Flood is a situation when there is flow of water in a river more than its capacity and the water overflows the levees and spreads in nearby areas.
Thus, flood is a part of normal conditions of flow of river water. The water spreads on the ground where the normal stream of water does not reach normally.
Flood is caused due to different reasons in different areas, but basically the use of the word ‘flood’ is made in the following situations:
1. Excess flow of water in the river more than its capacity within its banks so that water flows on the nearby land.
2. Amalgamation of Companion Rivers on the mouth of a main river in ultimate flood position.
3. Excess rains than the capacity of the river.
4. Existence of icebergs or landslide in flow route of river, which results in flow of water over the banks.
5. Coming of high waves from higher planes.
6. Heavy rains at local level.
7. Storm or cyclone.
Flood is a part of the natural water cycle. It is a natural catastrophe which is directly related to rains, which affects water management. In case it rains excessively in any area, rivers become imbalanced and come in spate, which creates a situation of flood. Effect of such an environmental situation can be seen on the ecology of such an area. The normal meaning of flood denotes continuous water logging of vast ground level for a long duration.
Hence, flood is such a natural incident which is the result of excessive rain. A total of 3.5 per cent area of the world is affected by floods, while it is inhabited by 16.5 per cent population. Out of the geographical area of 32.80 crore hectares, about 4 crore hectares (one-eighth of total area) is flood affected area.
About 80 per cent of rainfall in India occurs in summer through south-western monsoon. Brahmaputra and Ganga rivers of India give birth to more floods that affect large parts of Assam, Bihar, Uttar Pradesh and West Bengal. Due to insufficient disposal of rainwater, water management projects are not always successful and they result in creation of a situation of flood.
Causes of Flood:
Flood is the result of unfavourable combination of meteorological conditions and physical structure. Situation of flood is caused by rains in excess of disposal capacity of its catchment area. Mainly, nature is responsible for flood but flood is also caused due to human activities.
Thus, the following reasons are responsible for floods:
1. Large Catchment Area:
Large catchment areas conserve water in vast areas, due to which they receive water from different sources at different rates. When there is excess water, it results in flood situation. Mississippi and Missouri rivers of United States of America are of such nature, where devastating floods are caused.
In March 1927, floods had affected 18 million acres area here. Among other rivers with large catchment areas are Ascambia and Tennesse rivers of Alabama province, Colorado River of Arizona province. White, Red, Arkansas and Mississippi rivers of Arakansas province. El and Clamuth rivers of California province.
Savanna river of Georgia province, Wabash river of Indiana province, Niogi and Kansas rivers of Kansas province, Ohio river of Kentucky province, Potomac river of Maryland province, Missouri river of Missouri province, Dilawoeir river of New Jersey province, Miami river of Ohio province, Wilamet and Columbia rivers of Oregon province, Tennesse river of Tennesse province, Columbia river of Washington Province etc. Brahmaputra, Ganga, Kosi and Damodar are such rivers in India.
2. Tropical Cyclones:
Cyclones play an important role in floods caused in coastal parts. In India, Andhra Pradesh and Orissa coasts in the east and Gujarat coast in the west are highly affected by problem of floods due to cyclones. Cyclones cause high tidal waves whose water affects coastal ground areas. In November 1982 and 1983, cyclones had affected 27 dams of Saurashtra.
Floods in Indian River basins are also caused by rainstorms which are generally associated with low pressure systems like well marked lows, depressions or tropical cyclones. During past 109 years (1891-2000), over 1,000 tropical cyclones and depressions originating in the Bay of Bengal and Arabian Sea moved across the Indian subcontinent. Passage of such storms in quick succession over a river basin invariably leads to severe floods.
The soil generally gets fully saturated during the passage of the first storm. The rainfall following the subsequent storm leads to flood situation. Parts of the country, mainly coastal areas of Andhra Pradesh, Assam, Orissa, Tamil Nadu and West Bengal, experience such cyclones leading to extensive flooding. The flood due to super cyclone combined with heavy rainfall during October 1999 in the coastal belt of Orissa is an example.
Due to deforestation of high ground areas, soil erosion occurs very fast, which results in collection of silt in lower areas. Due to silt collection, water in rivers comes up resulting in lower storage capacity. In Bhabar area of northern Nepal, annual siltation is about 15-30 cm. at the base of rivers. Base level of Kosi river of Bihar is at a very high plane. Problem of siltation is there in Ganga and Brahmaputra also due to their tributaries. Due to such siltation, natural levees of the river are also formed.
4. Cloud Bursts:
Due to particular climatic conditions, some parts of the country experience sudden unprecedented heavy rain known as cloud bursts. During the year 2000, coastal districts of Andhra Pradesh experienced such a flood situation. Cyclonic circulations during monsoon conditions too sometimes lead to cloud bursts leading to floods.
In July 1981, such a condition developed over East Rajasthan. Many places around Jaipur recorded a three-day rainfall more than their annual normal average. Annual rainfall at these places varied from 550 mm to 700 mm. But during this period, the daily rainfall was 250-590 mm, two daily rainfalls were of 430-800 and three daily rainfalls were of 450-900 mm, which led to the flood.
In arid and semi-arid regions, sudden and unexpected torrential rain also causes floods because such areas normally get very little rain and natural catchment is not in proper condition. Hence, these rivers are not able to absorb heavy rain water.
For example, flood situation was caused in Jaipur city and nearby areas due to the unexpected 836.4 millimetre rainfall from 17th July to 21st July 1981. Clouds generally burst in hilly areas where wind rises up fast during the monsoon season and winds rising high do not allow water to drop. Water collects on the surface of the air.
Due to sudden rise and stoppage of air, a thick layer of water falls on the ground and its rate per hour is not less than five inches. On 16th July 2003, 100 persons had flown away in the Puliya nalla of Gadsa valley in district Kulu of Himachal Pradesh due to flood caused by cloud burst. Similarly, on 8th August 2003, 60 persons had died due to sudden flood caused by cloud burst in Kangdi nalla near Rohtang Pass.
5. Heavy Rainfall:
Heavy rainfall for long duration also causes flood situation because increase of quantity of water in upper catchment area cause flood in lower places. If time of concentration for water is less, severity of flood increases. This situation depends on size and morphology of the catchment area. Such flood is often caused in rivers of India.
Sudden and violent ecological storms also affect it. Rains of seasonal nature also give rise to floods, e.g., most of the rains during summer in tropical climate and during winter in Mediterranean climate cause flood.
Deforestation on a large scale in the upper catchment areas of rivers increases the intensity of flood. These anthropogenic factors are seen in two perspectives:
Due to deforestation, most of the ground surface in upper catchment area of rivers becomes naked, which reduces infiltration of water underground and surface water flow is the maximum. Dense forest cover prevents flow of water.
Forest canopy becomes an obstacle in allowing the drops of water to go the ground level. Shivalik, Lower Himalaya, Plateau of Chhota Nagpur and Western Ghats etc., have become a regular feature of flood. Similarly, Tista and Torsa (West Bengal), Gandak in Madhya Pradesh, Kosi and other rivers of Bihar, etc. are regular victim to floods. During last century, 86 per cent forest cover has reduced in India.
Natural incidents like earthquake also cause floods. After the earthquake in Assam in 1950, situation of flood was caused in river Brahmaputra, which affected large areas.
8. Inadequate Drainage System:
Flood is caused due to inadequate drainage system. Generally, rivers get lesser water, due to which their flow is obstructed and flood is caused during the monsoon period.
This flow route is obstructed in the following forms:
i. Slow Development of Drainage Channel:
Proper development of drainage channels has not taken place in western and north-western India. Due to absence of proper natural drainage system in arid and semi-arid regions, rain water does not get sufficient outlets during the monsoon season.
ii. Reduction in Carrying Capacity of Rivers:
Due to deposition of silt in the flow route of rivers, water carrying capacity of rivers is reduced. This silt comes from catchment area after soil erosion. As a result of it, a situation of flood is created in adjacent plain areas. Flood is caused in eastern Uttar Pradesh and north Bihar due to deposition of silt in Narayani and Kosi rivers due to soil erosion of hilly slopes of Himalayas.
iii. Obstruction in Natural Flow due to Landslide:
Because of landslide caused by earthquake or other natural reasons, natural flow of river water is obstructed. Sometimes, natural dams of rivers are damaged due to landslides, which causes a situation of violent floods. In 1978, landslide formed a dam amidst Bhagirathi in Himalayas, resulting in violent flood situation. In the year 2002, landslide caused flood situation in the upper portions of Yamuna River, and in 2005, in Sutlej river.
iv. Meanders on River Banks:
Formation of meanders and sand bars also cause floods. Normally, sand bars are formed on banks of rivers and they reduce capacity of water flow. Such a problem is found in delta areas of West Bengal and Orissa.
9. Faulty Agricultural Practices:
Planning Commission of India has divided India into 15 agro-climatic zones and recommended cultivation of suitable crops as per climatic conditions. Soil erosion increases due to faulty agricultural practices. Faulty irrigation methods also develop flood situation. Irregular irrigation system causes situation of water logging due to which the ground level is not able to absorb excess water and it causes flood.
The main reason for incessant floods and continuous increase in its frequency is reckless deforestation in catchment areas. Non-availability of funds for maintenance of modern water management practices is also one of the reasons. Havoc of floods directly affects water management system because the problem of water logging would arise in flood affected areas, due to which the quality of salinity and alkanity in soil would develop.