What is the Mamallan Reservoir project in Tamil Nadu and why is it being opposed? Explained

The story so far: The proposed Mamallan Reservoir in Chengalpattu district is being presented as a solution to two problems facing the rapidly urbanising southern parts of the Chennai Metropolitan Area — drinking water shortage and flooding. But the project is also being built in a landscape of marshes, salt pans, backwaters, and wetlands that supports fishing and provides feeding and roosting habitat for birds.