Chennai:Even as Chennai steps up its flood preparedness measures for the northeast monsoon, experts and environmental activists have questioned why the city continues to allow large volumes of rainwater to flow into the sea instead of conserving it for future use. They have called for an integrated rainwater harvesting network linking reservoirs, lakes, ponds and neighbourhood-level recharge structures to address the city’s recurring problems of flooding and water scarcity.

The concern has gained significance with the management of Chembarambakkam reservoir coming into focus following Chief Minister C. Joseph Vijay’s inspection on Monday. While controlled releases are necessary to maintain a buffer for incoming rainwater, experts point out that Chennai has no comprehensive mechanism to store or divert the surplus water for subsequent use.

According to water resources department officials, Chembarambakkam’s storage level is generally maintained at around 21 ft, against its full reservoir level of 24 ft, during the monsoon to accommodate further inflows. The surplus water is released into the Adyar river and eventually drains into the sea.

The reservoir had a water level of 14.76 ft as of 6 am on October 26, against its full level of 24 ft. Its storage stood at 1,488 million cubic feet (mcft), or about 41 per cent of its capacity of 3,645 mcft. Inflow was 150 cusecs and discharge, 208 cusecs.

V. Santhanam, a veteran environmental activist, said the city was losing an opportunity to conserve valuable water every time Chembarambakkam released its surplus.

“Every year, during the monsoon, water is released from Chembarambakkam reservoir to prevent a deluge. But there is no mechanism for saving this water once it is released. It simply drains into the sea through the Adyar river,” he said.

He suggested diverting surplus water to lakes in the southern suburbs and constructing a small reservoir to store the runoff. Interlinking lakes such as those at Thiruneermalai, Pallavaram, Chitlapakkam, Sembakkam and Kilkattalai could help conserve water, he suggested.

Experts said Chennai’s water-conservation strategy should extend beyond its principal reservoirs. Government data indicates that there are around 3,600 water bodies across the neighbouring districts of Tiruvallur, Kanchipuram and Chengalpet. However, surplus water from several of these water bodies eventually finds its way into rivers and the sea, without being adequately conserved.

“Instead of creating new reservoirs, the existing thousands of water bodies should be restored to their original capacity. There are several tanks along the ECR and OMR stretches spread across thousands of acres. If these are restored and utilised properly, they can cater to the water needs of the southern parts of Chennai,” Santhanam pointed out.

Chennai receives more than 1,000 mm of annual rainfall most years. When rainfall in the neighbouring districts within the Chennai Metropolitan Area is taken into account, the average annual rainfall is around 1,400 mm.

The city’s three major rivers — the Kosasthalaiyar in the north, Cooum in the central region and Adyar in the south — along with the Buckingham canal, form an extensive network of waterways. Experts argue that better management of this interconnected system could help reduce flooding while improving water storage.

Water conservation experts have also called for moving beyond conventional rooftop rainwater harvesting to capture runoff from roads, pavements, parks, playgrounds, parking areas and other open spaces.

The government recently launched ‘Thuli’, a rainwater harvesting campaign, and local bodies were directed to inspect existing systems in residential, commercial and government buildings, clean filter chambers and recharge pits, and restore defunct structures wherever technically feasible.

A senior official of the municipal administration and water supply department said rainwater harvesting was vital for a water-deficient state such as Tamil Nadu, particularly during the rainy season. The initiative would be a continuous process, involving periodic inspections and reviews, the official said.

Depending on local soil conditions, recharge trenches, recharge shafts, percolation pits, recharge wells and storage structures could be adopted to capture surface runoff.

According to a senior scientist at the Central Groundwater Board in Tamil Nadu, clay acts as an impermeable layer and hinders passage of stormwater deeper into the ground. So, he cautioned against adopting uniform designs across the city.

Darwin Annadurai, an environmental activist, called for closer monitoring of existing rainwater harvesting structures and the creation of underground storage facilities.

“Instead of storing rainwater only in available open spaces, underground storage points should be constructed. To improve the groundwater table, the rainwater harvesting structures already in place have to be monitored,” he said.

He suggested a separate agency to oversee the installation and maintenance of recharge structures, including borewells, at appropriate intervals along streets, public roads and highways, depending on soil conditions.

He also advocated rejuvenating existing water bodies while also creating new infrastructure. Saying that sponge parks offered a new approach to rainwater harvesting, he added that their design and implementation needed to be reviewed, as many had been constructed with concrete surfaces that obstructed the absorption of excess rainwater.

The objective, he said, should be to treat monsoon rain not merely as excess water to be drained away, but as a resource to be stored for the city’s growing drinking water needs.


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