13 January 2023
Joshimath: ISRO InSAR data reveals the scale of the issue
In the last 24 hours there has been a significant development in our understanding of the landslide problems at Joshimath in northern India. Yesterday, the ISRO had released a document that provides the results of two sets of InSAR analyses for the town. This was made available on the NRSC website, but frustratingly (and slightly bizarrely) it now seems to have been removed. There are images from that report on several news channels in India though, so its disappearance is quite weird.
One analysis examined deformation in the Joshimath area between April and November 2022, showing about 9 cm of movement over the seven month period. This is the resulting diagram:-
The second analysis examined deformation between 27 December 2022 and 8 January 2023:-
Now, I think it is important to put three caveats on this analysis. First, InSAR measures line of sight movement, not absolute deformation. Thus, to interpret the data properly it is essential to understand the line of sight but, with the document no longer available, this is not clear. Second, this is labelled as subsidence, but that is potentially misleading. The movement is clearly landslide movement, and so is likely to be parallel or subparallel to the slope (whereas subsidence might be interpreted as being vertical). Third, the magnitudes of deformation do not seem to conform to observations on the ground. Whilst not trivial, 5.4 cm of movement is insufficiently large to cause the damage seen. Images such as the one below suggest deformation that is much greater than this:-
I am unsure as to the cause of this anomaly – it could be the line of sight issue noted above, or the consequence of an initial analysis, or maybe the largest deformations occurred before 27 December.
However, the analyses provide key information. This appears to be a conventional landslide, almost certainly a reactivation of a part of the existing relict failure. The landslide is affecting the main part of the town, and extends close to the toe of the slope (the InSAR data may be unreliable in the very steep sections of slope close to the river). This might indicate that an element of the issue is toe erosion, raising the intriguing prospect that the Chamoli debris flow might have played a role. Some of the worst affected buildings, such as the two hotels that are being demolished, appear to be in the area of the lateral scarps. This is common.
The fact that the landslide is affecting the town and not the surrounding areas might hint that local factors – slope cutting, poor water management, inadequate engineering – could be playing a key role in the instability.
Of course the most important implication is that many properties at Joshimath are at risk. There are reports that the problem is continuing to get worse, with 723 buildings now considered to be unsafe.
Finally, there is a very interesting proposed paper on the NHESS website (Sundriyal et al., for comment 2023), submitted in late December, that considers the problems at Joshimath. This paper is open for comments. The paper highlights the state of instability and indicates that large amounts of deformation (in the order of metres) are possible. If this were to occur then a very large number of buildings in the town will be lost.
Sundriyal, Y., Kumar, V., Chauhan, N., Kaushik, S., Ranjan, R., and Punia, M. K. in review 2023. Brief communication on the NW Himalayan towns; slipping towards potential disaster, Natural Hazards and Earth System Sciences [preprint], https://doi.org/10.5194/nhess-2022-296, in review.