In November 2021 — one of the wettest Novembers in living memory — a good number of vehicular subways across Chennai suffered from “pleural effusion”, and none as alarmingly as the Rangarajapuram two-wheeler subway and Madley subway.
When other subways were discharged and joined duty, these two were still parked in ICU, an extended stay being unavoidable. The “accumulated fluid” being copious, the drainage was exasperatingly slow.
The systems for stormwater management around these subways had been caught napping during the heavy rains, and they had ended up being guzzlers like Cana Alberano from the “Fairy Tail” animie series.
What happened?
S Rajendran, chief engineer (general), who has handled many stormwater drain projects, remarks that in terms of depth, a vehicular subway would mark the lowest point in the area it is located, and rainwater runoffs would flow into the “trough”.
Usually, the first line of defence would be sufficient to clear these invading flood waters: In normal circumstances, the well or wells lodged in a vehicular subway’s bowels to take in the rainwater, and the pump house meant for pumping the water out and up to the stormwater drain systems above would be equal to the task.
During heavy rains, this arrangement could collapse like a pack of cards if the stormwater-drainage systems around the vehicular subway fail to deliver on their promise.
Rajendran notes that that is what happened in these subways, and the picture of inefficient drainage was starkly evident at the Ranjarajapuram subway.
22 vehicular subways in Chennai
Railway culvert 45
Rajendran notes that there was an evident reason for this failure.
“On the western side of the railway lines crossing the Rangarajapuram vehicular subway, there is Aziz Nagar, Subramanyapuram and Railway Border Road and the Arcot Road belt, which together constitute a huge rainwater catchment area. The rainwater has to “cross” the railway track (through available stormwater-carrying systems) and reach Mambalam High Road and go to Bazullah Road and Thirumalai Pillai Road and GN Chetty Road and reach Mambalam canal, in the process covering a distance of five kilometres.
Rainwater from this catchment area had only one culvert — railway culvert number 45 — to cross over and take this long course. This culvert is constricted, being just one-and-half feet to three feet wide, and two to three feet deep,” the chief engineer explains.
“Due to the magnitude of the rains last year, this culvert could not handle the influx of rainwater. The culvert is under the railways’ watch, and has to be maintained and desilted by them. There is a matrix of signalling cables on that section, making it vulnerable and dangerous, and any attempt by a GCC team to desilt the culvert could prove foolhardy.
Railways would obviously not allow GCC workers to desilt the culvert under the railway track,” Rajendran continues.
He notes that during the November 2021 floods, the Rangarajapuram subway acted somewhat like a culvert.
“But for the Rangarajapuram subway, people on the western side of the railway track would have been marooned. The two-wheeler subway had provided an unintended remedy. We pumped the stagnant rainwater to the other side — the eastern side of the tracks — where Mambalam High Road runs, and then Bazullah Road.
At that point of time, a flooded two-wheeler subway was not as significant as hundreds of houses being marooned,” Rajendran elaborates.
Rangarajapuram two-wheeler subway is one-kilometre from Duraisami Subway, and also one-kilometre away from Madley subway.
Madley subway | Photo Credit: File photo RAGU R
Madley subway
An almost similar situation showed up at Madley Subway.
“As West Mambalam was inundated, and the rainwater naturally flowed into Madley subway. We put huge pumps with capacities of 100 hp and 200 hp in Madely subway (as also in Rangarajapuram subway, and pumped out the rainwater). At Madley subway, the proximity to the Mambalam canal and the channels to that canal helped.”
The remedy
What are the remedial measures being taken by the Greater Chennai Corporation to ensure November 2021 does not repeat itself, at Rangarajapuram subway?
“Besides constructing stormwater drains along this course, we have identified two more railway culverts that were not used. We have discovered a more efficient way of using Culvert Number 44 and identified an abandoned culvert, which is Culvert Number 43. We have constructed all the drains leading to culverts 44 and 43. Apart from using the three culverts more efficiently — 43, 44 and 45 — we have undertaken work on constructing a drain to divert a part of the rainwater along the Railway Border Road, Kodambakkm railway station, Kodambakkam flyover, and towards Arcot Road, from where the rainwater would be directed into Trustpuram canal.
This drain work is proving to be challenging due to the narrowness of the service lane along the Rangarajapuram flyover,” he explains.
On dealing with inundation of Madley subway, Rajendran notes the stormwater drain networks being constructed in West Mambalam, particularly Ayodhya Road and Brindhavan Road, and the diversion of adjacent drains should address the problem.
“With construction of a skywalk over Madley subway under way in 2021, it was difficult to divert rainwater. With work on the skywalk on the Madley-subway section completed now, we would not face this problem. This time, we expect the Madley subway to not be inundated.”
