Saturday, November 29, 2008

"Landslides for dummies" by Dr Malay Mukul

We all know that the Himalaya has been formed by the collision of the Indian and the Eurasian plates and this process has been going on for the last 50 million years. The process of mountain building on our planet is accompanied by earthquakes and building of “topography” or relief resulting in formation of slopes. Once the slopes are built beyond a certain critical angle (that largely depends on how weak or strong the material or rocks/soils that make up the slope are) they strive to reach equilibrium by collapsing under the forces of gravity. This is essentially the landslide process that we encounter. Slopes can fail and slide at lower angles if the binding vegetation is removed or fluids get into the system. In an active mountain belt like the Himalaya, landslides are, therefore, an inherent product of mountain building where slopes are built to the point where they are no longer stable and then collapse to reach an equilibrium state. Landslides are, therefore, part of a natural process and we cannot stop or arrest this process no matter how hard we try. Our only option to avoid landslides completely is to move out of the hills and go to the plains. However, that is not a solution we seek at this juncture. So the next option we have is to try and understand the natural processes that lead to landslides so that we don’t facilitate them and develop an understanding of the risks involved in living on hill-slopes. To understand landslides an understanding of the geology of the region (preferably at the scale of our towns, villages etc.) should be the first order of business. While the detailed geology of the Darjeeling-Sikkim Himalaya is still in the process of being worked out, first-order features are more-or-less understood. First, there is almost a cake-like layering of rocks in Darjeeling-Sikkim Himalaya; the rocks are mechanically stronger as you climb up to the top of the cake. For example, rocks in the Teesta valley are weaker near the river than rocks higher up on mountain tops like Ghoom, Darjiling or even Delo. The Teesta has simply cut through this layering like a knife. Also, the layers in this cake are separated by particularly “weak” zones termed as faults which are basically large fractures which can be seen exposed on the surface but run deep underground. In these zones, the rocks have been crushed and powdered by natural processes and consequently they typically end up forming thick soils over the years. You cannot blast your way to solid rocks in fault zones. Needless to say, these are also zones that will slide easily and are difficult to stabilize. A typical example of this is the Birrik slide zone on NH-31A which has been active for decades now. A landslide in a fault zone is something which you cannot do much and the best strategy is to get the hell-out-of-the-way. Such zones have to be carefully identified and mapped at a scale of the habitat being planned. For example, in the Kalimpong area we have weaker rocks in the southern part of town and stronger rocks in the northern part of town (e. g. at Delo). So the fault zone separating the two layers of rocks must pass somewhere in between. This zone needs to be clearly identified and mapped in detail because this zone might hold the key to landslides around the Kalimpong area. This study is lacking at present. I fear that this zone might be heavily urbanized. While we cannot really do much to correct the presence of this geological zone, we can at least be careful to secure and stabilize it and keep out of this zone to the extent possible. Similar studies need to be carried out in the towns of Kurseong and Darjeeling and urban centres of Sikkim.

The bottom line is that as long as we live in the Himalaya, landslides will be part of our lives. We need to accept this and plan for it to the extent possible. Identification of geologically weak zones as well us having mitigation strategies in place would be the way to go. We cannot be thinking that all is well and adopt an ostrich mentality. We are not only vulnerable to landslides but also to large earthquakes and the worst case scenario would be a double whammy of both these hazards hitting us together. Great earthquakes cause multiple landslides as was evident in Kashmir during the recent earthquake. Unfortunately, we have chosen to ignore traditional wisdom and take backward steps in rapidly urbanizing our hills with concrete multi-storied dwellings. There will be a price to pay when the next big earthquake coupled with seismically induced landslides strikes the hills of Darjeeling and Sikkim. We know that for a fact from the Kashmir earthquake. The best we can do now is to minimize that price by acknowledging that we have a problem and taking remedial steps to the best of our ability.

Yes! We are living in a geologically precarious zone. We need to really map this zone in detail to begin with. Map the old and existing landslides closely next. Correlate the two and see what we get and have our mitigation strategies in place accordingly. That would be the strategy I would recommend. There are studies and reports on the entire Darjeeling-Sikkim area but that will not serve this purpose. We need very detailed work here specific to the urban area under consideration. It would also need to be done by a team of geologists, geographers and engineers who should be able to slant their specializations towards landslide hazard determination and mitigation. The work should be carried out in close consultation by social activists like you so that the deliverables can be directly used for policy making.

Personally, I think the best way to go about it would be to approach some Govt agency (like NDMA) for funding, get a team of motivated and qualified people together and start working on it ourselves rather than waiting for people to help us.


About Dr Malay Mukul

Dr Malay Mukul is perhaps the only scientist and geologist in CSIR Centre for Mathematical Modeling and Computer Simulation, at Bangalore who is fond of quoting John Lennon!
He did his Ph.D from Rochester University (USA) but much before that he did his schooling from Dr Graham’s Homes, Kalimpong, hence his intimate knowledge of this area. Malay who is the recipient of numerous awards and has published many papers and books, is still a frequent visitor to Kalimpong where his parents are settled.
For more on Dr Mukul please visit this link (

In the above letter sent to me by Malay, the title and italics are mine

praful rao

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