10 January 2023

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

GS-3: Disaster and disaster management.

Recently, Joshimath, in Uttarakhand, has been declared a landslide-subsidence zone after cracks appeared in many roads and hundreds of houses of Joshimath. As a part of the evacuation process, over 60 families living in uninhabitable houses were shifted to temporary relief centres set up in Garhwal by local administration. 

Land subsidence

• A gradual settling or sudden sinking of the Earth’s surface is called a land subsidence.
• It happens because of underground material movement such as the removal of water, oil, natural gas, or mineral resources out of the ground by pumping, fracking, or mining activities.

• It also happens due to natural activities such as earthquakes, soil compaction, glacial isostatic adjustment, erosion, sinkhole formation, and loess deposits.

• It can happen over a very small area like a corner in a yard to very large area like entire state.
• These changes can be measure through the Global Positioning System (GPS), satellite InSAR (Interferometric Synthetic Aperture Radar), repeated surveys of geodetic leveling, or installations of ground and water sensors.

Land Slide 

• Landslide is the rapid sliding of large mass of bedrocks.
• It is a type of "mass wasting," which denotes any down-slope movement of soil and rock under the direct influence of gravity.

Types of landslide

• There are 5 types of landslides. 

• Falls: Abrupt movements of masses of geologic materials, such as rocks and boulders that become detached from steep slopes or cliffs.

• Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling, which is strongly influenced by gravity, mechanical weathering, and the presence of interstitial water.

• Topples: Rotation about some pivotal point, below or low in the unit, under the actions of gravity and forces exerted by adjacent units or by fluids in cracks.
• Slides: Debris, rocks or soil slide through a slope.
• Spreads: Usually occur on very gentle slopes or flat terrain.

• Caused by liquefaction (sediments are transformed from a solid into a liquefied state)
• Triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced.

• Flows: There are five basic categories of flows that differ from one another in fundamental ways.

• Debris flow: A combination of loose soil, rock, organic matter, air, and water mobilize with less than 50 percent fines as a slurry.

• Caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes. 

• Debris avalanche: Very rapid to extremely rapid debris flow.
• Earthflow: A characteristic "hourglass" shape. 

• The slope material liquefies and runs out forming a bowl or depression at the head. 
• Usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. 
• However, dry flows of granular material are also possible.

• Mudflow (mudslides): An earthflow consisting of material that is wet enough to flow rapidly and that contains at least 50 percent sand, silt, and clay-sized particles. 
• Creep: Imperceptibly slow, steady, downward movement of slope-forming soil or rock. 

• Caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. 
• There are generally three types of creep: 

1. Seasonal: Movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature. 
2. Continuous: Shear stress continuously exceeds the strength of the material. 
3. Progressive: Slopes are reaching the point of failure as other types of mass movements. 

• Indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.

Landslide Vulnerability Zones 

• Very High Vulnerability Zone: 

• Highly unstable, relatively young mountainous areas in the Himalayas and Andaman and Nicobar
• High rainfall regions with steep slopes in the Western Ghats and Nilgiris
• The north-eastern regions 
• Areas experiencing frequent ground-shaking due to earthquakes
• Areas of intense human activities such as construction of roads, dams, etc. 

• High Vulnerability Zone: Similar conditions to those included in the very high vulnerability zone but intensity and frequency of the controlling factors are somewhat lesser than that. 

• All the Himalayan states and the states from the north-eastern regions except the plains of Assam. 

• Moderate to Low Vulnerability Zone: 

• Areas that receive less precipitation such as Trans-Himalayan areas of Ladakh and Spiti (Himachal Pradesh) 
• Low precipitation areas in the Aravali
• Rain shadow areas in the Western and Eastern Ghats
• Deccan plateau 
• Landslides due to mining and subsidence are most common in states like Jharkhand, Odisha, Chhattisgarh, Madhya Pradesh, Maharashtra, Andhra Pradesh, Karnataka, Tamil Nadu, Goa and Kerala. 

• Other Areas: The remaining parts of India, particularly states like Rajasthan, Haryana, Uttar Pradesh, Bihar, West Bengal (except district Darjiling), Assam (except district Karbi Anglong) and Coastal regions of the southern States.

Measures required to prevent landslide

• Increase forest cover in community lands and promote large-scale afforestation programmes
• Storage of the excess water in catchment areas to reduce the effect of flash floods and also recharge groundwater levels.
• Restrict the grazing of animals
• Reduce the urbanisation activities such as building dams or other commercial projects
• Adopt area-specific measures to deal with landslides
• Limit agriculture to valleys and areas with moderate slopes
• Control on the development of large settlements in the high vulnerability zones
• Terrace farming should be encouraged in the northeastern hill states where Jhumming is still prevalent.
• Increase Public awareness regarding preventive measures during landslides

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