Green roofs can help reduce floods in dense urban areas

Mumbai, Oct 31 (PTI) Planting small trees on roofs of buildings in dense areas can reduce floods in big and densely populated cities, according to a study jointly conducted by the Ahmedabad-based CEPT University and the Indian Institute of Technology Bombay (IIT Bombay). 
As buildings, pavements and roads made of concrete or tar increase, water-absorbent areas on the ground reduce, causing urban flooding and water-logging in low-lying and dense urban localities after heavy rainfall, it stated. 
Implementing small-scale distributed measures such as rainwater harvesting, rain gardens and green roofs that help reduce flood volume and control runoffs closer to their source, the study noted. 
According to the study, 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, and the excess water can help slowly recharge the rainwater harvesting system, avoiding fast runoff, it noted. Installing green roofs needs additional expenditure and increases the weight load on the building and also needs regular maintenance. 
However, in India, all buildings are not suitable for green roof installation. Slums and some low-cost housing having metal roofs or concrete sheets are not suitable for green roof installation, the study pointed out. 
“A significant contribution of the study is that it provides a realistic performance assessment in a highly dense urban area and quantifies the overestimation of runoff reduction. These overestimates arise from scenarios that consider all rooftops without evaluating the green roofing potential,” said CEPT University faculty Tushar Bose and IIT B Professors Pradip Kalbar and Arpita Mondal. 
The study also found that using green roofs can reduce the flood volume by 10 to 60 per cent depending on the percentage of green roof application for higher than usual rainfall of one in two- year frequency. 
However, the reduction is not linearly proportional to the application rate because flood volume also depends on the drainage network capacity, especially at high volumes of rainfall, it stated. The researchers observed that when less than 25 per cent of buildings have green roofs, the flood volume and runoff reductions can be as low as 5 per cent.