Chennai: The Tamil Nadu government’s recently-unveiled project to turn the Kovalam-Nemmeli backwaters into a freshwater reservoir named after the 7th century Pallava ruler Mamallan has run into severe opposition from fisherfolk, social activists and environmentalists. Invoking the rhetoric of climate change and resilience, the government justifies the project, claiming that it is essential to enhance Chennai’s drinking water supply. It asserts that it will recharge groundwater, mitigate flooding in the Kovalam sub-basin and arrest salinity intrusion.These stated benefits are stacked against the project’s social costs: biodiversity losses, lost fisher livelihoods and reduced availability of inexpensive fish for the coastal poor. The backwaters are an ecotonal patchwork of tidally influenced salt marsh, mud flats and mangroves. These not only host a unique assemblage of species, but also offer freshwater, marine and brackish water habitat for diverse flora and fauna. Seen from ecological or climate perspectives, estuarine marshes are more valuable than constructed freshwater wetlands; they dissipate storm energy, buffer tidal surges, and support higher biodiversity.Whether the reservoir’s claimed benefits adequately offset these social and ecological losses is a subjective valuation that depends on the premium one places on social justice and ecology. The validation of the asserted benefits, however, can be more objectively evaluated by examining project documents like the Environmental Impact Assessment. For that, understanding the lay of the land is essential.The locationBefore the East Coast Road (ECR) was built, the eastern stretch of the backwaters bordered a sprawling spread of towering sand dunes dotted by hamlets of fishers and small farmers. Dunes, with their permeable soil, are storehouses of freshwater and veritable aquifer recharge zones. These dunes have now been flattened by urbanisation and the ECR runs like a 6.5 metre-high ‘Great Wall’ along the marsh’s eastern fringe. To the west of the marsh are coastal floodplains; they are meant to protect Thaiyur, Thandalam, Poonjeri, Siruthavur and other villages in the southwestern side of the Kovalam sub-basin by receiving their floodwaters. The 3.5 metre-high Old Mahabalipuram Road (OMR) snakes across this landscape separating the villages from their floodplains. The backwater is hydraulically connected to the Bay of Bengal through the Kovalam and Kokilimedu estuaries to the north and the south respectively.Flood mitigationThe EIA identifies three reasons for the flooding of the western villages: raised bunds on the western side of the marsh, inadequate drainage outlets along the OMR causing upstream stagnation, and the Kelambakkam-Kovalam Link Road which blocks flow from the backwaters to the estuary.The EIA claims that dismantling the western bunds, and encircling the north, east and southern fringes of the backwater with 4.5 metre-high earthen bunds will allow freshwater inflow from the west, mitigating flooding, and prevent seawater ingress from the estuaries, creating a 17.71 sq. km freshwater reservoir. Surplus waters are to be diverted to the estuaries through two peripheral drains skirting the eastern and western edges of the backwater.Flood-mitigation claims are not supported by other information in the EIA. First, the backwater’s western bunds are the least significant of the three causes of flooding. Two other significant bottlenecks remain unaddressed. The OMR, with insufficient outlets, dams the drainage of water from the western catchment into the floodplains. Even more deadly, the Kovalam backwaters that receive a peak discharge of 25,000 cusecs – only slightly less than what was discharged from Chembarambakkam in December 2015 – are severely compromised. Google Earth measurements indicate that the Kelambakkam-Kovalam Link Road blocks a 1.7-km wide floodplain, forcing floodwaters to flow through a 16 m culvert to reach the estuary.Going by the information in the EIA, the project will not mitigate rain-induced floods. Hydraulically, inserting a full-level reservoir between an upstream choke point (Kalavakkam outlet through the OMR) and a downstream choke (Kelambakkam–Kovalam Link Road) does not repair the main controls on rain-driven flooding. The basic flood regime for villages west of the OMR remains unaltered, condemning areas like Paiyanur, Thandalam and Siruthavur to recurrent flooding; the conversion of a natural floodplain into a reservoir risks aggravating flood situations when the reservoir is full. The peripheral drain is useless because access to the macro-drain – the Kovalam estuary – is blocked at the Link Road choke point.Groundwater recharge and salinity intrusionFor the reservoir to recharge groundwater, two conditions must be met: the reservoir bed level must be above groundwater level to enable a downward flow; second, the bed must be made of permeable material (sand, silts, gravels or fractured rock). At the reservoir site, the first condition is met but not the second. The EIA reveals that the reservoir is underlain by low-permeability clay and clayey loam soils with slow infiltration.Any claim of substantial recharge through the bed is unsupported, as is the consequent expectation that the reservoir will arrest salinity intrusion. Contrarily, the EIA is replete with given data and missing data that implicate the reservoir’s role in aggravating salinisation of inland areas. The EIA’s disaster management plan only discusses extreme rain events and rain-induced flooding. It is blind to the biggest threat – the Bay of Bengal.The project site and study area are located in a “very high” flood inundation and cyclone risk zone. Severe cyclones can result in powerful storm surges pushing the seas to swell far higher than normal astronomical tides. Kovalam’s storm surge can top 3.5 metres. For this region’s topography, this portends danger. Adding this to normal tide levels (~1 metre) and wave run-up (i.e. how high the waves climb up the shore above the water’s normal level), one can expect waves to bring surges of up to 5.5 metres high. That would carry sufficient seawater and energy to spread across the entire backwater, with only the salt marsh vegetation to dissipate energy and protect the hinterland.But if the reservoir is built, 4,375 acres of storm surge buffer land will be lost. This will force the sea to use the estuarine channels as a throat to penetrate deep inland invading low-lying lands and flooding agricultural lands and aquifers with seawater. With warming seas, rising sea levels and intensifying storms, this risk cannot be taken lightly. Such surges also come with significant risks of seawater flowing over the 4.5 metre reservoir bund, frustrating the objectives of the project. Repeated overtopping can weaken the earthen bund causing catastrophic collapse and release of impounded waters. The EIA does not even consider such scenarios. The project was cleared by the state coastal zone management authority, despite glaring violations of the Coastal Regulation Zone rules. Photo: K. Saravanan, 2025.Drinking water promiseThe chief minister and proponents of the project point to drinking water scarcity as a justification for the reservoir. However, the government’s industrial policies are at odds with Chennai’s water predicament. For a water-scarce city, it is inviting the thirstiest of industries that aggravates water stress and does so without creating jobs.Chennai is fast becoming a hub for data centres clustered around Siruseri, with 234 MW of non-AI and AI-ready centres (see table). Data centres require uninterrupted supply of cooling water, even if monsoons fail. According to figures published by the International Energy Authority and World Economic Forum, this sector consumes between 0.8 to 2.6 billion litres per 100 MW annually. Just the existing data centres consume between 1.7 and 6 billion litres per year, enough to meet all domestic needs of 35,000 to 1.2 lakh Chennaiites. This consumption represents 3.7 to 12.8% of Mamallan’s full capacity. Curiously, the government claims Mamallan’s water will service Siruseri.If in-the-pipeline and publicly declared data centre proposals are included, the demand load of data centres is roughly 824 MW. If these materialise, that will add a cooling water demand of between 6 and 21 billion litres per year, or roughly 13 to 45% of the full reservoir capacity. Drinking water is a powerful justification for a city where adequate potable water is still a distant dream for a vast and marginal majority. Is the Mamallan reservoir for people or data centres?OperatorAI-Ready?Verified IT capacity (megawatts)CtrlS Datacentre ParkYes72Sify (Siruseri)Yes25.92Nxtra (Siruseri)No46STT GDCNo28Equinix (Phase 1)Yes3.24Sify (Tidel Park)No3.58AdaniConneX (Phase 1)No17NTTNo2Techno Digital (Siruseri)Yes36Total~234The above is a verified list of functional data centres sourced from annual reports, media reports and press releases.Causes for concernThe project was cleared by the state coastal zone management authority, despite glaring violations of the Coastal Regulation Zone rules. A concrete road was illegally laid for the chief minister’s foundation-laying ceremony. Project-affected villages were denied their constitutional right to pass resolutions condemning the project at the last Grama Sabha meeting. When the government wants something, law becomes the first casualty.If you are living in Chennai’s vicinity, you would be better placed worrying about these things rather than climate change and sea level rise – the institutional failure of our engineering departments, the political capture of the environment department, the state’s growing impunity in bypassing coastal protections, and the state’s powerful mobilisation of greenwash and climate-wash.Naming this ill-conceived reservoir after Mamallan is an insult to the hydro-engineering prowess of ancient Tamils. The Cholas and the Pallavas designed elegant, enduring structures not by offending water but by working with it. If the government truly wants to celebrate Mamallan, it should abandon this project and rejuvenate the salt marsh. The marsh must get its due budget of freshwater and seawater influxes and effluxes. For this, culverts of adequate capacity on the OMR must be designed by real engineers, and the Kelambakkam-Kovalam Link Road must be converted into a bridge. These two interventions are the only actions with a demonstrable flood-mitigation effect in the EIA’s own hydrological framework. As for drinking water, we must design the city to live within its limits; we must reject water- and electricity-intensive industries, and prioritise quenching the thirst of people and other life-forms.Nityanand Jayaraman is a Chennai-based independent journalist and social activist.