Floods in India can’t be entirely prevented. The approach to flood management is a combination of protection from floods of less severity: reducing the damage by flood forecasting; and disaster relief in case of floods of larger severity. Flood management options are typically divided into two types, structural- i.e. comprising some construction, of embankments, and flood control reservoirs; and non-structural, comprising flood forecasting, flood plain zoning, and disaster relief.
On average, every year India gets 4,000 billion cubic meters of water mostly through rainfall and some snowfall. However, this is the average, over a large number of years. In any given year, the rainfall, and hence the river flow, may vary from this average, on the lower side, or on the higher side. Such rainfall instances, very low or very high, are called hydrologic extremes. Floods in India are a result of hydrologic extremes. This short article explains the phenomenon of floods, and droughts, why these occur, and how to manage them.
Floods
The term ‘flood’ is commonly used to describe any inundation by water. But there are two distinct mechanisms that can cause the inundation. Rainfall takes place somewhere in the upstream catchment, and consequent high flow in the river may spill out into the habitation areas somewhere downstream. This is called a flood. The other mechanism is, a high rainfall may take place locally, and the rainwater may fail to drain out fast enough and accumulate in the city/village.
This is called drainage congestion. The inundation that takes place in many parts of Uttar Pradesh, Bihar, and Assam every year, floods. The inundation that took place in Mumbai in July 2005, or September 2017, was drainage congestion. In India, 33.5 m. Ha (million Hectares) of the area is flood-prone, and out of this, on average, some or 7.5 M.Ha is affected by floods every year. Floods in India are most common in Ganga and Brahmaputra river basins.
Causes of Floods:
A very heavy Rainfall in the upstream catchment causes a very large river flow. The width of the river through the city downstream is not adequate to carry that flow, and the water spills over, beyond the usual river banks.
Natural Lake Burst. A landslide takes place in the river and acts like a dam. Water accumulates behind it, creating a lake. As the water storage builds up, the landslide dam blocking the path of water bursts, and the accumulated water flows out in a short time, causing a flood.
Whatever may be the cause, a flood causes huge damage to property and also life-human and farm animals, destroys standing crops, deposits sand in the farms and renders them unsuitable for cultivation, destroys buildings, and also destroys roads/railway lines/bridges/communication links. A flood also destroys drinking water sources and is often followed by an outbreak of waterborne diseases.
Managing Floods in India:
Floods in India can’t be entirely prevented. The approach to flood management is a combination of protection from floods of less severity, reducing the damage by flood forecasting, and disaster relief in case of Floods in India of larger severity. Flood management options are typically divided into two types,
- Structural – comprising some construction of embankments, and flood control reservoirs
- Non-structural – comprising flood forecasting, flood plain zoning, and disaster relief
A flood control reservoir stores the incoming flood water and releases it slowly after the flood is over. Flood control reservoirs are the most reliable and long lasting protection from Floods in India. Many such reservoirs have been constructed in India and have very successfully provided long term flood protection. The most famous examples being Hirakud dam on Mahanadi, and a series of dams in Damodar valley.
Finally, if a flood does occur, relief operations are needed to rescue marooned people and provide them with shelter, food and water, and medical help.
Drainage Congestion:
Inundation in cities is usually due to the inability to drain out the rain water fast enough. Construction of buildings impedes the flow of water over the land; solid waste may choke the storm water drains, which are in any case not adequate, and in coastal cities, the problem is compounded if a heavy rainfall coincides with high tide. Mumbai was inundated on 29th August 2017. And at the same time, the city of Houston in the USA was also inundated, far worse than Mumbai, due to the same mechanism. It may sound harsh, but short-duration inundation due to drainage congestion, is a problem the cities may have to live with.
What is Not A Solution to floods?
We live in an era of self-proclaimed experts, and there is a lot of misinformation doing the rounds, about what causes Floods in India and how to manage them. Three of these need to be addressed.
- Dams do not cause floods. Dams moderate floods. The extent of moderation depends on how full the reservoir was when the flood started. But the flood released over the dam spillway is invariably less than the flood that would have been, had there been no dam.
- Forests are not the solution to Floods in India. The foliage of trees intercepts some rainfall; tree roots promote percolation, and trees act as impediments to water flow. But the impact of all this is perceptible only for small floods. For large devastating floods, the impact is insignificant.
- There is no such thing as ‘our ancestors had learned to live with the Floods in India. When the population was less, and the pressure on land resources was less, it was possible to simply live away from the river. Many villages are now located close to the river out of compulsion.
Floods in India
India is one of the few countries in the world endowed with reasonably good land and water resources. The average annual rainfall of the world is 840mm, whereas, in India, it is 1150mm. But the fact is that some parts of the country (ie. South and West) are facing water scarcity (drought) and some other parts (North and East) are getting Floods in India.
The reason is that the rainfall is erratic and gets only 3 to 4 months in a year (within 100 hours). Further, It is not uniform throughout the country. The per capita availability of rain (water) in different basins of the country is having vast variations. It varies from 18.417 m’ in the Bhramaputra basin, 3640 min west flowing rivers, 2546 min Mahanadi basin, about 660 min Cauvery basin and as low as 383 m’ in some east flowing rivers in Tamil Nadu.
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| Floods in India | Water Conservation System in India | Rainwater harvesting |
Flood Management in India
In India, flood damages are on the increase due to people’s encroachments of the water flow areas of the rivers and also due to silting of the rivers, since they are not maintained by the people or Government properly. Further, there is no water diversion to store the flood water flowing in the streams. The solution is to assess the problems of Floods in India by the agencies concerned.
For comprehensive flood and drainage management, it is necessary to assess the extent of the waterlogged area during and after the monsoon. The structural measures of flood management include constructing embankments including tanks to store excess (flood) water and channel improvements etc. The flood-affected areas are in North and North East parts of the country.