What’s clogging flood-ravaged Chennai’s storm water drainage system?

Besides the promise of rains, every monsoon in Chennai also evokes the dread of water logging and floods among residents. According to the Greater Chennai Corporation, the flat terrain of the city, which is only 2.0 m above mean sea level, and tidal effects are mostly responsible for flooding and water stagnation during monsoon.

Even though the corporation has been building storm water drains to alleviate the problem and avoid major stretches from getting inundated after a shower or two, the initiatives do not seem to have worked so far.

Experts attribute the failure to the scant knowledge of corporation officials about the major drain networks – both dating to the British era to the current ones – in the city, faulty design, and corruption.

No clue about British era drains

According to historians, in 1875, the British built Chennai’s first storm water drain – an 800 km stretch – under the Parish Venkatachala Iyer Street in George Town using brick and mortar.

“Much of this 800 km network continues to exist today, particularly in older and core city areas such as Broadway and Mylapore. In fact, it is widely acknowledged that areas where these drains have been maintained saw a lesser extent of inundation during the December 2015 flooding event,” says a 2019 report prepared by Chennai Resilience Centre.

The corporation, however, allegedly doesn’t possess any map or documentation on the network of these centuries-old British-era storm water drains. Experts told The Federal that the locations of these drains have been passed on from one generation of employees to another through word of mouth, instead of being officially recorded and passed on.

As far as the storm water drains of the post-British era are concerned, as of January 2021, the corporation had 8,835 drains across a network spanning 2,071 km (of a total 5,500-km stretch of roads in the city). A chunk of these drains were constructed under various projects including JNNURM, Chennai Mega City Development Mission, Integrated Storm Water Drain Project and Smart City Project.

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“Leave aside the British drain networks. It is doubtful whether the corporation has any maps on the current network of drains. Since the construction of storm water drains has become a money minting machine, it involves a lot of corruption. So it is not a surprise if the corporation lacks documentation on the same,” said Shekar Raghavan, activist on rainwater harvesting, who is fondly called as ‘Rainman of Chennai’.

Raghavan said the corporation is wasting taxpayers’ money by trying to construct storm water drains in places like East Coast Road, where there is no need for such drains as the sand can absorb the rainwater naturally.

‘Designs faulty, not updated with times’

According to the report by Chennai Resilience Centre, storm water drains were constructed in the pre-Independence era primarily “to carry excess surface runoff, as quickly as possible, from roads to the Bay of Bengal via canals and the Adyar and Cooum rivers”.

“This early objective is problematic today because it serves only one purpose: that of flood management. As Jameson and Baud (2016) point out, flood management in Chennai has not been thought about in an integrated manner that incorporates rain water harvesting, recognition of natural sinks and marshlands and contributions of the traditional eri (or tank) system to underground recharge,” the report said.

It is alleged that the existing storm water network fails to provide a holistic solution to the issue of flooding. The current network “offsets rainwater harvesting efforts by preventing rainwater seeping through into the ground”.

The report says that the storm water drain network was “built without basis” in a hydrological, topographical and meteorological study, quoting the Comptroller and Auditor General (CAG) report of 2016.

“In several areas, drains are built above the height of the adjacent road, defeating the purpose for which they were constructed. Some experts argue that a more appropriate design for Chennai would be the traditional eri system that was designed to mitigate flood risk through slow and gradual water movement through unpaved tank beds,” the report says.

The report also points out that the Chennai Corporation’s own study suggested constructing periodic groundwater recharge structures at 100 m intervals along the network and about difference in opinion between the corporation, Chennai Metro Water Supply and Sewerage Board on the illegal discharge of sewage into drains.

It is unclear how far these suggestions were incorporated and the mixing of sewage with drain water addressed. During the recent spell of rain, which left city spaces inundated, many newly-constructed storm water drains were seen to be damaged while sewage continued to flow into the drains.

“The storm water drains should harvest rainwater instead of disposing it. So they must have recharge wells which in turn will increase the groundwater table. The designing should be in a way such that the run-off water must fill ponds, tanks and lakes before draining into the sea. But the corporation is not taking these things into account,” said Raghavan.

Issues with desilting and shortage of drains

Following the 2015 floods, Chennai-based NGO Satta Panchayat Iyakkam in an RTI reply in 2018, found that only 1,894 km or 34 per cent of the city’s total road stretch of 5,500 km, had storm water drains. The coverage has now been increased to 2,071 km.

“If we have 5,500 km of roads, then we need an equal stretch of storm water drains. But there is a shortage of drains. The reason is the corporation instead of constructing new storm water drains, is demolishing and reconstructing the existing ones, and this paves the way for scams,” said Ranga Prasad, joint secretary of the organisation.

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He added that the corporation had constructed only 326 km of storm water drains in seven years between 2008-2009 and 2015-2016. But after the Chennai floods, within two years (between 2016-2017 and 2017-2018) it built 377 km of storm water drains.

“So the first two years after the floods, there is an increase in construction of new storm water drains, but after that it is unclear how many new drains were constructed. Besides, while focusing on new constructions, the corporation is not giving priority to the maintenance of existing storm water drains. Therefore the desilting is not properly done,” Prasad added.

The corporation officials were not available for comments.