Overview

A flood is an overflow of an expanse of water that submerges land.The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water.In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows or breaks levees, with the result that some of the water escapes its usual boundaries.While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless the water covers land used by man like a village, city or other inhabited area, roads, expanses of farmland, etc.
River and coastal flooding are the most frequently occurring natural disaster and are increasing in occurrence more rapidly than any other disaster. Urbanization and deforestation reduce the capability of the earth to hold excess water. As asphalt- and concrete-covered surfaces expand and open spaces disappear at the edges of metropolitan areas, it takes less rain to flood communities as water running over the pavement collects quickly and easily disrupts storm drain systems. Those areas which are most at risk for floods include low-lying areas, coastal regions, and communities on rivers downstream from dams. Flood waters are extremely dangerous; a mere six inches of swiftly moving water can knock people off their feet. In some places flooding is caused by excessive monsoons, while in others flooding is caused downstream from dams when reservoirs, which normally help to prevent downstream areas of rivers from flooding, are opened due to unusually high levels of rain to prevent the reservoir from overflowing the dam. Dam breaks are also a cause of catastrophic flooding. West Bengal has 37,660 sq. km flood prone area spread over 111 blocks where the total geographical area of the state is 88,752 sq. km. An analysis of the statistics of flood that occurred during last 41 years (from 1960 - 2000) shows that only on 5 occasions the state has not faced any severe flood. The total devastated area crossed 20,000 sq. km in 4 different years and the flood of medium magnitude i.e. between 2,000 to 10,000 sq. km. occurred on 10 occasions.

The Flood Problem

The flood problems of the state are of different nature at different regions. The rivers Teesta, Torsa, Jaldhaka, Raidak-I, Raidak-II etc. flowing through the districts of Jalpaiguri and Cooch Behar originate in the neighbouring country of Bhutan and the state of Sikkim and flows down to Bangladesh, another neighbouring country to meet the Bramhaputra at different locations. The rivers of the districts of Uttar Dinajpur and Dakshin Dinajpur originating at Bangladesh passes through these districts and then joins the Ganga-Padma at downstream of Farakka in Bangladesh. Both the places of origin and also the outfall of most of these rivers are in Bangladesh. The district of Malda through which the river Ganga flows receives its flood water from about 11 (eleven) States and is battered by the run-off flow generated from these vast areas. Ultimately the river flows down the Farakka Barrage to Bangladesh. Another portion of the Malda district receives floodwaters of the Mahananda, which again originates in the hills of the neighbouring country of Nepal and has some catchment area in the neighbouring state of Bihar and then passes through the district to join the Ganga-Padma at downstream of Farakka Barrage in Bangladesh.Major contributing factors to flood in North Bengal regions are the run-off because of heavy local rainfall, discharge of upper basin areas and also outfall condition in the neighbouring countries. The Mahananda and most of the rivers of Uttar and Dakshin Dinajpur districts get stagnated when the Ganga upstream and downstream of Farakka Barrage rules high thereby not allowing drainage of flood discharge during that period.

Analysis of Area Flooded against Years of Occurrence

Types of Flood

It is a temporary inundation of large regions as a result of an increase in reservoir, or of rivers flooding their banks because of heavy rains, high winds, cyclones storm surge along coast, tsunami, melting snow or dam bursts.

• Flash Floods
  It is defined as floods which occurs within six hours of the beginning of heavy rainfall, and are usually associated with cloud bursts, storms and cyclones requiring rapid localized warning and immediate response if damage is to be mitigated. In case of flash floods, warning for timely evacuation may not always be possible.
• River Floods
  Such floods are caused by precipitation over large catchment’s areas. These floods normally build up slowly or seasonally and may continue for days or weeks as compared to flash floods.
• Coastal Floods
  Some floods are associated with the cyclonic activities like Hurricanes, tropical cyclone etc. Catastrophic flooding is often aggravated by wind-induced strom surges along the coast.

River System In West Bengal

The Ganga-Padma river artery divides the state in two parts, north and south. Being a part of Ganga-Bhrammhaputra-Meghna basin, North Bengal is extremely flood prone. The rivers Teesta, Torsa, Jaldhaka, Raidak-I, Raidak-II and their numerous tributaries belonging to the Brammhaputra basin and flowing through a part of Darjeeling, Jalpaiguri and Coochbehar originate in the Himalayas of Sikkim and Bhutan and flows south-east to Bangladesh. A part of Darjeeling and the districts of Uttar Dinajpur, Dakshin Dinajpur and Malda are drained through the rivers of Mahananda, Dauk, Tangon, Nagar, Atreyee, Punarbhaba and their tributaries. These are part of Ganga basin. Except the Mahananda, all other rivers originate in the plain of West Bengal and Bangladesh and join the Ganga-Padma at downstream of Farakka in Bangladesh. The Mahananda emerges from the Nepal Himalayas. Malda through which the river Ganga Flows receives its flood water from about eleven states and batters by the run-off flow generated from these vast areas. In central and southern part of this state, there are certain distinctive features of drainage condition which gives rise to flood situation. Basin-wise there are a number of rivers on the right bank of the Bhagirathi-Hoogly. These are Pagla-Bansloi, the Dwarka-Bramhani, the Mayurakshi-Babla-Uttarasan, the Bakreswar-Kuye and also the Ajoy. They emerge from the Jharkhand Plateau an flow southeast to meet Bhagirathi-Hoogly. These rivers drain an area of 17,684 km. spread over the State Jharkhand and Birbhum, western part of Murshidabad and Burdwan. Originating from Ganga-Padma, the Bhairab-Jalangi-Sealmari system of rivers drain the eastern part of Murshidabad and meet the Bhagirathi at Swarupnagar in Nadia. Nadia is drained partly by Jalangi and partly by the Churni which is a part of Mathabhanga-Churni-Ichamati system, taking off from Ganga-Padma flowing southwest, to meet Bhagirathi on the east bank at Ranaghat. The other part viz. the Ichamati flows east through Bangladesh enters the district of Uttar 24 Paraganas, flows in the south direction to fall into the tidal creek of the Raimangal. Part of Howrah and Uttar and Dakshin 24 Paraganas are drained mainly by the Hoogly and its tidal creeks and other internal drainage canals. Burdwan, Howrah and Hoogly districts are mainly drained by Damodar and Bhagirathi-Hoogly Rivers. In the Damodar-Barakar basin system, the rivers originate at Chotanagpur plateau and flow down the plains of West Bengal to meet Bhagirathi. The Ajoy forms the border between Birbhum and Burdwan. Purulia and Bankura are drained by the rivers of Kangsabati, Kumari, Shilabati, Keleghai, Dwarakeswar and their tributaries. The Keleghai also drains Paschim Medinipur and a part of Purba Medinipur. These rivers originate from the western highland of the state and flow in southeast direction to form the tidal rivers of the Rupnarayan and the Haldi to meet the Hoogly on the west bank. The Rupnarayan forms the boundary between Hoogly and Purba Medinipur. A part of Purba Medinipur is drained by the river Subarnarekha originating from the Jharkhand Plateau and flowing in southwest direction to meet the Bay of Bengal in Orissa.

Causes of Flood

1. Excessive rainfall in river catchments or concentration of runoff from the tributaries and river carrying flows in excess of their capacities.
2. Backing of water in tributaries at their confluence with the main river.
3. Synchronization of flood peaks of the main rivers and tributaries.
4. Landslides causing obstruction to flow and change in the river course.
5. Poor natural drainage.
6. Cyclone and very intense rainfall.
7. Intense rainfall when river is flowing full.

The floods of West Bengal have special characteristics. Heavy rainfall at origin or catchment’s areas of main flooding rivers of this state cause flood, but these areas are mainly lying outside this state. The West Bengal is flooded by water from adjoining states or countries.

Major contributing factors to flood in Northern regions are the run-off because of heavy local rainfall, discharge of upper basin areas and also out fall condition in the neighbouring countries. The Mahananda and most of the rivers of Uttar and Dakshin Dinajpur districts get stagnated when the Ganga upstream and downstream of Farakka Barrage rules high there by not allowing draining of flood discharge during that period. Flooding Malda is caused by the rivers Fulhar-Mahananda-and Ganga. The Ganga, forming the southern boundary of the district, brings flood water from eleven states and Nepal. The Fulhar meets the Ganga upstream of Farakka.

The rivers of rivers of Bhagirathi-Hoogly basin generate flood because of high rainfall and limited carrying capacity of the river Bhagirathi from Jangipur in Murshidabad to Kalna in Bardwan. In this reach the Bhagirathi has discharge carrying capacity of maximum 1.3 lac. Cusec. But all these rivers if receive rainfall simultaneously in their catchment areas can generate run-off volume of any amount between 4-6 lac. Cusec. In this vast tract of land there is only the Massanjore dam to interfere with the natural flow of flood water.

The basic reason of flood in this zone is the shape of the catchment area, its steep slope from a high level plateau area sloping sharply down to aflat terrain near the outfall and also adverse outfall condition because of its limited intake capacity. This feature is again adversely affected by the tidal condition as is generally noticed in the month of September, when the Hoogly is in high tide condition. Delay in drainage causes more accumulation resulting in spread of flood in the upstream of the river system in the west and beyond Berhampur. Generally part of Murshidabad and Nadia suffer from flood because of three reasons –

1. High intensity rainfall in the basin area of Bhairabi-Jalangi-Sealmari itself,
2. Inflow of flood water from Ganga-Padma at its high spate and
3. Drainage congestion at its outfall because of the high stage of Bhagirathi.

Traditionally, Damodar basin was known to be a curse. The basin of river Damodar has a very special shape and this influences its flood pattern. The river has about 70% of its basin just upstream of Durgapur town. This upper catchments of Jharkhand plateau, above Durgapur, generates heavy run-off during high rainfall and is carried to Durgapur in a short time. From here, this discharge travels through the river, bifurcating at Beguahana. One branch, the lower Damodar with very small capacity, reaches the Hoogly on the west bank. The major discharge passes through the Mundeswari to meet the Rupnarayan. Any major discharge along the downstream of Durgapur Barrage may cause flood depending upon the outfall condition of the Mundeswari at Harinkhola. In Kangsabati river system, the Kangsabati Dam has a limited flood storage capacity which is very nominal. Any major spillway discharge from Kangsabati Dam may cause flood at lower areas downstream of Medinipur town depending on tide and downstream rainfall.

Occurrence of Flood

Blue Areas depict Flood Affected Area

Flood	North Bengal	South Bengal
Districts Affected by Flood	Cooch Behar, Jalpaiguri, Uttar Dinajpur, Dakshin Dinajpur, Malda;	Nadia, Howrah, Murshidabad, North 24 Parganas, South 24 Parganas, Hooghly, Burdwan, Birbhum, Paschim Medinipur, Purba Medinipur
Relatively scare Districts affected by Flood	Darjeeling	Puruliya & Bankura

Flood Among natural hazards, occurrence of flood ranks first in West Bengal which has become annual festival in the State. Almost all the districts are affected by flood from July to October. But flood is relatively scarce in Darjeeling in North Bengal and Bankura & Purulia in South Bengal. The detail is depicted in the following matrix. According to the Irrigation Department, 37.6 lakhs Ha of West Bengal (42.4% of the total geographical area and 69% of its net cropped area) has been identified as flood prone area; of this 29.8 lakh Ha ( i.e., 58% of the flood prone area) is Protected Area. Strong monsoon, rivers and floods are an integral part of Bengal’s characteristic ecology that shaped its civilization and culture and at the same time, cause of flood hazard and disasters for the society as a whole.

Flood Among natural hazards, occurrence of flood ranks first in West Bengal which has become annual festival in the State. Almost all the districts are affected by flood from July to October. But flood is relatively scarce in Darjeeling in North Bengal and Bankura & Purulia in South Bengal. The detail is depicted in the following matrix. According to the Irrigation Department, 37.6 lakhs Ha of West Bengal (42.4% of the total geographical area and 69% of its net cropped area) has been identified as flood prone area; of this 29.8 lakh Ha ( i.e., 58% of the flood prone area) is Protected Area. Strong monsoon, rivers and floods are an integral part of Bengal’s characteristic ecology that shaped its civilization and culture and at the same time, cause of flood hazard and disasters for the society as a whole.

Followings are the records of large FLOODS in West Bengal

Elements At Risk

Anything in the flood plains will get inundated. building built of earth, weak foundations, and soluble materials will collapse endangering humans and their property. Basements of buildings are under risk. Utilities such as sewerage, water supply, communication lines, and power are put at risk. Food stock in the godowns, agricultural fields, livestock, vehicles, machinery and equipments mounted on the ground, fishing boats are also put at risk.