The Telegraph
Wednesday , September 7 , 2016

Deluge drains civic machine

A girl walks through a waterlogged street in the Kasba area on Tuesday.(Sanjoy Chattopadhyaya)

Calcutta recorded "very heavy rainfall" for the first time this monsoon on Monday - more than 180mm - but fears of a rerun faded on Tuesday with the weather office ruling out torrential rain for now.

The forecast for Wednesday includes cloudy skies for the most part and one or two spells of light to moderate showers, which in meteorological terms add up to not more than 35.5mm of rain in a day.

Metro brings you up to speed on how the deluge on Monday stretched the city's drainage network and why the rainfall ebbed, contrary to official predictions.

How much did it rain?

Topsia topped the charts with 181.25mm over 24 hours till 8am on Tuesday, followed by Ballygunge with 172mm and Sealdah with 169mm. These calculations are on the basis of data provided by the pumping stations of the Calcutta Municipal Corporation (CMC).

The amount of rainfall didn't impact the city as much as the intensity of the downpour. In Ballygunge, 73mm came down in just three hours, starting 2pm. At this rate, Calcutta could have received 399.9mm, the average rainfall in July, in just 16 hours and 25 minutes.

The official rain count was lower than what the CMC data showed. The Alipore weather station recorded 131.9mm in 24 hours. The previous highest was 98.6mm on July 28.

How much water is that?

Average rain was 125mm, which translates to 5,147.5 million gallons. It would take 515 reservoirs of the size of the British-era Tallah tank to hold that much water.

How did so much water drain out?

There was high tide in the Hooghly till 3.56pm on Monday, because of which the lock gates could not be opened. Also, the water level of the river then was nine feet above normal. Civic officials said the lock gates could be opened only after 6.30pm, once the water level of the river had receded. This means that during the heaviest spell of rain, water didn't drain out from most parts of Calcutta.

The CMC can drain out 1,500 million gallons of water when all of its 200 pumps are running. These pumps had been switched on during the rainiest phase - between 2pm and 5pm - and again at night between 11pm on Monday and 2am on Tuesday. Two hundred pumps would take three-and-a-half-hours to drain out 5,147.5 million gallons, according to the CMC.

What spared the city more heavy rain on Tuesday?

Bangladesh again came to Bengal's rescue, with the low-pressure area shifting towards the neighbouring country. "Maximum rainfall occurs to the north and east of a low-pressure area. Since this low-pressure area has shifted, Calcutta is now to its west, which is why rainfall has declined," a weather scientist said.

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