Guwahati: Over the past decade, densely populated cities in India have faced a troubling rise in urban floods, leading to extensive damage to infrastructure, property, and loss of life. Rapid urban expansion has worsened this situation, as impermeable surfaces like concrete and asphalt replace natural, water-absorbing areas.
These floods also bring severe health risks, increasing the likelihood of epidemics and infections in densely populated regions.
Traditional flood management strategies, such as stormwater drains and storage silos, require costly and extensive infrastructure. In contrast, smaller, sustainable measures like rainwater harvesting, rain gardens, and green roofs present promising, cost-effective alternatives that address runoff control directly at the source. Evaluating their effectiveness in the Indian context, however, remains a challenge.
A recent study, led by Dr. Tushar Bose from CEPT University, along with Professors Pradip Kalbar and Arpita Mondal of IIT Bombay, focuses on the effectiveness of green roofs as a flood mitigation strategy in Ahmedabad’s densely populated Odhav area. Funded by the Government of India’s Science and Education Research Board, the research—published in the Journal of Environmental Management—demonstrates that green roofs can reduce flood volumes by up to 60% in high-density urban areas during extreme rainfall events.
Green roofs consist of a shallow layer of soil over a waterproof membrane and drainage system, allowing trees to be planted on rooftops. These green roofs not only help cool buildings in summer but also absorb rainwater, which can then recharge rainwater harvesting systems gradually, reducing runoff and mitigating flood risks. While green roofs require additional expenditure, maintenance, and increase the building’s weight load, they represent a scalable approach to flood mitigation.
Previous studies have evaluated combined strategies like rain gardens, infiltration trenches, and green roofs, primarily in Western countries, but few studies focus solely on green roofs in India. Additionally, not all buildings in India are suitable for green roof installation; low-cost housing and slum areas with metal or concrete sheet roofs, as well as industrial buildings, are typically unsuitable.
For this study, the team assessed suitable rooftops for green roof installation, factoring in land use, local rainfall patterns, terrain, and natural water drainage paths to calculate potential runoff and flood volumes in specific sub-catchments. They developed a computer model that simulated water flow in the area and used it to estimate runoff and flood volumes under various heavy rainfall scenarios and green roof implementation rates.
The study examined the impact of green roofs covering 25%, 50%, and 75% of suitable rooftops, revealing that flood volumes could be reduced by 10-60%, depending on application rates and drainage system capacity. The findings indicate that green roof benefits are amplified with increased coverage, creating a collective impact that significantly curbs urban runoff. This research offers policymakers a city-specific blueprint, suggesting that green roofs could play an essential role in urban flood resilience planning.
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