173mm in 15 hours: city struggles to stay afloat.......Overnight downpour sinks Kolkata....

A waterway called Camac Street on Saturday afternoon. Picture by Pradip Sanyal

Rith Basu and Deepankar Ganguly, TT, Calcutta, Aug. 1: Large parts of Calcutta went under water as the city received as much as 173mm of rain in 15 hours, from 10pm on Friday to 1pm on Saturday.

The spring tide in the Hooghly at 1.17am and again at 1.40pm on Saturday worsened the situation as water in Tolly's Nullah overflowed and clogged drainage channels in Kalighat, resulting in prolonged waterlogging in many parts of south Calcutta. Kalighat Road was under waist or knee-deep water.

The weather office predicted heavy rain in south Bengal in phases over the next 48 hours as Cyclone Komen, which has turned into a depression, stayed put at Jessore in Bangladesh, 102km from Calcutta, for most of Saturday.

The depression may enter Nadia by Saturday night and weaken into a well-marked low pressure area before crossing over to Jharkhand.

Metro decodes the phenomenon that caused Saturday's deluge:

How much rain did Calcutta get?

A steady shower that started on Friday evening continued through the night. The Calcutta Municipal Corporation's figures reveal the city received 118mm of rain in eight hours, from 10pm on Friday to 6am on Saturday. Another 55mm was recorded between 6am and 1pm, taking the total rainfall to 173mm in 15 hours, since 10pm on Friday.

Why the waterlogging?

Calcutta's drainage system can drain out only 6mm of water an hour. To drain out 173mm in 15 hours, 11.5mm would have to be drained per hour, which is twice the capacity of the drainage system.

However, even "6mm per hour" remained a far cry for most part of the day because of the spring tide in the Hooghly.

Large parts of the city - including Hospital Road, Sarat Bose Road, Southern Avenue, Kali Temple Road, CR Avenue (off Muktaram Babu Street), Thanthania, Rashbehari Avenue, Amherst Street and College Street - were under water even a couple of hours after rain had stopped around 1pm.

What caused Tolly's Nullah to overflow in Kalighat?

The water level in the Hooghly rose from 5.6ft to 23.3ft, 1.6ft more than what was expected, under the combined impact of the spring tide and Cyclone Komen, which turned into a depression after making landfall in Bangladesh on Thursday.

The Hooghly water flowed over the lock gate in Kidderpore and entered Tolly's Nullah during high tide on Friday night and Saturday afternoon, raising the water level of the canal. The water blocked underground sewerage channels and innundated Kalighat Road and other thoroughfares.

On Sunday, the tide can be even higher.

What caused the rainfall on Friday and Saturday?

The Komen effect. The system has been constantly sucking moisture from the Bay of Bengal.

What lies ahead?

"We expect heavy showers in south Bengal on Sunday and Monday, especially in the western districts. Calcutta should get decent rainfall, too," said Devendra Pradhan, of the India Meteorology Department, Calcutta.

Were flights or trains disrupted?

Flights were not disrupted though several hangars (areas where planes are parked) were waterlogged. The Calcutta-Lalgola-Hazarduari Express was cancelled. Local trains ran late from Sealdah and Howrah stations.

Three deaths

A 55-year-old man died after a part of a two-storeyed house at 10 Hindusthan Park, off Gariahat, collapsed around 10pm. Amitabha Kar was lying on a sofa on the first floor when the ceiling caved in. A civic official said incessant rain over the past few days might have triggered the collapse. Debashis Kumar, a mayoral council member, said the Calcutta Municipal Corporation had declared the structure "condemned" at least 10 years ago.

Asif Iqbal died after being electrocuted while trying to roll up the wet iron shutters of his shop in Topsia. "A wire running through the shutter wet with rainwater seems to have caused the electrocution," an official said. Halima Khan, 43, died after a wall of her mud house in Howrah's Panchla collapsed on Friday following the continuous rain.

TNN, 2 Aug 2015, KOLKATA: The city woke up to that sinking feeling on Saturday morning as overnight rains coupled with high tide in the Hooghly turned roads into waterways and houses into islands. 

Those who ventured out for work found public transport nonexistent and suburban commuters suffered as trains ran late in both Sealdah and Howrah sections. At the Kolkata airport, water entered the tarmac and apron areas and flights were delayed by 10-15 minutes due to slow movement of aircraft. 

With most people marooned at home, civic bosses hopped through knee-deep water to reassure households that this was a one of its kind situation. Officials at KMC control room were busy attending phone calls from VIPs asking for submersible pumps to flush out the water. 

But the flush-out operations didn't help either. City canals, already full to the brim, suffered a back thrust as the Ganga was on surge due to high tide. As usual, pumps installed at the 20 drainage pumping stations in the city didn't function to their full capacity. In many places, locals opened manholes, adding to the danger.