The recent floods in the Adyar river have left many residents wondering about flooding in areas that are usually not vulnerable.
The reason is that the Chembarambakkam reservoir, which chiefly contributed to floods in Adyar river, received the highest inflow in 100 years on December 1, much more than the capacity fixed based on flood data of 60 years.
The reservoir’s capacity was last improved in 1996 to handle an inflow of 33,400 cubic feet per second (cusecs). This equals to nearly 10 lakh litres per second. Sources say that the reservoir received was more than 35,000 cusecs on the midnight of December 1.
“If 35,000 cusecs were released into the river, it would have only taken three hours for the flood. But, the discharge was moderated to 29,400 cusecs that took six to nine hours for the floods to flow into the river. We could not have discharged lesser volume of water as the inflow was beyond the reservoir’s capacity to handle,” said the official source.
Discharge is moderated according to the inflow and the rainfall over the waterbody. Normally, the reservoir’s levels are maintained two feet lower than the maximum level during the rainy season. This is to accommodate stepping up storage when the inflow is less.
Former officials recalled that the reservoir was improved to discharge 21,000 cusecs in 1980s and later to 33,400 cusecs. Even weirs, structures that allow surplus water to flow automatically, were converted into regulators to moderate huge inflow of over 33,400 cusecs in 1996.
Experts suggest that an assured monthly supply of water be created to manage the drinking water needs of residents during summer even if a minimum volume is released daily from the reservoir as a preparatory measure prior to the monsoon.
Mapping of flooding zones are essential to educate people living along the banks on the vulnerability of their area during such deluges, a former official said. A legislation to safeguard reservoirs and surplus courses from encroachments is the need of the hour to prevent such calamity. “Such legislation should also cover patta lands if proven that the structure is built on a waterbody or surplus course,” sources added.