As if the downpour weren’t enough, there’s strong tide stirring too

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By OUR BUREAU
  • Published 21.08.13
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Calcutta, Aug. 20: Heavy rain lashed Calcutta through the day today and the Met office warned of more in the next 24 hours because of a stubborn weather phenomenon that was refusing to budge till late in the night.

Between 8.30pm Sunday and 8.30pm Tuesday, Calcutta received 206mm of rain, the wettest spell this season. Parts of the city were inundated, unable to drain out water at a pace to match the downpour. Tomorrow could be worse because of the Shanrashari Baan, a kind of spring tide that will put the river in spate.

A warning of heavy rain has been sounded across all south Bengal districts where at least four persons have died so far.

The Telegraph tries to explain the twin threats from the leaking skies and bulging river.

What is causing the rain?

A low-pressure area that had formed over the Bay of Bengal sucked in moisture furiously and turned into a depression as it closed in on the Bengal coast. It hit land and inched close to Calcutta overnight.

On Tuesday afternoon, its epicentre lay to the north-west of the city. It showed signs of movement (very slow, the Met office said) towards Odisha/Jharkhand in the evening.

As it moves further from the sea, it is likely to weaken.

What is a depression?

It’s an area where the atmospheric pressure is much less than the usual sea-level pressure. So, moist air from the sea rushes towards it from all directions. This moisture rises and condenses into clouds, bringing rain.

What was its effect on Calcutta?

On Monday, the rain gauge at Alipore measured 96.6mm. On Tuesday, it clocked 110mm, the decade’s second heaviest rain in an August day.

A schoolgirl and her parents were killed in Hind Motor, about 15km from Calcutta, when she touched a partially submerged and electrified iron gate at her apartment on the way home from school. An elderly woman was crushed under a tree at Entally in central Calcutta.

Is heavy rainfall expected on Wednesday?

When the Met office predicts heavy rain, it means between 65mm and 125mm in 24 hours. But even if it rains like today, the consequences could be worse on Wednesday.

Why?

A very strong tide, locally called the Shanrashari Baan and described by marine experts as the Perigean Spring Tide, will be headed Calcutta’s way from the Bay on Wednesday night.

What’s that?

A Perigean Spring Tide comes in when the moon is closest to the earth and its gravitational pull at its strongest. During regular high tide, the water level rises seven to eight feet. In a Shanrashari Baan, it swells by about 16 feet.

During these, water from the Hooghly flows into the city through nullahs and floods low-lying areas like Kalighat. When the river is in spate, there’s no question of the floodwaters flowing out of the city into it.

The Kulti Gang, to which water from the city’s east flows out, is also heaving. The lock gates there could not be opened until Tuesday evening and some of the pumps in the Ballygunge and Chowbaga stations had to be kept idle.

When will the tide hit?

It is supposed to arrive Wednesday’s full-moon (Rakhi Purnima) night but the river has already started swelling. A marine expert warned the spring tide could be severe when the river level is already high and there’s more rain coming.