Why is Joshimath sinking

The obvious question in the minds of most people following the incidences in Uttarakhand relates to why is Joshimath sinking.

Joshimath in Chamoli district of Uttarakhand has been sinking or witnessing distress of slope material since 1970. Active and severe ground subsidence however initiated just after the New Year in 2023 and portions of the town hitherto considered free from the problem of slope distress were affected – Sinnghdhar, Marwari, Sunil and Manoharbagh.

This is attributed to the breach of a water bearing horizon in the higher reaches of the slope comprising palaeo-landslide material deposited over dip slopes.

This water bearing horizon having been drained out, active subsidence is deduced to halt soon, but piping of fines can aggravate the problem. Planned drainage together with measures to minimise subsurface infiltration of rainwater are therefore recommended, together with measures for addressing water shortage related issues that are expected during the forthcoming summer season.

Prologue

The Himalayan orogenic belt is the result of ongoing north-northeasterly drift and underthrusting of the Indian Plate beneath the Eurasian Plate. This rendered the terrain highly sheared and prone to erosion. High relative relief and precipitation further add to the instability of the slopes.

Uttarakhand 

Central sector of the Himalaya houses the state of Uttarakhand in India that has particularly high vulnerability to natural hazards, especially landslides and flash floods that devastated many parts of the state in 1880, 1894, 1970, 2010, 2012, 2013 and 2021.

Chamoli district

Chamoli district of Uttarakhand is one of the most disaster prone district of Uttarakhand and apart from 1999 Chamoli Earthquake the district has witnessed landslides and flash floods in 1893, 1894 and 1970 besides recent flooding of Alaknanda, Pindar and other rivers in 2013 and Dhauliganga – Rishiganga floods in 2021.

Chamoli district at the same time houses one of the most revered Hindu and Sikh shrines – Badrinath and Hemkund Sahib respectively, as also many other tourist and adventure sports destinations including Auli and Valley of Flowers.

The district is therefore visited by pilgrims, tourists and adventure enthusiasts in large numbers, which enhances the risk of hazards to which the district is exposed.

Joshimath

Being located on way to  Badrinath, Hemkund Sahib, Auli and Valley of Flowers – Joshimath is winter abode of Lord Badrinath, seat of Shankaracharya, staging area for forward posts along Sino-Indian border and Tehsil headquarter. 

Located on Rishikesh-Badrinath national highway Joshimath therefore has administrative, religious, spiritual and strategic significance.

Joshimath is situated over a large palaeo-landslide, which is evident from the composition of the slope material; gneissic boulders scattered all along the slope in sandy–silty matrix, with in situ rocks exposed at the toe of the slide along the left bank of Alaknanda.

The palaeo-landslide material has been exposed to aggregational and degradational forces of nature for a long time and these have modified the Joshimath slope into a number of distinct facets that have developed their individual stability conditions.

Some portions of Joshimath town started to subside at a fast pace in the first week of January 2023 and the subsidence has not seized since then.

This communication is an attempt to analyse this issue to better visualise the causative factors, and suggest practical remedial measures.  

Slope instability around Joshimath

Though highlighted by media after aggravated pace of ground subsidence in January 2023, distress of slope material is no new phenomenon for Joshimath and official records suggest that the problem has been persisting since 1970s.

Mishra Committee

Antiquity of slope instability around Joshimath is evident from the fact that the Government of Uttar Pradesh formally constituted a Committee under the chairmanship of the Divisional Commissioner M.C. Mishra in 1976 to investigate the causes of instability and suggest remedial measures.

Even though Mishra Committee did not identify any area as having been particularly affected by slope instability, it described subsidence as being localised in nature.

Poor surface and subsurface drainage arrangements were held responsible for aggravating the problem in the area around Joshimath that was described as being situated over a naturally unstable old landslide material.

Besides drainage measures Mishra Committee suggested avoiding agriculture on the hill slope so as to minimise infiltration of rainwater.

Haphazard development

There exists nothing to show if the recommendations of the Mishra Committee were ever acted upon.

It is therefore no surprise that most infrastructure – public or private – affected presently by ground subsidence was constructed after 1976 despite Mishra Committee having specifically recommended taking stock of load bearing capacity of the soil before allowing construction, and restricting heavy construction in the area.

Instability aggravated

Dhauliganga that meets Alaknanda at Vishnuprayag to the northeast of Joshimath experienced a major flood related devastation on 7 February 2021.

The area also experienced particularly heavy precipitation between 17 and 19 October 2021.

These incidences – former destabilising the toe and latter facilitating increased groundwater infiltration – are perceived to have some role in ground subsidence in the area around Joshimath.

Multi-institutional committee

In August 2022 the state government constituted a multi-institutional Committee of experts drawn from GSI Dehradun, WIHG Dehradun, IIT Roorkee, CBRI Roorkee and USDMA Dehradun to investigate the causes of ground subsidence around Joshimath and suggest remedial measures.

The observations and suggestions of the Committee were however no different from those made more than four decades back by Mishra Committee. The Committee after detailed field investigation of the area, however considered Ravigram, Gandhinagar and upper portion of Suneel ward in Joshimath as being the worst affected by ground subsidence.

Other areas safe

Even though Ravigram, Gandhinagar and upper portion of Suneel ward of Joshimath faced the problem of ground subsidence and consequent distress of structures other wards of the city were perceived as being safe, which is evident from heavy infrastructure including Joshimath – Auli ropeway being sited in these areas.   

Silt laden water in JP Colony

In the night of 2 January 2023 heavy discharge of silt laden water was witnessed from the hill slope in the JP Colony in Marwari ward of Joshimath that was observed to be more than 500 lmp and was highly pressurised.

This was accompanied by reports of active subsidence and ground fissures from many areas in Joshimath. These had severe adverse impact on the built environment of these areas. As observed the structures were depilated to an  extent that local administration had to vacate many residential areas and shift the residents to temporary relief camps.

As observed, ground subsidence and fissures were particularly restricted to Suneel, Marwari, Singdhar, Manoharbagh wards of Joshimath.

Surface and subsurface characteristics

Joshimath is located over a thick cover of old landslide material that is deposited over the dip slopes, wherein in situ rocks are exposed along the course of Alaknanda river and to the west of Singdhar ridge. The slope forming material has large gneissic boulders embedded all along the slope in sandy–silty matrix.

Movement of subsurface water

The slope material around Joshimath, being both highly porous and permeable, facilitates absorption and subsurface percolation of significant proportion of rainwater before draining the same into Alaknanda river at lower altitudes.

Though following subsurface slope geometry the bounders embedded in the slope material provide resistance to free flow of subsurface water and modify subsurface flow characteristics. Finer clay particles thus get deposited at favourable sites shielded by large boulders where percolating water gets an opportunity of remaining transitorily stagnant.

The impervious layer

Continued deposition of fine clay particles created an impervious layer within the slope material and thus a water bearing horizon came into existence within the slope material.

This impervious layer impeded downward movement of water, which had two implications. Firstly, percolation of water to the strata below was restricted, which protected the same from piping effect of percolating water. Secondly, restricted downward flow of water ensured that the slope material below the impervious layer remained relatively dry as compared to other areas witnessing unrestricted percolation of water.

Distress in other areas

Though the area below the impervious layer was protected, other areas were exposed to piping effect of percolating groundwater. Slow but ongoing subsidence was therefore witnessed in areas not falling in the safety net of the impervious layer.

As evident from the observations of various Committees, Ravigram, Gandhinagar and Upper reaches of Suneel wards of Joshimath were affected previously by ground subsidence. And the same is evident from written complaints to the local administration by the residents of upper Suneel ward of Joshimath in November 2021.

Impervious strata breached

Though exact cause is hard to figure out, but in the night of 2 January 2023  the impervious layer got breached. Huge amount of water stored in the water bearing horizon above thus gushed downslope eroding relatively dry material initiating an underground movement of silt laden water whose erosional capacity increased downslope. This water punctured the surface in JP Colony in Marwari ward of Joshimath.

This water thus eroded the very strata that was hitherto protected by the impervious layer.

Ground subsidence aggravated 

The fast erosion as evident from more than 500 lpm of silt laden discharge at JP Colony resulted in creation voids in the subsurface that on collapse were observed as ground fissures, differential settlement of ground and distress in the structures.

The erosion was particularly high at places where the slope material was devoid of large boulders – roads and other areas levelled for construction fit into this category and these were the places where major subsidence was observed.

Impervious strata and the water bearing horizon

Field observations suggest Marwari, Singdhar and Manoharbagh as being worst affected by ground subsidence. It is thus deduced that the impervious surface traversed through elevations higher than these wards and the resulting water bearing horizon extended upwards towards Auli.

The way forward

With impervious layer breached and water bearing horizon drained out as is evident from drying out of the discharge at JP Colony in Marwari ward, the problem of ground subsidence being faced presently is perceived to end.

The instability introduced would however take long time to heal and therefore the affected area needs to be kept free of anthropogenic interventions.

Piping effect and subsidence

The impervious layer having breached, no one can assure if nature is to heal it in near future. This however implies  that presently there exists no resistance to free flow of water.

The safety net provided by the impervious layer having gone, the rainwater is to subject areas around Marwari, Singdhar and Manoharbagh to piping action and consequent creep and subsidence as has been witnessed previously in Ravigram and Gandginagar.

Drainage measures

It is recommended that efforts be made to minimise infiltration of rainwater below the surface and ensuring its safe disposal together with household wastewater.

Efficient drainage and sewer lines have therefore to be put in place and soak pits have to be done away with. At the same time agriculture be discouraged on the upper slopes, and flat surfaces be made impervious.

Ground readjustment

With water drained out the water bearing strata higher reaches of the slope are to readjust, collapse and settle down and therefore ground subsidence is to be expected in those areas.

Regulated development

Joshimath is deduced as being culmination of long drawn negligence and apathy, not only of administration but also the masses as every around knew and witnessed the ground below them crumbling – at least since the Mishra Committee report. 

Both bearing capacity and stability of the ground tend to vary at short distances, particularly in the hills and therefore locally relevant lands and building bye laws are required to be put in place with a sound mechanism for ensuring their compliance – incentives for those complying and stern and definitive action against violators with no scope of compounding. 

Water shortage

With water bearing strata gone, the areas around Joshimath are to face shortage of water, particularly during the summers when with the arrival of tourists and pilgrims the demand would be at its peak. Discouraging people to night halt around Joshimath and making alternative arrangements for water supply are suggested.