Land Subsidence Threat in India’s Megacities

Syllabus: GS1/ Problems related to Urbanization

Context

• A recent study titled “Building Damage Risk in Sinking Indian Megacities” has revealed that approximately 878 km² of land across five major Indian cities—Delhi, Chennai, Mumbai, Kolkata, and Bengaluru—is subsiding.

What is Land Subsidence?

• According to the National Oceanic and Atmospheric Administration (NOAA), Land subsidence refers to the gradual sinking, lowering, or collapse of the Earth’s surface due to the removal or loss of subsurface materials, such as soil, groundwater, or minerals.
• It can occur naturally (Karst processes, tectonic activities or Soil compaction) or be induced by human activities (mining or construction activities).
• Across the world, many cities are slowly sinking from tropical megacities like Jakarta in Indonesia and Manila in the Philippines, to India’s own Joshimath, which recently faced land subsidence.

Key Findings

• Population and Infrastructure at Risk: Nearly 1.9 million people and over 23,000 buildings face severe damage risk if current trends persist for 50 years.
• Current Damage: Over 2,400 buildings in Delhi, Mumbai, and Chennai are already at high risk of damage.
• Geologic Variation: Cities built on soft alluvial soils (Delhi, Kolkata, Chennai) are more vulnerable than those on igneous or metamorphic rock formations (Bengaluru).

Causes of Land Subsidence

• Excessive Groundwater Extraction: Continuous withdrawal of groundwater causes aquifer compaction, leading to ground-level sinking.
• Weight of Urban Infrastructure: The increasing load from high-rise buildings in densely built-up areas accelerates ground deformation.
• Inefficient Urban Planning: Unregulated construction on reclaimed or soft soil zones amplifies vulnerability.
• Climate Stress: Irregular rainfall patterns and reduced groundwater recharge due to urban sealing worsen the problem.
• Natural Causes: Geological faults, tectonic activities, and the dissolution of underground rocks (e.g., limestone in karst areas).
• Unregulated Dumping of Waste: Municipal solid waste piles exert heavy pressure on land, especially if dumped on weak or marshy soils.

Impacts of Land Subsidence

• Infrastructure Damage: Cracks in buildings, road deformities, and disrupted pipelines or drainage networks.
• Flooding Risk: Lowering of land height, especially in coastal cities like Mumbai and Chennai, heightens flood exposure during heavy rains or storm surges.
• Economic Costs: Repair and reconstruction expenses may rise substantially in coming decades.
• Compounded Hazards: Land subsidence can intensify the impacts of earthquakes and sea-level rise, posing multi-dimensional threats to urban safety.

Way Ahead

• Soil testing and simulation models should be used to predict the likelihood of subsidence. Installing InSAR (satellite radar interferometry) and ground sensors could help prevent disasters in densely populated areas.
• Urban Hydrogeological Zoning: Mandatory mapping of soil and groundwater characteristics is needed before any large-scale construction.
• Infrastructure Design: Strengthen building codes to address differential ground movement and adopt resilient foundation technologies in soft-soil regions.

Source: DH