Urban jungle built atop waterbodies

CHENNAI: Even after 2 weeks, the swath of destruction left behind by Cyclone Michaung continues to haunt Chennaivaasis. Though rainwater has receded everywhere, denizens are still struggling with putting back their homes piece by piece.

While a lot has been discussed and debated on the ferocity of the storm, the inefficiency of the newly-built stormwater drains in enabling the free flow of water, and the administration’s lackadaisical attitude towards rescue and rehab of those stranded in the flood, there are 2 main factors that contributed to the city’s inundation. Firstly, the Greater Chennai Corporation (GCC) and Water Resources Department (WRD) had failed to desilt the waterbodies, increase their capacity, and build additional check dams. Secondly, uncontrolled urbanisation and lack of urban planning in the city prevented the free flow of excess rainwater.

Saturated groundwater level

In 2015, the city received less rainfall during the southwest monsoon (SWM) and intense showers during the northeast monsoon (NEM).

However, this year’s SWM was intense in the city, so much so that Chennai’s groundwater level was saturated. Add to this the cyclone-induced downpour on December 3 and 4, there was no way for the groundwater level to recharge.

“During SWM, almost 75% of rivers and lakes were brimming. Since there was no flood moderation in the catchment areas during the NEM, the waterbodies reached their maximum capacity and couldn’t prevent flooding,” explained S Thirunavukarasu, former PWD assistant executive engineer.

The intense rainfall occurred over upstream and in the catchment area, which led to water stagnation, as continuous downpour did not allow time or space for the excess water to drain. It has become one of the main reasons for the inundation.

“Each flood is unique. The field dynamics, rainfall distribution, and end boundary conditions (tide and storm surge) are never the same. Plus, the changes in land use and rapid urbanisation in that last decade need to be factored in,” stated Tamil Nadu weatherman Pradeep John. “In 2015, the city received 500-550 mm of rainfall in the upstream areas of the Adyar catchment. This resulted in a very high inflow to the river. Downstream areas got an average of 350-400 mm, and most of the downpour was in a day.”

In November this year, the soil had been saturated with a never-before-seen 1,000 mm rainfall. In upper catchment areas in Adyar, it was the heaviest downpour in history.

“Similarly, this year, the downstream received the same amount of rainfall in the Adyar catchment. Also, around 550-650 mm of extreme rains raised the water level in the Cooum upper catchment. That’s why the areas from where the excess water drains into the Cooum were waterlogged due to elevated flow in the river,” added Pradeep.

Missing lakes, ponds

Many lakes and ponds are missing in the city, especially in north Chennai where at least 20 waterbodies are missing in areas like Vyasarpadi, Manali, and Manali New Town. These neighbourhoods have been severely affected by the cyclone.

“If a waterbody is missing, and even if squatters have encroached the space, you can identify it as it would be lower than the ground around it, and would take longer for the excess water to recede,” pointed out Thirunavukarasu.

During the 2021-NEM, north Chennai was adversely impacted due to the missing waterbodies. Many had disappeared more than 50 years ago due to rapid urbanisation in the neighbourhood.

For instance, Dr Ambedkar Arts College at Vyasarpadi was built on Vyasarpadi Lake and the remaining waterbody was occupied by housing quarters.

Similarly, Ayanavaram residents who have been living there for 45 years have not seen a single waterbody, as the Ayanavaram Lake had disappeared 50 years ago. When waterbodies are encroached by both government and private entities, it leads to 99.9% inundation.

“Most of the surplus from tanks flows to the Cooum River, Otteri Nullah, and Buckingham Canal. But, over a period of time, buildings have come up at the Otteri Nullah, which shrunk in size. If there were no encroachments in the nullah, it would have carried 3,500 cusecs of water during the recent showers. Now, imagine a similar situation in other waterbodies in north Chennai,” he added.

Thirunavukarasu also explained that the State government had failed to increase the carrying-capacity of lakes and ponds, which is another major reason for the flood in 2015 and 2023. For instance, though the capacity of Otteri Nullah shrunk 50-60 years ago, no steps have been taken by the department to expand its carrying capacity.

“Hundreds of waterbodies are missing in the city. But, at present, we have at least 4,000 lakes. The State government must ensure their protection and maintenance,” he opined.

Encroachments

While encroachments across the city have been rampant, the channel link between each lake has been encroached, which led to flooding in the city. Almost 25-30% of encroachments surged in the last decade.

“One of the most affected areas in the city is Pallikaranai, as the Anai Eri in the area has been encroached by a 7-storeyed building where 1,000 families live currently. In the recent rains, the compound wall was damaged and the entire area was flooded. The cost of damages would be around Rs 2,000 crore for the residents,” stated Thirunavukarasu.

As the city is built over waterbodies and wetlands, Chennai often suffers from flooding during the monsoon. This would be the new normal for the residents until a permanent solution is provided by the State government.

On the other hand, climate change has played a vital role in recent years bringing sudden downpours during the monsoon, opined G Sundarajan, an environmentalist.

“There was a long tank from Mambalam to Villivakkam, which had been encroached on by the private and government entities a few years back. Now, only a small part is visible in Chetpet. People have been encroaching waterbodies for over 200-300 years in the city, especially the flood plains,” he added.

Similarly, the Virugambakkam Canal and Captain Cotton Canal in Vysarpadi have become a mess with sewage discharged from nearby residential and commercial buildings.

“To ensure waterbodies are not encroached for residential and commercial purposes, the electricity department should deny power supply. The State government should decide and take preventive measures to prevent flooding in the coming years,” opined Sundarajan.

Additionally, the WRD must take steps in a zero-budget scheme by increasing the waterbodies’ capacity themselves instead of giving it to private contractors, experts suggested. If the lakes and rivers are desilted, it would store adequate rainwater during the monsoon season, and ease the inundation situation in the city.

Officials’ PoV

The WRD mentioned that the restoration of the Adyar and Cooum rivers has been almost completed. Many encroachments are yet to be cleared due to pending court cases, but those will be removed soon.

“We’ve planned to increase the storage capacity of waterbodies to hold adequate water during the monsoon seasons,” said a senior WRD official. The department is proposing a new design for the check dams that can help redirect about 50,000 cusecs — more than three times as that of the existing structures. This will help improve storage in the reservoirs that supply drinking water to the city, and also prevent damage to the check dams during heavy downpours.

A proposal to renovate the check dams on Cooum and Kosasthalaiyar rivers was submitted. “Similarly, a proposal to construct new reservoirs in Ramenjeri, Nemam, Tiruninravur, and Sriperumbudur has been submitted. After completion, it will add to the source of drinking water for the city,” added the official.