Mumbai: With floods in big and densely populated cities becoming more frequent, the need for implementing sustainable, small-scale measures such as ‘green roofs’ that help control runoffs closer to their source has increased.
To study the effectiveness and quantify its benefits, Tushar Bose, faculty, CEPT University, Ahmedabad and Prof Pradip Kalbar and Prof Arpita Mondal at the Indian Institute of Technology Bombay (IIT-B) evaluated the performance of ‘green roofs’ in reducing floods in dense urban areas.
To study green roofs, which are created by planting trees on rooftops of buildings in a shallow layer of soil over a waterproof membrane and a drainage system, the researchers chose the Odhav area of Ahmedabad, Gujarat. They divided the total area of 100 hectares into 19 sub- catchments to create the model. They identified buildings suitable for green roof installation.
Buildings with roofs of metal or concrete sheets and industrial buildings are not considered suitable. They considered land use, local rainfall patterns, the terrain and natural pathways of the water drainage of each area to calculate the runoff and flood volume in the sub-catchments.
“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,” the authors said.
Further, the researchers created a computer model that mimicked how water flowed in the area. Using this model, they calculated the runoff and flood volume assuming different scenarios of heavy rainfall events and various percentage values of green roof implementation.
The data that the researchers used included the model of the terrain of the area, soil type, stormwater network layout, and whether the land is unused or used for buildings, gardens or other purposes, obtained from the Ahmedabad Municipal Corporation and in-person surveys; and rainfall data from IMD.
The IIT-B team considered scenarios where green roofs were installed in 25%, 50% and 75% of the suitable buildings. They calculated the flood volume reduction for a total of 36 scenarios, where they considered three green roof application rates (25%, 50%, and 75%), four extreme rainfall event frequencies (extreme rainfall events once in 2, 5, 10, and 25 years), and three scenarios of extreme rainfall event duration (2, 3, and 4 hours).
They also explored the minimum application rate needed for each of the 12 scenarios created by varying the extreme event frequency and duration. The researchers also calculated the uncertainties in the model predictions.
The study found that using green roofs can reduce the flood volume by about 10-60% 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.
The researchers observed that when less than 25% of buildings have green roofs, the flood volume and runoff reductions can be as low as 5%. When the green roofing is more, there is a collective effect over the whole area and flood volume is reduced.
The study also quantifies the uncertainty in the prediction of flood volume reduction and finds that the rainfall intensity contributes maximum to the uncertainty of runoff reduction.
The findings of the study, which was funded by the central government, can help policymakers make informed decisions about city-specific green roof implementation.