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Effective flood management in Ahmedabad requires more than green infrastructure, revealed a joint study by Indian Institute of Technology Gandhinagar (IITGN) and the University of Cambridge, UK.
The joint research, conducted by Angana Borah (a PhD student) and Prof Udit Bhatia from the Department of Civil Engineering at IITGN, and Prof Ronita Bardhan from the University of Cambridge to find data-backed scientific answers to the role of popular alternative flood adaptation measures, revealed that contrary to the current and widely adopted perception, benefits of uniform green infrastructure interventions for urban flood management are marginal.
Even at a 24 per cent application rate per sub-catchment area, green infrastructure interventions such as green roofs and permeable pavements do not exhibit a significant impact on peak flow that contrasts with earlier studies suggesting flood volume reductions up to 96 per cent, the findings show.
The study underscores the necessity for location-specific models or green infrastructure (GI) solutions and enhancing conventional urban drainage infrastructure to mitigate the risk of urban flooding during excessive downpours.
“The results underscore an essential point that the reduction in flooded areas does not directly result in a decrease in peak flood depth within the zones. This highlights the importance of considering variations in flood characteristics such as depth and extent, when evaluating the efficacy of flood management strategies,” the study states as shared by IITGN.
Explaining the significance of the work, Prof Udit Bhatia, Assistant Professor of Civil Engineering at IITGN, said, “With accelerating urbanisation, flood risks are becoming increasingly pervasive. This study highlights the necessity for location-specific models that critically evaluate the effectiveness of flood mitigation strategies such as green infrastructure. Universal solutions are ill-suited to address the unique challenges posed by rapid urban growth and climate change.”
The results reinforce the need to strengthen and expand our cities’ existing stormwater drainage systems to counter urban flood crises instead of relying on alternative pathways like green infrastructure, he added.
The research team chose the west zone administrative division of the Ahmedabad Municipal Corporation (AMC), a rapidly growing metropolitan city with historical relevance and India’s first UNESCO World Heritage City, to conduct the study.
Despite having stormwater drainage infrastructure coverage of 55 per cent, Ahmedabad has experienced severe flooding in the past few years owing to the monsoon rains. “Pragmatic urban greening solutions for climate action are critical in Indian heritage cities as they are cauldrons of traditions and culture that can disappear in the changing climate,” says Prof Bardhan.
Discussing the mainstream solution for this urban challenge, she adds, “As cities grapple with the increasing incidence of urban flooding, conventional drainage systems — once the backbone of urban flood management — are frequently overwhelmed. This inadequacy is compounded by two critical factors: the rapid and extensive urbanisation that modifies land topography and disrupts existing drainage patterns and the escalating pressures from climate variability and change. Together, these forces render traditional infrastructure insufficient. Hence, we need to strengthen the conventional urban drainage systems and augment them along with green infrastructure solutions to ensure improved resilience against flooding and further environmental changes.”
The team carried out comprehensive field surveys using a hydrodynamic model — a mathematical tool that explains the flow of water in any given system such as rivers, lakes or floodplains and cities using theories of conservation of mass and momentum — after major flood events in various areas in the west zone a day after the flood levels receded.
The study also integrated high-resolution remote sensing data from satellites to validate the model. This specific method of urban flood modelling, particularly concerning green infrastructure adaptation, is a novel and pioneering endeavour in the Indian context and can be used to analyse urban flood management interventions in any densely populated city.
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The study evaluated four types of green infrastructure — green roofs, permeable pavements, bioretention cells and vegetative swales — in Ahmedabad. Vegetative swales are engineered structures designed with multiple layers of soil for the purpose of growing vegetation while also aiding retention, infiltration, and evapotranspiration, essential for the reduction of flood peaks and flood extents.
After scrutiny, the researchers used several metrics to compare the advantage/efficacy of one green infrastructure with another regarding their ability for peak flow reduction and flood volume reduction.
“In the face of increased incidences of erratic weather and resultant flooding in rapidly urbanising cities, effective urban flood management strategies have become both a challenge and an urgent requirement. While many large cities around the world, including India, have employed alternative solutions like Green Infrastructure (GI) interventions, its effectiveness in mitigating urban flooding and water logging during torrential rains remains a puzzle,” the study highlights.
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