Parts of south Calcutta were deluged with more than 100mm of rain in just two hours on Thursday afternoon, causing widespread flooding and traffic chaos that lasted well into the evening.

The downpour was strikingly uneven across the city. While southern neighbourhoods like Ballygunge recorded 107mm and Jodhpur Park saw 105mm of rainfall, northern areas received barely a drizzle — Maniktala got just 10mm and Belgachhia less than 5mm. For context, meteorologists classify anything over 60mm in 24 hours as heavy rain.

The heaviest rainfall occurred between 1pm and 3pm, leaving major thoroughfares submerged under knee-deep water. By 3pm, key areas, including Syed Amir Ali Avenue, Ballygunge Phari, Park Street, Sarat Bose Road, and Rashbehari Avenue, were flooded.

The timing couldn’t have been worse. At 2.30pm, schoolgirls at a Ballygunge bus stop stood on benches to keep their feet dry. By 3.30pm, the intersection near Deshapriya Park was under almost knee-deep water.

Many areas remained waterlogged hours later. Even at 7pm, Southern Avenue, Pratapaditya Road, and sections of Rashbehari Avenue were still flooded.

Traffic crawls

The flooding brought the city’s transport network to a virtual standstill. A routine 4.5-km drive from St Xavier’s College on Park Street to Gariahat took over an hour and 15 minutes. Children returning from school were trapped in traffic jams that lasted hours.

Senior police officers reported that among the worst-affected areas was a stretch of Syed Amir Ali Avenue near Panchanantala, where rainwater accumulated on both sides of the road. Vehicles crawled along Theatre Road, Suhrahwardi Avenue, around Golpark and Southern Avenue, officers said.

“Accumulated rainwater slowed traffic on stretches of Ananda Palit and CIT Road near Entally, on Gobinda Khatik Road and parts of China Town, Prince Anwar Shah Road and NSC Bose Road in Tollygunge,” said a senior officer. “Evening traffic slowed down, leading to tail-ups at some points.”

In the city’s southeast, water accumulated on a stretch of Prince Anwar Shah Road Connector, between Gangulypukur and the approach to Jibanananda Setu, forcing vehicles to slow down and move in a single lane across on both flanks.

In Bhowanipore, ward councillor Ashim Basu joined police officers to manage traffic as cars struggled through waterlogged streets.

On the Parama flyover, traffic was affected on both flanks. In the evening, long queues of vehicles inched forward, with the tail-back reaching the Park Circus seven-point crossing and stretching till Gariahat.

Commuters had a harrowing time as overcrowded buses crawled on flooded stretches across south Calcutta.

Why the flooding

The flooding resulted from multiple converging factors. Heavy rain coincided with high tide in the Hooghly River, forcing the Kolkata Municipal Corporation to shut drainage sluice gates from 10am to 4pm. This prevented rainwater from flowing out to the river.

“The high volume of rain in a short time, together with high tide, prevented the water from being drained out,” said a KMC drainage department official.

The city’s infrastructure limitations became starkly apparent. Calcutta’s underground drainage system is designed to handle only 6mm of rainfall per hour — a fraction of Thursday’s deluge.

Weather patterns

Meteorologists attributed the intense rainfall to the monsoon trough positioned over south Bengal and a cyclonic circulation over Gangetic West Bengal. “The convergence of the clouds was more over the southern parts of the city,” a Met official said.

The monsoon trough currently passes through Jaisalmer in Rajasthan, Datia in Madhya Pradesh, Ranchi in Jharkhand, Bankura, and Digha before extending into the east-central Bay of Bengal.

Expect more rain

Similar weather conditions could affect the city again on Friday, though the intensity is expected to decrease from Saturday onward.

The Met office has forecast heavy to very heavy rainfall (7-20 cm) for Bankura, Purulia, and West Midnapore districts on Friday. Heavy rain is also expected in Jhargram, East Midnapore, North and South 24-Parganas, and Howrah. Several other south Bengal districts, including South 24-Parganas, West Midnapore, East Midnapore, Howrah, Hooghly, and Birbhum, also experienced significant rainfall on Thursday.

Additional reporting by Subhajoy Roy and Kinsuk Basu