Urban flooding is a perennial problem in several Indian cities that poses various challenges to their growth and development. However, a study conducted jointly by CEPT University, Ahmedabad and Indian Institute of Technology (IIT), Bombay has found that low-impact development (LID) strategies such as having a ‘green roof’ is a potential way out of this menace.
Green roofs can reduce up to 62 per cent of the flood volume and 24 per cent of the runoff, said a group of three senior faculty members who conducted the study ‘Performance and uncertainty assessment of green roofs for urban flood reduction in a high-density catchment in Ahmedabad, India’ — published in Journal of Environmental Management.
A green roof or living roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems.
In the study, green roofs have been evaluated for a catchment in Ahmedabad, following an extensive traverse survey to map all the city buildings and their uses. From the potential of green roofs, three scenarios — with 25 per cent, 50 per cent and 75 per cent of roofs being turned into green roofs — have been evaluated for three storm durations (two, three and four hours) and four storm frequencies (two, five, 10 and 25 years).
“Results show that the achieved reduction in flood volume is between 9.9 per cent and 62.2 per cent for two-year storm frequencies. However, reduction in flood volume with an application of green roofs is non-linear as the volume can also get influenced by drainage network capacity during high-intensity rainfall,” Tushar Bose, faculty (Technology) at CEPT University said.
This analysis was based on data on land use, topography, soil type, stormwater layouts, stormwater discharge and rainwater collected to set up a hydrological-hydraulic model.
Further, the maximum reduction in runoff for a four-hour-long storm is 24.1 per cent — for a two-year storm frequency. The runoff reduction decreases with increased intensity of rainfall and increases with the application rate of green roofs. “When green roofs are applied to less than 25 per cent of the buildings, in all the cases except two-year frequencies, the runoff reduction remains less than 5 per cent,” Prof Bose added.
A catchment in Ahmedabad city’s Odhav area was a selected area in the study, based on stormwater lines as well as the merging of contiguous sub-catchments’ draining into the Arbudanagar pumping station, thus forming a catchment. This selected study area, covering 100.1 ha in total, is prone to frequent flooding and has a considerable built-up area, making it a case fit to demonstrate typical urban catchments.
The runoff reduction represents the decrease in discharges compared to the baseline scenario, with no application of green roofs for the same intensity of rainfall at the discharge point (pumping station). The reduction in flood volume is the dip in overflow volume from the system, compared to the baseline scenario with no application of green roofs.
Based on the stormwater network, data collected from the Ahmedabad Municipal Corporation (AMC) along with discharge data from the pumping station is collected for 2020 at an hourly frequency, and used for calibration and validation. Further, hourly rainfall data was collected from the India Meteorological Department (IMD) from 1979-2018 and the AMC from 2018-2021.
Pradip Kalbar, Interdisciplinary Programme in Climate Studies, IIT Bombay, meanwhile, said, “To address this challenge, some approaches have been tried, such as the interlinking of lakes in cities and stormwater silos. These are capital-intensive and reactionary, and thus, do not integrate with the local catchment. To identify appropriate solutions for building resilience in cities, there is a need to apply life-cycle perspectives and sustainability assessment methods.”
Researchers believe the study is likely to build confidence in the acceptance of LID strategies, thus increasing their application as an adaptation strategy. Taking the study forward, the researchers plan to supplement this with another paper on the implementation of green roofs.
“We are looking at the policy linking, for structural changes — at least in new buildings in the future. There are other co-benefits of green roofs, such as bringing down temperatures, reducing cooling demand from ACs, and climate change mitigation in cities. The idea is to bring transparency in the developing resilience for flooding in cities,” Kalbar added.
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