Intense rainfall spotlights the fault in Kolkata’s aging, clogged drainage systems

On September 23, Kolkata received one-eighth of its annual rainfall in only six hours. At least eight people died due to accidental electrocution.
It led to severe waterlogging in many parts of the city and drainage took time as old sewage and drainage systems in the city were filled with plastic and other debris.
Experts call for repairing and de-clogging drainage systems and improving the carrying capacity of canals, as the frequency and intensity of rainfall events increase due to climate change.

On September 23, as Kolkata was preparing for West Bengal’s biggest annual festival, Durga Puja, it encountered roughly 12% of the city’s average annual rainfall in just six hours. From midnight to dawn, it rained 234.2 mm causing severe waterlogging that the city had not experienced in almost four decades.

Kolkata is used to cyclonic storms bringing the city to a standstill with fallen trees and electric poles. This time however, there was no cyclone. The rain mostly stopped around 8.30 a.m. Yet, water alone paused civic life — from flight operations to railway and bus services. This cloudburst-like situation, where rain falls heavily in a short period, is new to the city located in the world’s largest deltaic landscape, the Bengal delta. Kolkata was unprepared.

Informal settlements, ground floors and basements of houses, high rises and hospitals were all flooded with drainage water or overflow from waterbodies. People were stranded in multi-storey apartments without electricity or an operational elevator for hours. Damaged cars, motorbikes, and vans were left in the middle of roads. Loose, live wires created safety hazards.

Items from grocery and garments to electronics and electrical goods worth millions of rupees were damaged in stores and markets. Thousands of books got drenched in the heritage book market of the College Street area.

At Netaji Nagar in the southern part of the city, a bicycler lost balance while navigating through a flooded road. He fell on an electric pole and was instantly electrocuted. Fearing electrified water, none dared to rescue his body, which remained half-afloat for hours.

A small book stall owner saw her entire shop washed away on the night of September 22. She managed to rescue a few items and is now storing them, hoping to clean and resell them. Image by Subhrajit Sen.

The adjacent drainage canal, Adi Ganga or Tolly’s Nullah, largely choked, had overflowed. The local people attributed the choking to the massive concrete pillars of overhead metro rail tracks erected in the middle of the canal.

As electrocution deaths were reported — at least eight — from different parts of Kolkata, West Bengal chief minister Mamata Banerjee advised people to stay indoors. She announced work-from-home for all government employees, except those on disaster response, and urged private sector companies to allow their workforce to work from home for two days.

Soon, it emerged that Kolkata’s drainage system was not functioning in many parts. The underground drains and the canals clogged with primarily plastics. By September 24, roadsides had turned into dump yards for heaps of garbage, predominantly plastics that had been extracted from the drainage system.

The Calcutta High Court has taken suo motu cognisance of electrocution deaths and has asked for reports from the civic authorities on the status of drainage system and repair work and from the power supplier, the private company Calcutta Electric Supply Corporation (CESC), regarding safety and maintenance.

Meanwhile, the chief minister, while blaming the lack of dredging in the Hooghly river, an arm of the Ganga that forms Kolkata’s western boundary, asked, “Where will the water go when the Ganga is already full because no dredging has been done?”

River dredging is the process of removing sediment, silt, and debris from the bottom to increase its water-carrying capacity and ease ship navigation. However, researchers have warned about the negative impacts of dredging on the ecosystem of this heavy silt-carrying river.

Dipak Bhowmik, a 62-year-old paper seller, says he lost almost ₹50,000 worth of paper stock due to water damage Image by Subhrajit Sen.

Not an aberration

The 98 mm rainfall between 3 a.m. and 4 a.m. on September 23, fell slightly short of the 100 mm per hour threshold for qualifying as a cloudburst.

Between June 1 and September 25 this year, Kolkata and its neighbouring districts of Howrah and North 24-Parganas have received 27%, 28% and 27% excess rainfall than normal, respectively.

Scientists say such events can no longer be considered as aberrations; rather, they need to be seen as part of the new normal, where extreme weather events like drought, flood, heat waves and cyclonic storms are expected to increase, either in frequency or intensity.

According to Mahesh Palawat, Vice President, Meteorology and Climate Change, Skymet Weather, a private weather forecast service, the Bay of Bengal has been witnessing immense warming on account of rising temperatures, leading to more evaporation.

Under such conditions, a low-pressure area attracts more moisture from the oceans, making the weather activity more intense. “While this kind of rainfall is unusual, we need to be ready for more such events considering the impacts of global warming,” Palawat said.

Over the past seven decades, Kolkata has recorded the highest urban warming among global megacities — 2.6 ° Celsius between 1950 and 2018. According to a 2024 UN report, Kolkata, along with eight other megacities (cities exceeding 10 million inhabitants in 2025) — Dhaka, Cairo, Karachi, Lahore, Lagos, Tokyo, Osaka and Los Angeles — could be up to 1°C warmer by 2040.

This is not good news for Kolkata. “Every degree rise leads to a rise in moisture content by 7%,” Kartiki Negi, lead of climate impacts at Climate Trends, a New Delhi-based research and advocacy group, told Mongabay India. Increasing moisture content, in turn, can intensify weather activities.

Negi said that the extreme rain event in Kolkata needs to be seen in the context of the rapid rise in climate change-led extreme weather activities, including cloudburst-like events. “With climate change continuing to raise temperatures, especially the catastrophic combination of land surface and ocean heating, we need to equip ourselves better with technology interventions like early warning systems, etc.” Negi said.

Stagnant water being drained out using a pump. Following the floods of September 23, slum areas, ground floors and basements of homes, hospitals and other buildings in Kolkata were flooded with drainage water or overflow from waterbodies. Image by Subhrajit Sen.

The challenges

Kolkata sits between the river Bhagirathi-Hooghly on its west and swamps and marshes to its east. Drainage congestion is a routine affair during monsoons, when Hooghly river, the tidal creeks and the swamps overflow. The low elevation (20 feet above sea level on an average) and flat, saucer-like topography worsens Kolkata’s drainage issues. This is one of the reasons why the threat from sea-level rise also knocks on Kolkata’s door.

The city slopes downwards towards the east. New townships and roadways in the eastern fringes have impacted natural drainage paths. The once-mighty Bidyadhari that runs parallel to Hooghly river has ceased to be a river and has turned into a mere sewage channel, which again is gasping for breath due to the dumping of waste and encroachments.

Besides, the East Kolkata Wetlands (EKW) — a Ramsar Site or wetland of global importance that played a key role behind natural sewage treatment and flood control in Kolkata — has its wetland areas shrunk from 91.2 km2 in 1991 to 33.4 km2 in 2023.

The climate action report for the city, released earlier this year, noted that Kolkata is one of the world’s most climate vulnerable cities — facing increasing threats from heat waves, flooding, sea level rise and Category 3-5 cyclones (extremely severe to super cyclones).

Corporation workers clean after the floods on College Street. The floods made it apparent that Kolkata’s drainage system was not functioning in many parts. The underground drains and canals were clogged with plastic waste, and roadsides had turned into dump yards with plastics that had been extracted from the drainage system. Image by Subhrajit Sen. — Corporation workers cleaning up after the floods on College Street. The floods made it apparent that Kolkata’s drainage system was not functioning in many parts. The underground drains and canals were clogged with plastic waste, and roadsides had turned into dump yards with plastics that had been extracted from the drainage system. Image by Subhrajit Sen.

The fault in the drainage system

The heavy rain spotlighted Kolkata’s old and stressed drainage system, dating back to 1742 for the Maratha Ditch and 1820 in the case of the Circular Canal, which has become inadequate. The city’s climate action plan suggests that the existing drainage network needed to be upgraded and expanded, the canals restored and properly maintained.

Following the September 23 flooding, Kolkata mayor and state urban development minister Firhad Hakim, who was seen on the streets in waist-deep water, said that finalising the climate action plan and improving disaster response is one of their top priorities.

Tarak Singh, member-mayor-in-council of the sewerage and drainage (S&D) department of Kolkata Municipal Corporation (KMC) said that water logging remained beyond one day in only a few pockets. “In 2024-25, we have removed over 2,00,000 metric tons of silt from the city’s underground sewer,” Singh said. “Pumping stations, the number of pumps, mechanised desiltation… every capacity has been significantly enhanced. The September 23 rain would have left any megacity waterlogged,” he added.

A senior KMC official, who did not want to be named because they were not authorised to speak to the media, said that in some areas, the logged water could not be cleared despite continuous pumping, as the outflow outlets were full and the discharged water kept flowing back into the same place. “The number of pumping and lifting stations have increased over the past few years, but in many congested areas, there is absolutely no space for setting up new pumping or lifting infrastructure,” said the official.

According to Pankaj Kumar Roy, professor and former director of the School of Water Resources Engineering at Jadavpur University, the KMC authorities are upgrading and repairing the drainage system, which is why the water retention time was only one day in most parts of the city. “However, given the possibility of such intense rainfall events recurring, a greater focus is required on the carrying capacity enhancement of the outlets, usually canals. Repair, desiltation and de-clogging of the drainage system is necessary, but no less important is the desiltation of the canals,” Roy told Mongabay India.

The first step should be to identify the outlets where the pumped water would be discharged from waterlogging prone areas, and assess the carrying capacity of those outlets. “Assessing the carrying capacity of outlets where the pumped water would be discharged and restoring their carrying capacity to the required level is absolutely necessary to prepare for cloudburst-like situations,” he said.

Banner image: One of Kolkata’s newly planned zones, home to high-rises and office complexes, gets flooded. Much of it, however, has been built on encroached land from the East Kolkata Wetlands, a natural lowland. Every monsoon, residents and office-goers face severe problems because of the area’s poor drainage system. Image by Dipanwita Saha.