Half of Chennai’s typically flood-free regions could face significant flooding if a sudden cloudburst or flash flood occurs, a risk that becomes acute during the monsoon season from October to December.
This threat arises from the National Highways Authority of India (NHAI) dumping debris into the Cooum River as part of the Rs 5,885 crore 20.9 km double-decker Port-Maduravoyal corridor project. The Cooum River, crucial for draining central Chennai areas, has been heavily obstructed. Entire sections of the river are fully blocked in Koyambedu and Thirumangalam, with over 50% of the river obstructed from Chetpet onwards. Given that 15.5 km of the expressway spans the river from Koyambedu, previously safe areas like Anna Nagar, Kilpauk, Chetpet, and Egmore now face potential flooding.
The National Green Tribunal has taken up this issue on its own initiative, with a hearing scheduled for July 26. A Water Resources Department official noted that there seems to be no alternative to depositing debris in the Cooum, as per NHAI’s requirement for bringing in piling rigs for the riverbed excavation. The NHAI plans to dig 35 metres below the river, raising concerns about whether the debris can be cleared before the monsoon or if pile foundations can be placed quickly enough. Experts warn that the Cooum’s capacity to handle a flow of 40,000 cubic feet per second (cusecs) will be drastically reduced, with the average breadth of 60-70 metres shrinking to 20-30 metres in some areas. Given that the river is fed by four canals and over 100 stormwater drain outlets, flooding is deemed inevitable.
Water Resources Department officials have stated they will remove the debris if the NHAI fails to do so before the monsoon. They have a Rs 50 crore security deposit from NHAI to cover debris removal. Additionally, Anna University is conducting a flood response study for disaster mitigation, examining the impacts of debris-filled roads and advising on building retaining walls, evacuating residents, diverting water, and increasing bund heights, with implementation planned from July.
When fast-flowing floodwaters hit debris-filled areas or pillars, they could potentially flood residential regions adjacent to the river. “Based on velocity studies, retaining walls will be constructed in critical locations to mitigate flooding,” said a Water Resources Department official. This situation highlights the urgent need for strategic planning and timely action to mitigate flood risks and protect Chennai’s residents and infrastructure from the impending monsoon threats. The effectiveness of these measures will be crucial in ensuring the city’s resilience against severe weather conditions.
