What’s flooding Chennai’s T Nagar? Mainly, the Rs 110 cr stormwater drain network

While the Chennai Corporation rehauled the stormwater drains in T Nagar, the drains are either too uneven, disrupt gravitational flow of water, or too shallow to hold the extra rain water that the city received this month.
People wading through flooded street
People wading through flooded street

T Nagar was arguably the worst hit zone within the Greater Chennai Corporation (GCC) limits in this year’s rains. After it flooded for the third time this month (last on 26 November), the locality’s residents have raised some pointed questions, chief among them being: “What did the Chennai Corporation do with the Rs 200 crore investment under the Smart Cities Mission?”

A prominent central zone locality in the city, T Nagar made news when it was identified for area-based development under the Union Government’s Smart Cities Mission. It was touted to become Chennai’s first model neighbourhood. The GCC in collaboration with the Smart Cities Mission Ltd spent a total of Rs 200 crore to flood-proof T Nagar in 2019 and 2020.

The civic body invested Rs 110 crore on constructing new stormwater drains and linking old ones, and an additional Rs 80 crore to restore the Mambalam Canal, which drains surface water runoff from T Nagar, Puliyanthope, West Mambalam and areas as far as Ashok Nagar into the Adyar river.

But come November 2021 and the northeast monsoon, many of T Nagar’s landmark roads – Pondy Bazaar, Bashyam Road (Smart City area), GN Chetty Road, Bazullah Road, Usman Road and Habibullah road – were under water.

Faulty stormwater drains

To counter public outrage, the GCC released maps of the stormwater drain (SWD) networks in all the 200 wards of the city for the first time. The function of stormwater drains is to collect rain water runoff from roads and concrete surfaces and direct it to the nearest natural channel of water, ensuring that water doesn’t stagnate on the streets.

However, a quick look at T Nagar’s SWD map shows that an uneven drain network and inadequate size of some of the drains triggered waterlogging.

“Many aspects such as terrain, elevation levels, amount of rainfall runoff and the location of natural water channels have to be studied before designing stormwater drains. It is based on how much rainfall is received that the capacity of the drain is fixed,” Dayanand Krishnan, civil engineering expert and Geographic Information System (GIS) consultant told TNM. However, it is unclear whether T Nagar's terrain and elevation levels were studied before constructing the Storm Water Drain (SWD) system, as many of the drains have been found to lie outside the contour line.

Dayanand studied the storm water drains of T Nagar and Ashok Nagar and realised that the drains were built without alignment after a contour study of the area. A 'contour study' marks points of equal height on a map, and drains need to be built in a way that that water flows from the higher surfaces or gradient to lower levels (expressed in Mean Sea Level or MSL) until it finally reaches a natural water channel which is on the lowest MSL, explained Dayanand. But in T Nagar, unaligned storm water drains have led to water logging due to  reverse flow in over 11 streets. "In all these areas, water is moving from a lower SWD to a higher SWD and therefore water has to climb uphill to flow," he adds. 

In T Nagar, reverse flow has been found on Raghaviah Road, Thirumalai Pillai Road, Vijaya Raghava Road, Gopalakrishnan Road, Rama Street, and a big stretch of GN Chetty Road.

Map of T Nagar's stormwater drain system released by GCC

The most prominent reverse flow was identified at GN Chetty Road. Here, water flows from A 24 (manhole) to A 23. However, while A 24 is at an elevation of 5.835 m, A 23 is at 6.053 m. “This means that A 23 is 0.218 m higher and the water level near A 24 has to reach this level for it to flow into A 23,” Dayanand explained. Similarly, at the point connecting Coats Road to Raghaviah Road, E 21 (manhole) is at an elevation of 5.838 m while E 20 is at 6.651 m, leading to reverse flow and waterlogging. 

Inadequate drain capacity

Narrow drains cannot carry enough water and excess runoff spills out on the road. A look at the map shows that the drains on Dr Nair Road and Thyagaraya road in ward 136 are too small. According to a survey conducted between January 25-28, 2021, the depth of drain A 29 on Dr Nair Road is only 0.116 m or 4.5 inches.

“This makes it too narrow to even carry a quantum of the rain water received even on normal spells, let alone the kind of rains we witnessed on November 7,” Dayanand said.

Typically, the drains are designed to hold 5-7 cm of hourly rainfall. However, in November, Chennai received close to 100 cm of rainfall, which is 14 to 20 times higher than the drain capacity.

Dimension chart of T Nagar's SWDs

Stormwater drains should measure at least 1 metre by 1 metre on small streets, while on major arterial roads such as Pondy Bazaar, Venkat Narayana Road or GN Chetty Road, macro drains of sizes 2 m by 2 m should be built. But a quick look at T Nagar’s SWD map shows that eight of 32 drains under ‘A’ classification are less than 1 m deep and wide, while the dimensions of 10 more drains are unknown.

GCC’s response

Speaking to TNM, a senior GCC official confirmed that drain capacity on some streets in T Nagar, including Bazullah Road, was less, which caused severe inundation. However, the official denied that there was any reverse flow.

“No drain pipe can be built considering the highest levels of rainfall. What we saw this month was torrential rains. But with optimum design of drains, rain water should drain within 3-4 hours of flooding in an urban locality. The problem arises when the water does not drain even after 4 hours,” the official said.

Speaking about the GCC’s efforts to drain water, the official added that over the last 10 days, Corporation officials had added extra pipes along the GN Chetty Road flyover and on Bazullah Road to increase capacity and drain water from these roads faster. “We didn’t need traffic diversion on GN Chetty Road because we managed to drain out the water quickly over the last 10 days,” he explained.

However, a locality wide audit is necessary to fix waterlogging in each street. “Some drains need to be augmented, others have to be cleared of garbage and silt. Some new commercial buildings are located close to very old drains that cannot carry enough water, etc,” he added

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