Indian megacities face sinking risks

India’s largest cities are slowly sinking, according to a new study published in Nature Sustainability and highlighted by a Mongabay-India (MI) report. This is due to excessive groundwater extraction and soil compaction, putting people and buildings at risk. Experts warn that land subsidence, when combined with sea-level rise and extreme rainfall, could worsen flooding and infrastructure damage in both coastal and inland cities.

Land subsidence, or the gradual sinking of the ground, occurs when underground water is pumped faster than it can be naturally replenished, causing soil and rock layers to compress. Using eight years of satellite radar data, researchers found that parts of Delhi, Mumbai, Chennai, Kolkata and Bengaluru are subsiding due to groundwater extraction and soil compaction, a process where the ground compresses as water is drawn out from underground layers. Delhi and Chennai face the most worrying levels of subsidence.

The study titled ‘Building damage risk in sinking Indian megacities’ states that across the globe, building collapses emerge as a recurring disaster, causing substantial loss of lives and properties. Between 1970 and 2020, more than 180 disastrous building collapse events were reported worldwide, killing more than 330 people per year on average. These tragic incidents are not immediately related to seismic events or climate extremes but are most prevalent in developing countries. Building damage poses serious safety risks, causing substantial financial losses worldwide. Engineering shortcomings are commonly cited as the cause of long-term structural failures, often neglecting the exacerbating role of land subsidence. The study used a geotechnical measure called angular distortion (the degree to which one part of the ground tilts or sinks compared to another) to identify where buildings are most likely to develop cracks or stress. This measure captures how unevenly the ground beneath a structure settles. Even minor differences in settlement between adjacent areas can cause strain that weakens foundations over time.

The MI analysis of the study points out that the study mapped 878 square kilometres of land showing signs of subsidence and found that nearly 1.9 million people are exposed to areas where the ground is sinking faster than 4 millimetres each year. More than 2400 buildings across Delhi, Mumbai and Chennai are at high risk of structural damage from ongoing land subsidence. City-level projections show that Chennai and Delhi could be the worst affected, followed by Kolkata, Mumbai, and Bengaluru. The paper projects that more than 23,000 buildings could face a very high risk of structural damage within the next 50 years if current trends continue. Although less than five percent of urban areas fall under high-risk zones, these tend to be densely populated neighbourhoods, making the potential damage disproportionate.

Land subsidence interacts with other urban risks such as flooding and earthquakes, amplifying the impact on already overburdened infrastructure, the study finds. The MI report says that the authors of the study recommend that India’s building codes and city planning systems evolve to address subsidence risk. While the National Building Code, 1970 includes provisions for foundation safety in new construction, it does not yet account for long-term ground deformation. Experts note that India’s building regulations focus mainly on soil stability during the design and construction phase, with little provision for long-term monitoring of ground deformation. Once projects are completed, there are no formal mechanisms to track gradual sinking or foundation stress, leaving most urban infrastructure unmonitored. As the MI report states, scientists say the way forward lies in combining groundwater governance, flood management, and satellite-based monitoring to protect infrastructure and lives in a rapidly urbanising nation.

The study’s abstract points out that the results highlight the compounding risk of infrastructure damage from subsidence, assisting policymakers to develop resilience plans and adaptation strategies that prioritize mitigation and maintenance spending. It adds that current and future infrastructure damage risk can be exacerbated through the compounding effect of multiple factors, including climate change, land subsidence and human activities. Weather extremes directly influence infrastructure vulnerabilities, and as they become more frequent and intense in the face of climate change, their toll is increasing. Thus, the compounding effect of land subsidence, climate change and weather extremes forms a complex and critical nexus that can exacerbate the risk of structural damage.