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July 4, 2024Cloudburst in Kochi Exposes Urgent Need for Effective Stormwater Management
Kerala faces significant challenges in collecting accurate rainfall data due to a shortage of rain gauges, adversely affecting the design of stormwater infrastructure.
Key Takeaways
- Effective stormwater management requires a comprehensive approach integrating rainfall analysis, engineering, environmental science, policy, and community engagement.
- Kerala faces significant challenges in collecting accurate rainfall data due to a shortage of rain gauges, adversely affecting the design of stormwater infrastructure.
- Cities need to reduce runoff by converting impervious surfaces into pervious ones and incorporating rainwater harvesting systems, as extensive concrete surfaces increase runoff coefficients.
- Reconstructing existing drainage systems according to guidelines and improving administrative efficiency and transparency are crucial for developing effective stormwater management plans and enhancing urban resilience.
On May 28, the city of Kochi experienced an intense cloudburst, resulting in over 100 mm of rainfall per hour and significant waterlogging across various parts of the city. This event highlights the critical issue of urban water stress exacerbated by climate change and underscores the importance of how governing bodies address these challenges. Urban water stress is exponentially increasing in India, manifesting as both floods and droughts. There is a pressing need to move beyond the common excuse of “climate change” and focus on actionable steps that cities can take to mitigate these impacts.
The immediate response to such events often revolves around questions of how to tackle changes in weather conditions or reduce the intensity of a cloudburst. While altering weather patterns is beyond human control, there are infrastructural and planning measures that can be implemented to manage and mitigate the impact of such extreme weather events.
The IRC:SP:50-2013 guidelines by the Indian Road Congress provide a comprehensive framework for designing stormwater drainage systems for road networks. This guideline emphasises the importance of system planning prior to the commencement of design for the successful development of stormwater drainage. Effective stormwater management requires a comprehensive approach that integrates rainfall analysis, engineering, environmental science, policy, and community engagement to address the multifaceted challenges posed by stormwater runoff. However, Kerala faces a significant challenge in collecting rainfall data due to a shortage of rain gauges. According to the Kerala State Disaster Management Authority (KSDMA), the state lacks sufficient rain gauges to collect accurate rainfall data. The India Meteorological Department (IMD) currently operates only a few automated rain gauges in Kerala, gathering daily rain data from just 68 stations, which results in inaccurate rainfall forecasts.
According to the Manual on Stormwater Drainage Systems by the Ministry of Housing and Urban Affairs (MoHUA), the rain gauge density for an urban area with a population above 1 million should be 1 rain gauge per 5-10 sq.km. For the Greater Cochin Development Authority (GCDA) area, which spans 632 sq.km and had a population of 2.11 million according to the 2011 census, this translates to a requirement of 63 to 126 rain gauges. Currently, Ernakulam district, which includes Kochi, has only 15 rainfall data observatories managed by the IMD. This discrepancy in the number of rain gauges results in inaccurate rainfall data, adversely affecting the design of stormwater infrastructure.
Without precise rainfall records, the design of stormwater drains often relies on generalised rainfall intensity values ranging from 12 mm/hr to 20 mm/hr, which are not specific to the area. This inadequacy becomes glaringly evident during extreme events like a cloudburst with a rainfall intensity of 100 mm/hr.
To address high-intensity rainfall events, cities need to reduce the volume of runoff water from each parcel of land. The peak flow for which stormwater drains are designed is directly proportional to the runoff coefficient, which describes the area’s ability to absorb or shed water. Urban areas, with their extensive concrete surfaces, have high runoff coefficients (the runoff coefficient adopted in a fully developed area is 1.0), resulting in significant runoff.
One effective strategy is to reduce the runoff coefficient of the catchment area by converting impervious surfaces into pervious ones. By converting concrete roofs (runoff coefficient of 0.95) into green roofs (runoff coefficient of 0.17) and incorporating rainwater harvesting systems in all parcels of land without exceptions (mentioned in the Kerala Municipal Building Rules, 2019) based on plot size or built up area, 60-70% of the runoff water from individual land plots to the road network could be allowed to settle into the ground. Similarly, by introducing lawns (runoff coefficient of 0.10) and parks/playgrounds (runoff coefficient of 0.25) alongside the current asphalt roads (runoff coefficient of 0.70-0.95), the volume of stormwater entering the drainage system from streets can be reduced by 50-60%. Moreover, planting more trees can help intercept rainfall, with trees acting as natural umbrellas and soil
with trees allowing 10 inches/hr (254mm/hr) rain to infiltrate into the ground. Approximately 67% of rainfall intercepted by trees returns to the atmosphere through evapotranspiration, thereby reducing the amount of water reaching the ground.
Reconstructing existing drainage systems, traffic islands, and open spaces along roadways in line with the IRC:SP:50-2013 guidelines and the Manual on Stormwater Drainage Systems is crucial. Such reconstruction should incorporate accurate, area-specific rainfall data and focus on effective system planning. A shift is also required at the administrative level to enhance efficiency and transparency among departments responsible for stormwater management. This includes developing stormwater management plans that account for design return periods (the average period of time after which it reoccurs for a given rainfall intensity or more and corresponds to a particular duration of time) and ensuring that infrastructure is designed to handle specific rainfall events.
The recent cloudburst in Kochi serves as a stark reminder of the urgent need for comprehensive stormwater management strategies. By adopting a holistic approach that integrates accurate data collection, green infrastructure, and effective system planning, cities can better manage the impacts of extreme weather events and enhance urban resilience in the face of climate change.
(Nikhil Ali is Research Associate, Centre for Public Policy Research.)
Views expressed by the author are personal and need not reflect or represent the views of the Centre for Public Policy Research.
Nikhil Ali is an Associate, Research at the Centre for Public Policy Research. He completed his graduation in Civil Engineering from Sree Narayana College of Engineering and is a seasoned Civil Engineer with working experience at Tata Realty and Infrastructure Ltd. With a passion for urban planning, he acquired his master's degree in Urban Planning from Hindustan Institute of Technology and Science, Chennai. His expertise lies in Urban Mobility, land use planning/analysis, and water-sensitive planning.
