Planting small trees on roofs of buildings in dense urban areas can reduce flood volume and runoff.
In the past decade, floods in big and densely populated cities have become more frequent. They are increasingly causing damage to property and infrastructure, and loss of life. As buildings, pavements and roads made of concrete or tar increase, water-absorbent areas on the ground reduce. After heavy rainfall, water flows rapidly in large volumes and quickly accumulates in low-lying areas. Risks of epidemic breakout and infections after the floods recede are higher due to the dense population over a large area. Any damage to centres of commercial and national importance situated in cities can also potentially affect a larger population.
Floods in cities are very different from rural floods in their nature, causes and effects. They need different analysis and mitigation methods. Cleaning stormwater drains, adding stormwater silos that can act as local storage and avoid runoff, interlinking lakes and installing water pumps to remove accumulated water are some measures that the administration is implementing centrally. However, these methods involve large infrastructure changes and are expensive. Implementing small-scale distributed measures such as rainwater harvesting, rain gardens and green roofs that help control runoffs closer to their source is more sustainable.
Small-scale and sustainable measures cost less than large infrastructure changes. However, it is important to study their effectiveness and quantify their benefits. In one such effort, Tushar Bose, faculty, CEPT University, Ahmedabad and Prof Pradip Kalbar and Prof Arpita Mondal at the Indian Institute of Technology Bombay (IIT Bombay) evaluated the performance of ‘green roofs’ in reducing floods in dense urban areas. The study was published in the Journal of Environmental Management. The project received funding from the Science and Education Research Board, Govt. of India.
Green roofs are created by planting trees on rooftops of buildings in a shallow layer of soil over a waterproof membrane and a drainage system. Green roofs can keep the building cooler in summer and absorb rainwater. Excess water can help slowly recharge the rainwater harvesting system, avoiding fast runoff. Installing green roofs needs additional expenditure and increases the weight load on the building. It also needs regular maintenance. So, the advantages of implementing green roofs must be carefully evaluated against their cost.
Previous studies assessing the effectiveness of combined strategies such as rain gardens, infiltration trenches and green roofs are available for Western countries. There are very few studies that assess the performance of only green roofs, especially in the Indian context. In India, not all buildings are suitable for green roof installation.