Unit 3.9 - Floods-Causes and Hazard Management Steps

A flood is an overflow of water that submerges land that is usually dry. It is  a covering by water of land not normally covered by water.

[Image of flooding from Satellite imagery- See the area on September 19 (bottom) before the floods and on October 12 (top) after floods.the image is of  northwestern Bangladesh in October 2005.  ]

Flooding may occur as an overflow of water from water bodies, such as a river, lake, or ocean, in which the water overtops or breaks levees, resulting in some of that water escaping its usual boundaries,or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these changes in size are unlikely to be considered significant unless they flood property or drown domestic animals.

Floods can also occur in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers. While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry.

Some floods develop slowly, while others such as flash floods, can develop in just a few minutes and without visible signs of rain. Additionally, floods can be local, impacting a neighborhood or community, or very large, affecting entire river basins.

Why Floods Occurs:

Floods are caused when the volume of water exceeds the normal carrying capacity of a river. During the rainy season, rain water flowing past small rivulets joins the main current. The volume of water flowing into the main current thus far exceeds the channel capacity. The excess water overflows the banks and spreads on the surrounding areas.

Rivers flow past the plains and discharge water into the sea. In case there is excess inflow, the rivers cannot discharge the increased volume into the sea at their existing speed. That is how riverine floods are caused.

Rain is not the only cause for flooding. Floods also take place when a cyclone strikes the coasts. While rains are a natural phenomenon, there are certain other factors too that lead to flooding of rivers. A river flowing through a hilly region may be blocked due to a landslide. In that case, the water will flow into the neighbouring areas.

[Blue shades portions indicate areas of increasing rainfall intensity and thereby more prone to floods in India]

Silting reduces the channel capacity of rivers. If a sea tide and the riverine flood happen at the same time, the severity of the flood is compounded. The pressure of population is leading to human encroachments on river banks. The rivers now have little or no space left for expansion when the volume of water they are carrying increases substantially. The net outcome is flooding wherever the river finds space along its banks.

Flood Prone Regions:


[Green portions indicate areas prone to floods in India]

Floods are a worldwide phenomenon. For centuries the river Hwang-ho was named as the Sorrow of China. In India, North and Eastern parts are more prone to floods. India’s 60% of river flow is accounted for in the Indo- Gangetic- Brahmputra plains. This basin accounts for a near 47% of the country’s population spread over 15 States.

 River basins of Narmada, Tapi, Mahanadi, Godavari, Krishna and Cauvery are also prone to floods. However, flooding in these areas of the North-West and Central regions is not as severe as in the North-East. The monsoon period (June to September) has maximum floods. This is the time of the years when there are heavy rain falls and the rivers have to carry unduly heavy discharge.

Effects of Flooding:

Floods cause immense damage to life and property. During floods, the affected regions are cut off from the rest of the country.

The worst affected are the farmers and the economically backward communities. The farmers lose their crops. They have to move out along with their cattle and the agricultural operations are totally disrupted. Most farmers are not well off economically and as such take much longer time to re-establish themselves once the flood waters have receded.

(see article : Worst flood in 29 years hits Assam)

Preparing for the Floods:

Certain precautions need to be taken well before the expected time of floods. You have to be evacuated. So know the route to the nearest shelter point. Carry a first aid kit. Diarrhoea caused by infected water is most common in flood relief camps. Your first aid kit should include a torch, fresh water, food items like parched grains, biscuits etc. candles, matchboxes, kerosene, stove, umbrellas and clothing stored in a waterproof bag.

When Flooding Appears:

1.  Flood warnings are broadcast over Radio and even mobile loud speakers.

2.  Besides your family, you may have to shift your cattle to a safer destination situated at a higher point or slightly distant from the flood prone area. Keep them in readiness.

On Evacuation:

1.  You may have to be away from your home for quite some time.

2.  Turn off electricity.

3.  Block the outgoing drains to prevent overflow.

4.  Place clothing, beddings and other household goods on tables or beds to save them from flood waters.

5.  Lock your house.

6.  Never enter flood waters. They may be deep or the current may be too fast.

During floods:

1.  Be alert against snakes that may have come out from their holes.

2.  Use boiled water

3.  Keep yourself prepared against the outbreak of diarrhea.

4.  Keep the surroundings disinfected. Use bleaching powder for that purpose.

5.  Do not eat food that has been soiled with flood waters.

6.  Be very particular about what you eat and drink. Epidemics are not uncommon at relief camps.

See Also :

Worst flood in 29 years hits Assam