India, US study reveals the gradual sinking of the capital Delhi
Satellite analysis shows ground subsidence of up to 51 mm per year, with thousands of buildings at risk
From our correspondent
NEW DELHI - Delhi is sinking. This is not being said by the growing number of detractors of India's less livable capital, but by a group of American researchers who published a study entitled 'Sinking Indian Megacities' in the scientific journal Nature Sustainability. The data collected by applying a technique called Synthetic Aperture Radar (InSAR) to 1,299 satellite images made it possible to measure the subsidence of the soil on which India's major cities rest. Leading the way is Delhi, followed by Chennai, Mumbai and Kolkata. The megacity with the least problems is Bangalore, due to the fact that it is largely built on a rocky surface.
According to the study data, in some southern and eastern parts of the Indian capital, the ground level is dropping at the rate of 51 millimetres per year. This is not the 15-25 centimetres of some areas in Jakarta (it is no coincidence that Indonesia is building a new capital in Borneo), but it is still a rate that can affect the stability of buildings, particularly when they are located near the border between areas that sink at different rates. Cross-referencing the data with Google's Open Buildings v3 database, the researchers identified thousands of high-risk buildings: 11,457 are in Delhi, 8,284 in Chennai, 3,477 in Mumbai, 199 in Kolkata and 112 in Bangalore.
The causes are many, but according to the authors of the research, five factors are decisive: the over-exploitation of the water table, which lowers the pressure exerted on the ground from below; the increased weight of buildings (in Delhi, old houses of one or two storeys are continually being demolished to build 15-metre high buildings); irregular rainfall that does not regenerate the water table; population growth, with the related increase in water consumption and the number of wells; legislative delays in adapting building regulations to the new realities.
Some factors are more difficult to govern, but this does not mean that the trend cannot be reversed. The study showed that in Dwarka - a western district of the Indian capital - the ground level is rising by 15 millimetres per year. The credit for this goes to the policies undertaken locally to collect rainwater and regenerate the water table.
