What is land subsidence that caused Joshimath sinking?

Joshimath has been flooding the headlines as the sinking hill town forces over 600 families living in uninhabitable houses in the area to evacuate to temporary relief centres.

The Uttarakhand town was on Sunday declared a land subsidence zone but cracks had begun to show months earlier. DH brings you the low-down.

What is land subsidence?

Subsidence is a gradual settling or sinking of the Earth's surface, which can be brought on by both natural and human forces.

Read | Joshimath a grave reminder that we are messing with the environment: Experts

As per USGS, the phenomenon occurs when large amounts of groundwater have been withdrawn from certain types of rocks, such as fine-grained sediments. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rocks falls in on itself.

Why is Joshimath sinking?

In the case of Joshimath, 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 panel discovered that land subsidence in the area had induced structural defects and damage could be observed in almost all wards of Joshimath.

Experts have also attributed the subsidence to man-made factors such as the National Thermal Power Corporation's Tapovan Vishnugad Hydro Power Project.

Rampant and unplanned infrastructure development has made the "fragile Himalayan ecosystem even more vulnerable", experts noted, adding that climate change has further added to the damage.

When the cracks began to show...

The factors leading to Joshimath's "sinking" built up over a long period of time.

Read | Govt apathy to repeated warnings main cause of Joshimath crisis, says expert Chandi Prasad Bhatt

Speaking to PTI, director of the Wadia Institute of Himalayan Geology Kalachand Sain pointed at Joshimath's vulnerable foundations. The town, he noted, was developed on the debris of a landslide triggered by an earthquake more than a century ago.

Other factors adding to its subsidence could include Joshimath's location in seismic zone V which is more prone to earthquakes, besides gradual weathering and water percolation, all of which combined reduce the cohesive strength of the rocks over time.

According to a survey conducted by the municipality of Joshimath, cracks appeared in over 500 houses in the town over a year, rendering them inhabitable.

Commenting on the current state of affairs, residents of the hill town have also stated that they started noticing cracks in houses after the 2021 floods in the region.

Sain reiterated that the downflow of the Himalayan rivers coupled with heavy rainfall, characteristic of the area, in addition to the flashfloods in the Rishiganga and Dhauliganga rivers last year might have aggravated the situation.

Moreover, being the gateway to Badrinath, Hemkund Sahib, and skiing destination Auli, Joshimath has seen several haphazard and unplanned construction activities mushroom in recent years. In addition, the heavy influx of pilgrims and tourists as well as regular movement of troops and heavy vehicles, may have led the already vulnerable town to slowly succumb to the pressure.

Now what?

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 microzonation of the town, replanning of its drainage system and rainwater outlets, and assessment of rock strength among other measures be made a priority.

Construction activities related to NTPC's Tapovan-Vishnugad hydro power project, Char Dham road project and Helang bypass by the BRO have also been halted.

(With agency, DH inputs)