Why dams cannot control such devastating floods
Dams subsume low and mediumlevel floods, but during high to very high ones, their safety cushions fall short
Devastating floods in Odisha (2011), Uttarakhand (2013), Tamil Nadu (2015), Bihar (2016) and now in Kerala prove that reservoir dams on rivers, contrary to claims, cannot prevent floods. In fact, they create devastating floods.
Here’s why: Often dams are claimed to be multi-purpose projects, and prevention of floods is cited as one of the purposes of dams because they have something called a flood cushion, which gets filled only when a river is in spate. But floods don’t have any uniform dimension. There are low floods, medium floods, high and very high floods. Many of these are cyclical in nature with high floods taking place in 25, 50 or 100 years. But in this era of climate change, these cycles have gone awry: high to very high floods visit us far more often than before.
Thanks to their flood cushions, dams subsume low and medium-level floods. But these harmless floods should not be controlled because they replenish floodplains with fresh nutrient-rich soil, boost fisheries and rejuvenate the riparian flora and fauna. Such floods also recharge aquifers. Farmers, too, welcome floods for these reasons. The prevention of low and medium-scale floods also provides a false sense of security to the State and people, leading to occupation of flood plains for raising housing colonies, highways, schools, colleges, commercial establishments and even airports as has been done in many cities in India.
But when high to very high floods come, the flood cushion proves ineffective and counterproductive, necessitating the dam manager to release as much water as possible in as little time as possible to save the structure from damage. Such emergency releases from the dam result in mayhem downstream, since the real flood cushion — the flood plains — are occupied. Unfortunately, rainfall, river and nature are blamed for the mess.
The pro-dam lobby often cites the example of Tehri dam in Uttarakhand. They claim it successfully controlled the very high flow in the river Bhagirathi in 2013. Yes, that’s a fact. But it is also a fact that the event took place in June, when the dam reservoir did not have much water, and so could accommodate the sudden high inflows. What if the extreme event had taken place during the monsoon months of July-September when the reservoir would have been full? I am yet to get a convincing answer to this question.