Joshimath crisis a case of sliding, not subsidence: Experts
The holy town of Joshimath is sliding. Geologists, who have identified multiple causative factors and a possible trigger for rapid subsidence over the past few weeks say that technically, this is “sliding” not “subsidence”.
The holy town of Joshimath is sliding. Geologists, who have identified multiple causative factors and a possible trigger for rapid subsidence over the past few weeks say that technically, this is “sliding” not “subsidence”.
“The phenomenon we are seeing in Joshimath cannot be called subsidence in geological terms,” explained Kalachand Sain, Director, Wadia Institute of Himalayan Geology (WIHG), explaining that what’s happening in Joshimath is an instance of the surface sliding down.
Sain rattled off possible reasons for this.
“One, the foundation rocks are some crystalline rocks and here are several structures built on them and an entire habitation is settled. “
“Two, Joshimath is situated between two important tectonic faults—the Munsiyari thrust to the south and the Vaikrita thrust to the north which have a thrusting influence. It is a kind of a shear zone so the rocks are relatively weak in strength. Very long back, some 6,000 years ago there were some landslides possibly linked to earthquakes in this zone. It is a very seismically active area. “
”Three, in February 2021 there was the Rishiganga deluge linked to a glacier breach which led to a flow of debris in Alaknanda and erosion in several regions and toes were formed. It also contributed to the weakness in the region.”
“Four, was there water seepage from a source? Lubrication may have triggered it. The linkage with construction of hydropower plant etc need to be ascertained with certainty through geophysical imaging which is a critical matter of study immediately. Such rapid sliding is seen when there is lubrication or water flow.”
According to the US Geological Survey, land subsidence occurs when large amounts of groundwater are withdrawn from certain types of rocks, such as fine-grained sediment. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rocks fall in on itself. Land subsidence is most often caused by human activities, mainly from the removal of subsurface water.
“It appears that there has been an aquifer burst because there is flow of water from under the surface. But without enough evidence it cannot be said what has caused it. This is why investigations are critical at this stage to fix accountability. Once subsidence starts it can occur over several months and we know that this sliding also started months back but now it has picked pace. The entire region may be impacted due to this,” said a geologist associated with Geological Survey of India who did not wish to be named.
Several experts have suggested that recent construction of tunnels for National Thermal Power Corporation’s Tapovan Vishnugad hydropower project may have led to an aquifer burst. The Union government and NTPC have refuted suggestions that tunnelling associated with the National Thermal Power Corporation’s Tapovan Vishnugad hydropower project led to an aquifer burst which further led to sliding of the region.
“The ongoing crisis in Joshimath is primarily because of anthropogenic activities. The population has increased manifold and so has the tourist landfall. Infrastructure has also amplified and has been unchecked. However, the town does not have a proper drainage system. Besides gradual weathering of fine material between the rocks of debris, water percolation has decreased the cohesive strength of the rocks over the time. This has resulted in landslides, triggering cracks in the houses. Secondly, construction of these tunnels for hydro power projects is being done through blasting, creating local earthquake tremors, shaking debris above the rocks, again leading to the cracks. Government has not learnt anything from the 2013 Kedarnath floods and 2021 Rishi Ganga flash flooding,” said Professor Y P Sundriyal, Head of Department, Geology, HNB Garhwal University in a statement.
An NTPC spokesperson was dismissive of these theories: “ It’s obvious that the subsidence is not linked to our project.”
To be fair, experts also say holding NTPC’s NTPC’s hydropower project alone responsible for the crisis does not make sense. “Subsidence occurs when natural drainage channels are blocked. It’s a clear case of natural drainages being blocked and water finding its own way. In simple language, nature is readjusting to the changes made. Several experts have flagged that this region in Uttarakhand and several places in Himachal Pradesh are in sensitive geological areas. But there has been too much of construction—hotels, homes, tourism. It’s too naïve to blame NTPC alone immediately. A clear assessment is needed of past drainage patterns and how that has changed now,” said Anurag Linda, Assistant Professor, Central University of Himachal Pradesh.
