show talus (loose to semi-consolidated rock debris) at altitudes suitable for human settlement and so are rarely preferred.
With better construction technology and population increase, the development of infrastructure and growth of settlement is inevitable on such conducive slopes. Past experiences show that solely administrative and civil engineering approaches have not yielded any fruitful results in terms of maintaining sustainability. It is necessary to re-evaluate such developmental activities in critical terrains, keeping in mind the historical and existing geological, geomorphological and climatic attributes. It is high time that all the hill area establishments are geologically investigated to calculate the possible risk and hazards, and accordingly fine-tune the existing code of practice for building construction. The stability of any slope requires managing the surface and subsurface water and gaining an understanding of the bedrock surface and other geological features as well as the climatic zone, so that one can plan the distribution of superficial load and necessary safety measures.
Upslope areas should be investigated for landslide and avalanche or debris flow, to avoid deadly incidents like the landslides in Boi and Batseri, Himachal Pradesh, which occurred last year. Downslope or valley establishments must, at the same time, be checked for risk of high-level floods due to cloud bursts, GLOFs or LLOFs. A policy mandating geological inputs during pre-construction, construction stage and post-construction monitoring of proposed or existing settlements and projects is also needed.
(As told to Rohini Krishnamurthy. This article is only an abstract form of observations based on more than two decades of field experience in the Himalayas and literature available on the internet. It is not conclusive in any way and requires further research. Any critical site will require detailed methodical geological and geotechnical investigations.)
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