Joshimath crisis: What is land subsidence and why does it happen?

Subject: Geography

Context:

Almost a week after cracks appeared in many roads and hundreds of houses in Joshimath, Uttarakhand, authorities on Sunday declared it a landslide and subsidence-hit zone.

What is land subsidence?

According to the National Oceanic and Atmospheric Administration (NOAA), subsidence is the “sinking of the ground because of underground material movement”.

Major causes for land subsidence:

Natural factors: Land subsidence happens due to gradual or sudden natural compaction or collapse of soils due to reasons like –
- Tectonic activities (e.g. earthquake and faulting).
- Volcanic activities.
- Landslide
- Formation of sinkholes.
- Thawing of permafrost.
Anthropogenic causes:
- Compaction of aquifer systems due to extensive groundwater withdrawals: When water is extracted from aquifers, the clay between pockets of water collapse gradually, leading to land subsidence.
- Development of underground infrastructure such as metro, tunnels, etc.
- Excessive underground mining of minerals, oil, and gas.
- High load of constructions such as high-rise buildings

Joshimath land subsidence: MC Mishra committee report:

The possibility of such an incident happening in the region was first highlighted around 50 years ago when the MC Mishra committee report was published and it cautioned against “unplanned development in this area, and identified the natural vulnerabilities.”
According to experts, Joshimath city has been built on an ancient landslide material– meaning it rests on a deposit of sand and stone, not rock, which doesn’t have high load-bearing capacity.
This makes the area extremely vulnerable to ever-burgeoning infrastructure and population.

What can be the reasons behind Joshimath’s subsidence?

an expert panel set up by the Uttarakhand government found that several pockets of Joshimath are “sinking” due to man-made and natural factors.
The exact reason behind Joshimath’s land subsidence is still unknown.
Experts suggest that the incident might have occurred because of unplanned construction, over-population, and obstruction of the natural flow of water and hydel power activities.
The lack of a proper drainage system might have also contributed to the sinking of the area.
Not only this, the area is a seismic zone, which makes it prone to frequent earthquakes.
Residents have also blamed NTPC’s Tapovan Vishnugad Hydro Power Project for the incident.

Land subsidence in Uttarakhand region:

The towns that could suffer a Joshimath-like fate include Karnaprayag and Gopeshwar in Chamoli district (where Joshimath is situated); Ghansali in Tehri district; Munsiari and Dharchula in Pithoragarh district; Bhatwari in Uttarkashi district; Pauri; Nainital and several other towns.

What can be done now?

Construction activities related to NTPC’s Tapovan-Vishnugad hydropower project, Char Dham road project and Helang bypass by the BRO have also been halted.
While land subsidence cannot be reversed, it can be slowed, and in some cases, it can be checked significantly.
Once the evacuation of Joshimath is completed, experts have suggested that micro-zonation of the town, replanning of its drainage system and rainwater outlets, and assessment of rock strength among other measures be made a priority.