Floods: Concept, causes and complications

According to the Indian Meteorological Department (IMD), flood is a meteorological situation when water of a river begins to flow above the danger mark. The danger mark is fixed on the basis of average flow of the river over 25 to 50 years during monsoon period.

Drought is the situation, according to the IMD, having less than 5 cm rainfall for four continuous weeks any time between mid-May and mid-October. If the rainfall is less than 5 cm for any month (four continuous weeks), it is a meteorological drought situation.

Flood prone areas are located practically in all parts of the country excluding the internal plateau regions. However, there has been varying tenure of flood and from that point of view, eastern India, northeastern India and eastern coastal plains are the worst affected areas as far as flood is concerned.

The IMD has divided flood into four tenures of flood and affected geographical regions:

Six-week flood region: Assam valley

4-6-week flood region: Eastern Uttar Pradesh, Bihar plain, West Bengal and isolated plain areas of the Northeast – Loktak basin and Tripura basin.

1-4-week flood region: Eastern coastal plain, Punjab, western Uttar Pradesh, Delhi Union Territory, South Gujarat especially Narmada and Tapi basins, and Kashmir Valley.

Less than one-week flood region: Central and northern Gujarat, Rajasthan, Haryana and peripheral parts of peninsular plateau.

There are some rivers, which are highly flood prone. Brahmaputra and Ganga cover about 60 per cent of flood prone areas of the country. Some flood prone rivers are Teesta, Kapoli (a tributary of Brahmaputra), Dihang and Burhi Dihang (known for flash floods).

In the Ganga system of the Himalayan rivers, the main flood causing rivers are, Mahananda, Kosi, Gandak, Burhi Gandak, Kamla Balan, Saryu, Ghaghra, Gomati, Ramganga and Yamuna. Kosi was once called the sorrow of Bihar.

In the peninsular India, the main flood causing rivers are, Son, Punpun, Ajay and Damodar, which was known as the sorrow of West Bengal once. At present, both Kosi and Damodar rivers have been managed and controlled to some extent but others continue to play the same role.

Among the east coast rivers, Mahanadi, Godawari, Krishna, Cauvery and Brahmi are highly flood prone. Among westf-lowing peninsular rivers, Tapi and Sabarmati are highly flood prone.

In the Punjab plains, Sutlej and Beas are highly flood prone while in Haryana Ghaggar and Yamuna bring floods. The Jhelum is flood prone in the Kashmir Valley.

Besides these rivers, there are several short-distance rivers, which flow on high slopes and create flash floods during monsoon season.

Total area under flood-prone category, according to the Seventh Five-Year Plan papers is 320 lakh hectares where flood happens every year. Total flood-affected area is 2.42 crore hectares where flood occurs but not every year.

Causes of flood

Flood is the manifestation of many geographical factors and, to many geographers and geologists it has become an ecological factor. This does mean that floods are bound to come in this part of the world every year. It is a flood-ecology region.

Factors

1. Erratic behaviour of monsoon: Sometimes the catchment area receives very high rainfall and then flood arrives. In the western India – Rajasthan, Gujarat and Haryana –torrential rain brings sudden outburst of rain which leads to flashfloods. In contrast, in the North-East, even the regular rainfall brings heavy rainfall and that is responsible for the flood situation in the region.

2. Geomorphology: Flood occurs due to geomorphological profile of central plain of India. Brahmaputra Valley is a narrow ramp valley. Similarly, the Ganga basin is sandwiched between the Himalayas and the peninsular plateaus. Both river valleys are bordered by steep rising slopes while the plains are among the flattest ones in the world.

This topographical feature brings rapid run-off from the catchment areas while the run-off in the plain moves at a slow speed. Consequently, these river basins overflow bringing flood over large areas during monsoon season.

3. Deforestation: Deforestation in the Siwalik Himalaya has increased the volume of the run-off and therefore frequency and tenure of floods in many of Himalayan rivers.

Several peninsular rivers, including small ones, have become flood prone in recent years due to loss of trees in the past few decades, e.g. Ajay river has become flood-prone due to deforestation in the catchment area. Jhelum was not flood-prone earlier.

Mahananda was not so acute in bring floods. But at present all these rivers bring floods almost every year and that is related to deforestation. Several plateau districts of Jharkhand and Madhya Pradesh have fallen under flood due to deforestation and rapid run-off in these areas.

4. Soil Erosion and Siltation: These are responsible for increased flood-frequency mainly in the central plain of the country. According to the UNEP, River Kosi is the second-most affected siltation affected river in the world, only after Hwang-ho of China.

Bed siltation occurs due to soil erosion in the catchment area and the deposition of silt in the plains owing to rivers’ inability to carry the load any further. It makes the valley shallow and whenever there is voluminous flow of water, it brings flood and water spreads in the surrounding areas.

5. Drainage Pattern: It is responsible for the flood in the central plain. Most of the rivers make meandering courses. So, overflow brings waters of different rivers close and vast areas, thus, get inundated.

Punjab does not witness spread of water because of parallel, man-made, drainage of rivers. Although, the rivers often flow over danger mark during flood season but water does not spread. In Punjab, the doab line is always free from flood due to parallel pattern of drainage but the meandering courses of rivers in Bihar and Uttar Pradesh, and braided courses in all delta regions create floods.

Most of the rivers have wide V-shaped valleys. So, the depth of the valley is not great and any increase in the volume of water leads to overflowing and inundation of the neighbouring areas.

Again, some of the rivers have created natural levees which protect one side of the valley from flooding but the other bank is flooded and causes flooding over low lying areas adjacent to that bank. Bihar plain is noted for this type of flood. Western banks of rivers in Darbhanga have natural levees.

In the central Bihar, there is also flood due to overflow of the main consequent river. Its overflow prevents the inflow of tributary water channels. In fact, the water of the main river, Ganga, begins to enter the course of the tributaries and the tributaries’ water is pushed back and water spreads over vast areas, e.g. Tal area flood in South Bihar and Jharkhand, Chaur area flood in West Bihar and West Bengal. Chaurs are the floodplains situated in the abandoned courses of rivers. They form ox-bow lakes during floods.

6. Shifting Course of Rivers: Such flooding is mainly important in regions where rivers have a history of shifting their course. Rivers like Kosi, Teesta, Gomati and Mahananda are facing this problem.

Kosi is now believed to have settled in its course but others are still active. All these rivers have westward shifting courses. These rivers have a tendency to migrate to the west. Hence, the west banks of these rivers are constantly being eroded and the east bank extends to a huge lowl-ying floodplain. These rivers cause phenomenal spread of water beyond eastern banks onto the vast plain.

The rotation of the earth is held indirectly responsible for the westward shifting of these rivers and the associated floodplains. Rotational impact brings westward shift of rivers. The eastern banks become low lying and vulnerable to floods.

Eastern coastal plains receive floods not simply due to deforestation and rapid run-off but also because of cyclonic rainfall and greater havoc is caused by the combination of these factors. If there is a high tide situation along with cyclone, rainfall causes great damage.

Cyclones bring rainfall and high tide water on the coastal plains as well as increase the velocity of the sea waves. Tidal water enters the mouth of the rivers pushing back the river water. The water level of the river increases. The river goes in a spate. The flow of the river is reversed and lowl-lying floodplain is taken over by the floodwater. Such tidal flooding is seen in the Mahanadi and the Godawari.

Western coastal rivers generally receive flash floods. The duration of such floods is about one week. The situation mainly occurs due to deforestation along the Western Ghats. Loss of vegetation means less obstruction to run-off.

Rapid run-off leads to greater water volume in the river valleys than the channel is capable of carrying off to the sea. This pushes the water level up in the river valleys leading to overflow and inundation in the neighbouring areas.

7. Human Factors: Floods are also caused by human factors. Ironically, flood control programmes and some other developmental programmes have indirectly contributed to greater spreading of flood. Although, multi-purpose vally projects and other power generation projects have controlled floods in the basins of some of the rivers and regions. But the problem has emerged in a different way.

Due to siltation on the beds, the reservoirs’ capacity has reduced and whenever excess water is released from that reservoir through canals and other outlets, flood is caused. Flood water even enters the areas where flood had never come previously.

The canals also face siltation problems, which get compounded by the seepage of river water from the sides of the canals into neighbouring agricultural lands. These agricultural fields become unproductive due to excess of water and behave like marshy land and are waterlogged at times even if water is not pulled from these canals. The excessively wet lands are not conducive for agriculture.

This type of flood brings greater loss as this is a relatively new phenomenon and people dependent on such lands are neither prepared nor equipped to deal with such a situation. In 1995, Banka district of Bihar turned into a wasteland due to the opening of two reservoirs – Chandan and Orhni.

Governmental approach to construct embankments for the protection of settlement and agricultural lands has also deteriorated the situation in many parts of the country. Now, the approach is being modified. Such programmes are now being called flood protection programmes instead of flood control programmes.

Thus, several new regions, e.g. in northeastern towns of Bihar, many new settlements have fallen in the trap of flood. Flood control measures have brought new dimensions in floods in India.