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Friday, September 17, 2021

Extra rain and poor urban planning: Why Chennai went under water

Indian Express explain the October-December northeast monsoon that brings rain to Tamil Nadu at this time — and how it became so deadly this year.

Written by Amitabh Sinha , Arun Janardhanan |
Updated: November 19, 2015 7:22:57 am
Commuters carry their motorbikes on a submerged cart as they wade through a waterlogged subway in Chennai. (Source: AP photo) Commuters carry their motorbikes on a submerged cart as they wade through a waterlogged subway in Chennai. (Source: AP photo)

Why is it raining in Chennai now? Isn’t the monsoon season over?

India receives more than 75% of its annual precipitation from June to September. The rest is spread out through the rest of the year, but mainly in the months of October, November and December.

June-September is the season of the southwest monsoon. After the first week of September, the monsoon begins to “retreat”, and the process continues until around October 15, even though the southwest monsoon season is officially declared over on September 30.

Tamil Nadu and parts of Kerala, Andhra Pradesh, Telangana and Karnataka, however, see a second period of rainfall — in October, November and December. This is the northeast monsoon, also known as “winter monsoon” — which is nothing but the ‘retreat’ of the monsoon from the northeastern part of India, in the northeast-southwesterly direction. This ‘retreat’ is delayed compared to the retreat from north and northwest India, and produces rain until December in Tamil Nadu and parts of peninsular India.

The northeast monsoon accounts for as much as 48% of Tamil Nadu’s annual rain. Some coastal areas get about 60% of their annual rain during this season. Together, the five meteorological subdivisions of Tamil Nadu, coastal Andhra Pradesh, Rayalaseema, Kerala and south interior Karnataka receive 30% of their annual rainfall during the northeast monsoon season.


How much has it rained in Tamil Nadu this season?

The state has received 39.3 cm rain (area weighted) since November 1; Chennai alone has received 92.2 cm (area weighted). In the 24 hours between November 15 and 16, Chennai got 24.6 cm rain — the most it has got over any 24-hour period in the last 10 years. The November 15-16 rain beat the November 2005 24-hour rain of 14.2 cm by a long way, according to Thambi Narayanan, Deputy Director-General, Meteorological Department, Chennai.

pivsWas this rainfall entirely unexpected?

Not quite. The Met department had predicted that the northeast monsoon rain in the five subdivisions would be 111% of the normal. Tamil Nadu was expected to get 112%.

Also, there the northeast monsoon has some typical characteristics: it does not bring continuous rain over a long period, but results in periodic spells of 3-4 days of fairly heavy rainfall. Again, the deviation from normal rainfall is much bigger in the northeast monsoon — sometimes as much as 80% to 90% (see graph), which results in heavy downpours on some days. In November 1976, Chennai recorded 45.2 cm rainfall in a 24-hour period. In 1985, it got 25 cm over two days.

SW monsoon was deficient this year, so why is the NE monsoon excessive?

These are driven by different atmospheric systems, and do not always follow the same pattern. In addition, the El Nino effect, which has a negative impact on the southwest monsoon, has often been associated with good rainfall in the northeast monsoon. Ironically, therefore, while Chennai has gone under water and the Test match in Bengaluru has had to be abandoned after four continuous days of washouts, farmers in large parts of the country are battling drought.

Was the extra rain also a result of a cyclonic wind system?

Heavy rainfall was brought by a low pressure system, whose formation over the Bay of Bengal was first sighted in the last week of October. This was also associated with an upper air cyclonic circulation extending up to 3.6 km above mean sea level. The pattern of rainfall that the low pressure system brought was erratic. It rained 1.5 cm in Chennai on November 8, 13.6 cm on November 9, 6.1 cm on November 12, and 15 cm on November 13. The next two days saw only 1 cm and 1.3 cm of rain. Then, on November 16, it poured.

Could Chennai have handled the rain response better?

According to police, 178 people have been killed across Tamil Nadu since October 28. In Chennai, tall claims of the corporation and the government about flood preparations were washed away, clogged stormwater drains showed the expensive desilting procedures of recent months hadn’t worked, and the flooding underscored the trend of reckless illegal construction that has shut outlets for water. Studies show at least 300 water bodies have been converted into residential areas. Most waterways, tanks and reservoirs are choked with silt, and their flow channels and banks have been encroached upon.

Some 16 channels and 43 minor drains in the city need to be redesigned, and a grid of stormwater drains joining the Buckingham Canal and Cooum river needs to be built as a longterm solution to flooding, an expert at the Chennai Metropolitan Development Authority said. “An early warning system prepared by Anna University with rain gauges in 40 of the most flood-prone areas, sending signals to the control room, remains a dream,” he said. Data from digital maps of the city, showing missing links in the stormwater drains and sewage canals, are hardly used, he said.

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