CHENNAI: A group of 22 environmental experts, including ecologists, hydrologists, ornithologists and marine biologists, has written to M. K. Stalin, urging the Tamil Nadu government to withdraw the proposed Mamallan Reservoir project in the Kovalam–Nemmeli backwater system. In their letter, the scientists warned that the project would cause irreversible damage to a critical coastal wetland and undermine both ecological stability and local livelihoods.
The Mamallan Reservoir has been proposed as a major new source of drinking water for Chennai, with the state government projecting it as the city’s sixth reservoir aimed at capturing surplus floodwater and easing chronic water shortages. Planned across a low-lying coastal stretch between Thiruvidanthai and Kokilamedu, the project would convert a large expanse of existing brackish water marshland into a freshwater storage facility. Authorities have argued that the reservoir would significantly augment urban water supply, particularly during periods of scarcity.
However, the scientists’ intervention highlights what they describe as a serious underestimation of environmental risks. According to the letter, the Kovalam–Nemmeli backwater system is not a degraded or expendable water body but a functioning coastal wetland that performs multiple ecological roles. These include acting as a nursery for fish and other marine species, supporting migratory and resident bird populations, regulating salinity and nutrients along the coast, and serving as a natural buffer against flooding, erosion and storm surges. The experts cautioned that altering the system’s hydrology and salinity by converting it into a freshwater reservoir would dismantle these functions in a short span of time.
From an ecological standpoint, the scientists warned that the shift from a brackish estuarine system to a freshwater reservoir would lead to the collapse of habitats that have evolved over decades. Species adapted to saline and semi-saline conditions would be unable to survive, triggering cascading effects across the food web. The loss would not be limited to biodiversity alone, they said, but would also affect traditional fishing communities that depend on the wetland as a breeding and feeding ground for fish and shellfish.
The letter also raised concerns about the decision-making process behind the project. The signatories questioned whether adequate baseline studies were conducted before approvals were granted, pointing to the absence of publicly available, comprehensive assessments on hydrology, biodiversity, groundwater dynamics and long-term coastal impacts. They argued that without such data, the true costs of the project remain poorly understood, increasing the risk of unintended and irreversible consequences.
As an alternative approach, the scientists urged the state to consider recognising the wetland as a protected coastal ecosystem, proposing the name “Mamallan Lagoon.” They called for a shift towards water management strategies that work with natural systems rather than replacing them, such as wetland restoration, groundwater recharge, demand-side management and decentralised water storage. In their view, these measures could strengthen water security while preserving ecological integrity.
The Tamil Nadu government has yet to issue an official response to the letter. The intervention by the scientific community has, however, sharpened the debate around the Mamallan Reservoir, placing questions of environmental sustainability and scientific due diligence alongside the state’s pressing need for urban water infrastructure. As the project moves forward, the decision before policymakers is increasingly framed not just as one of water supply, but of how development in sensitive coastal landscapes is planned and governed.
