About two decades ago, a research project by the Centre for Environmental and Water Resource Engineering, IIT Madras estimated that about 650 waterbodies existed in the Chennai region.
More than half of them were located south of River Adyar. At present, as the second Master Plan for Chennai indicates, only a fraction of them exists. Most of the waterbodies within the city have vanished and only a few remain in the immediate periphery.
According to records of the Water Resources Department (WRD), the area of 19 major lakes has been shrunk from a total of 1,130 hectares to nearly 645 hectares and hence reduced their storage capacity.
M. Kaarmegam, former director of the Centre for Water Resources, Anna University, said: “There were 16 tanks downstream of Retteri called Vyasarpadi chain of tanks. Kodungaiyur tank was one among them. Now, there is no sign of them. There was also a tank in Thirumangalam area.”
Maduravoyal Lake, which was once spread across 120 acres has now shrunk to 25 acres. Encroachments and misuse of lake bed were the reasons, he added.
S. Narayanan, treasurer of Kazura Garden Residents' Welfare Association, Neelankarai, recalled that there were over 13 waterbodies in the area until a few decades ago. “Many of them have been encroached upon and buildings have come up. There are only two lakes now. Even a pond in our colony has been encroached,” he said.
The consequence of this rapid loss of waterbodies has not only reduced the extent of collective water harvesting, but also severely impacted flood management within the city. The principal cause of local flooding in many areas, it emerges, is the mismanagement of waterbodies and impairment of linking canals.
For instance, the Virugambakkam drain, which was 6.5 km long and drained into the Nungambakkam tank, is now present only for an of extent of 4.5 km. The remaining two km stretch of the drain is missing. Nungambakkam tank was filled and built. This along with the loss of Koyambedu drain has resulted in the periodic flooding of Koyambedu and Virugambakkam areas.
This phenomenon is now repeating in the suburbs. The surplus channels connecting various waterbodies in western suburbs such as Ambattur and Korattur have been encroached upon. The waterbody in Mogappair has almost disappeared. Lake beds often serve as make shift dumping yards and cesspool. This has resulted in inundation of neighbouring localities.
The Veerangal Odai that connects the Adambakkam lake with Pallikaranai marsh ends abruptly after 550 m from its origin and the remaining part is not to be seen. This causes inundation in places such as Puzhithivakkam and Madipakkam.
S. Mohan, professor, Environment and Water Resources Engineering, IIT Madras, cautions that loss of waterbodies and channels not only induced flood but also increased saltwater intrusion. As a thumb rule, he said, every one metre of water-head in a water body can push sea water laterally by 40 meters.
The waterbodies thus function like a protective ring. But for the presence of Buckingham Canal, saltwater would have intruded further west and affected more residential areas, Mr. Mohan explained. Restoration and proper maintenance of the tanks are critical to Chennai's future, he emphasised.
Some of the tanks, because of negligence have silted at the rate of 2 to 3 mm every year and some have been lost due to encroachments.
A way forward would be to create a scientific inventory of waterbodies and delineate flood zones within the city. The flood zone will have to be identified based on the location of the waterbodies, natural drains, water shed area and it has to be a no building zone, said Mr. Mohan.
According to sources in the WRD, only 19 of the 29 existing major waterbodies can be restored. Others such as the Ullagaram, Adambakkam, Thalankanacheri, Mogappair and Senneerkuppam tanks cannot be restored, they say. The case of lost water channels is even worse, they have totally disappeared.
All is not lost say the officials. The waterbodies in Madhavaram and Korattur can be fully restored. The water resources along with that in Ambattur and Porur could be used as storage points as they would have a capacity of 600 million cubic feet of water.
Once rejuvenated, the 19 waterbodies would have a combined storage capacity of 1,000 mcft. At present, the city reservoirs have a storage capacity of 11,000 mcft, officials of the Water Resources Department said.
The department is also in the process of improving the waterways and surplus courses and creating straight cut canals from various waterways to the Cooum under the Jawaharlal Nehru National Urban Renewal Mission. This could substantially improve flood management within the city, officials added.