Climate modeling studies indicate that there will be more spells of intense rainfall and frequent instances of extreme rainfall. This prediction is coming true, particularly in the central sector of the Himalayan belt, where there is pronounced decline in winter precipitation (both rain and snow) and prolonged spells of drought in summer broken by cloudbursts in localized areas. Moreover, there is perceptible shifting of the seasons.
Intense and extreme rainfalls are making the floodways of all rivers vulnerable to greater floods, particularly in east-central, eastern and northeastern India. A floodway is that part of the river basin/valley which experiences at least one-foot inundation by floodwaters at least once in a 100-yr period. The compulsion of pressure on land due to expanding urbanization, increasing industrialization and uncontrolled growth of population has resulted in indiscriminate occupation of, and attendant obstruction in the floodways, in practically all the floodplains in India.
All kinds of construction such as residential buildings, commercial and industrial complexes, road embankments and bridges impede the free flow of floodwater moving in its own waterway.
Since there will be more frequent spells of intense and extreme rainfall, the people occupying the floodways have to brace for the disasters that the floods will bring. Whatever we do, the rivers will reoccupy their natural waterways – the floodways – during spells of heavy discharges.
In order to cope with the hazards of floods, it is imperative that floodways of all major rivers are identified and precisely delineated in the 1:50,000 scale topographic maps. Remote sensing and study of aerial maps will be helpful. The second step is to make bypasses for excess floodwaters through canals skirting the settlements on the floodway. There is a need to have and implement strictly the floodway regulations.
Furthermore, the inevitable consequence of voluminous accumulation of sediments is the progressive decrease in the carrying-holding capacity of the channels. In a number of areas, the channels are almost choked. Therefore, if there is even slightly higher than normal discharge, the floodwater spreads beyond the limits of its pathway.
The construction of embankments (levees) to contain floods has considerably aggravated the situation, because the phenomenal accumulation of sediments has raised the level of the channel beds between the levees. The channel beds are now higher than the floodplain.
It is time to realize that building embankments or levees on the banks of the channels is not the solution to the problem of flood hazards. Embankments away from the channel on the higher edge of the floodways offer a better option. Channelization of the floodwaters would prove more effective.
If the excess discharge can be distributed through a network of canals linked with rivulets and streams, the hazards of floods will considerably diminish. Not only would these canals and streams provide passage to excessive discharges, but also serve as temporary storage of floodwater for a short duration. Desilting of channels periodically, if not regularly, would also provide some relief.
Lastly, the occurrence of floods is a normal phenomenon, and the disaster they cause in the floodways of rivers cannot be regarded as an expression of nature’s wrath. Since people unknowingly and indiscriminately occupy the natural waterways of rivers, they must pay the price for coming in the way of the furious flow of rivers in spate.
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