Monday, Dec 05, 2022

Explained: Why is Chennai under water?

Unusually heavy rain has exposed the city’s broken urban planning, revealed its stolen natural waterways, and exposed its tolerance of illegal construction.

Almost the whole of Chennai city (map on the right) is flooded. The low-lying area between Velachery and Sholinganalur is especially badly affected. (Source: Reuters) Chennai floods: Almost the whole of Chennai city (map on the right) is flooded. The low-lying area between Velachery and Sholinganalur is especially badly affected. (Source: Reuters)

The catastrophic flooding in Chennai is the result of the heaviest rain in several decades, which forced authorities to release a massive 30,000 cusecs from the Chembarambakkam reservoir into the Adyar river over two days, causing it to flood its banks and submerge neighbourhoods on both sides. It did not help that the Adyar’s stream is not very deep or wide, and its banks have been heavily encroached upon over the years.

Similar flooding triggers were in action at Poondi and Puzhal reservoirs, and the Cooum river that winds its way through the city.
Watch Video: Why Is Chennai Under Water? (app users click here)

While Chief Minister J Jayalalithaa said, during the earlier phase of heavy rain last month, that damage during the monsoon was “inevitable”, the fact remains that the mindless development of Chennai over the last two decades — the filling up of lowlands and choking of stormwater drains and other exits for water — has played a major part in the escalation of the crisis.

On the evidence of what is currently unfolding in Chennai, city authorities would appear to have the abandoned the concept of stormwater drains — the fundamental instrument of flood-fighting — altogether over the years. Experts at Chennai Metropolitan Development Authority (CMDA) pointed out that the key parameter of rain intensity measure — which ought to be at least 1 inch per hour — has been ignored while planning multicrore drainage projects.

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chennai floods, why chennai has floods

While stormwater drains are supposed to be planned on the basis of detailed topographical data, their linkage with water bodies, construction along their course, and the design of roads have rarely been seen as part of a whole. As a result, drains constructed over the past decade have repeatedly proved inefficacious — and showed up problems of poor urban planning nearly every monsoon.

Planning officials said contractors are rarely briefed on the topography or the flood character of sites. A top civic official said even the water log data of the last 10 years are often not considered as officials “hurry to complete works” before the allocated funds lapse. A former CMDA planner said mandatory standards based on data on sea level and water flows are not followed, resulting in situations like Koyambedu, the neighbourhood that saw expensive stormwater drain projects, but has still gone under.

Across Chennai, illegal construction has been making neighbourhoods unrecognisable — what may have been a tank, lake, canal or river 20 years ago, is today the site of multistorey residential and industrial structures. There are over 1.5 lakh illegal structures in the city, according to a report submitted by CMDA to the Madras High Court. Despite several HC orders ordering their demolition, the buildings stand — often after appeals to the Supreme Court, and due to the inefficiency of the CMDA’s legal wing. Hundreds of stay orders against demolition orders have been obtained by both business houses and individuals.


As the illegal structures sprouted in the city and suburbs, over 300 water bodies disappeared. The irreversible destruction of the city’s natural water paths can be seen in the flooding in Mudichur, Velachery, and several other areas that have come up on wetlands or river basins.

After a major flood in 2005, Chennai had commissioned a project to prepare laser terrain maps, scanning the entire city from a helicopter, instead of depending on topographic maps. But this project remains unimplemented — just like the early warning system that was prepared by the Department of Remote Sensing at Anna University to understand the run-off pattern of rainwater.

El Niño to blame, other factors too

An exceptionally strong El Niño, along with a rare “coincidence of various factors”, has resulted in the heavy rain in Tamil Nadu this northeast monsoon season, officials at India Meteorological Department (IMD) said.


The El Niño phenomenon — an unusual warming of surface waters in the equatorial Pacific Ocean — is “intensely strong” at present, Additional Director-General of Meteorology (Research) at IMD, B Mukhopadhyay, said. “In a strong El Niño year like the present one, the summer (southwest) monsoon is adversely affected, while the northeast monsoon or the winter monsoon is favourably affected.”

However, Mukhopadhyay added, “the present very heavy rain is exceptional and not normal.”

The 2015-16 El Niño could turn out to be the strongest ever recorded — in fact, by one measure, it has already reached that milestone. In mid-November, the sea surface temperature in the central tropical Pacific was 3 degrees Celsius warmer than normal, the largest positive deviation in recorded history. To officially beat the 1997-98 El Niño as history’s worst, sea surface temperatures must stay at these levels for three months.

Other than the El Niño, a strong upper air divergence and high moisture content at lower levels, have contributed to the rain, Mukhopadhyay said. “These two factors, along with formation of low pressure systems have resulted in the heavy rainfall,” he said.

First published on: 03-12-2015 at 12:20:02 am
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