Whenever a major flood occurs, with accompanying loss of life, livelihoods and property, it generates a strong feeling that something should be done about this problem. Starting from the proposition that the problem, i.e., floods, should not be allowed to happen again, administrators and engineers decide that the river needs to be controlled, and come up with proposals for embankments and dams. This was what happened in the 1950s when Bihar was laid low by exceptionally heavy floods, and Nehru, appalled at the destruction and human misery, wanted something to be done urgently. Once again, in 2008, when Bihar is reeling under floods, we hear demands for a high dam on the Kosi in Nepal as a flood-control measure. Prima facie, that seems reasonable. Where is the fallacy in this?

First, we must distinguish between avoidable and unavoidable floods. When Delhi or Mumbai get heavily flooded with the first monsoon showers, the cause is inadequate or badly designed or poorly maintained or carelessly blocked drainage systems, or the ill-considered diversion of natural drainage channels. These are floods caused by human error or negligence. Floods are also sometimes caused by bad dam management, and these too are man-made. Periodical river-floods are different— they are natural phenomena — and could be seasonal floods because of heavy monsoon rains; flash floods from cloud-bursts; floods resulting from the sudden release of waters held up by mountain landslides; and so on. These cannot be prevented. They can be of varying intensities, and the regular floods, i.e., those other than freak events, are classified by probability of recurrence (once in 20, 100, 1000 or10000 years). When the flood waters come, the river needs space to spread and accommodate them. That space is known as the natural flood-plain of the river. If we occupy the flood-plain or build structures on it, or try to jacket the river within narrow confines, we are asking for trouble.

... contd.