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It is an intense vortex or a whirl in the atmosphere, characterized by high winds rotating about a calm center of low atmospheric pressure in anti-clockwise direction in the Northern Hemisphere and in clockwise direction in the southern Hemisphere. This center moves onwards and pressure increases outwards. The force of the pressure in the center and the rate at which it increases outwards gives the intensity of the cyclone and the strength of winds.

Cyclones form in certain favourable atmospheric and oceanic conditions. There are marked seasonal variations in their places of origin, tracks and attainment of intensities. These behaviours help in predicting their movements. Pre and Post mosoon storms are more violent than the storms of the monsoon season.

The coastal stretch of West Bengal is necessarily highly vulnerable to cyclone. The phenomenal storm surge in coastal West Bengal is due to its peculiar bathymetry and nature of coastal belt. The northern part of the Bay of Bengal is very shallow. The coast is also landlocked on three sides. As a result when a very severe cyclonic storm or a hurricane approaches the coast, the enormous storm surge generated by the wind pressure submerges the coastal belt at the time of the storm crossing the belt. The frequency of storms crossing this belt is also high. Another peculiar characteristic of this coast is that it is crisscrossed by innumerable rivers and rivulets, with the elevation of the islands about 4 to 5 meters. The seadykes and embankment are not strong enough to resist strong wind-driven waves and naturally cave in during depression / cyclonic storm situation.

Onset Type:
They strike suddenly although it takes time to build up. They can be tracked on the development but accurate landfall is predictable barely within few hours. The onset is extensive and often very destructive.

Cyclone Accompanied by Sea Wave:
A heavy cyclone accompanied by a sea wave, is the worst kind of disaster which may occur in this delta. Disasters of this kind have caused appalling mortality in the past and will possibly do so again. Practically, nothing can be done to avoid them but fortunately they are not frequent.
They are most likely to occur at the beginning or at the end of rains, i.e., either before the winter paddy is planted or at a time when it is almost ripe. Under such circumstances the damage to crops may be small in comparison with the mortality among men and cattle which may be enormous. The maritime districts of West Bengal are liable to storm waves but the districts of 24 Parganas has suffered most severely.

Elements at Risk:
All lightweight structures and those built of mud, wood, older buildings with weak walls and structures without proper anchorage to the foundations will be at great risk. Settlements located in low lying coastal areas will be vulnerable to the direct effects of the cyclones such as wind, rain and storm surge. Settlements in adjacent areas will be vulnerable to floods, mud-slides or landslides due to heavy rains. Other elements at risk are fences, telephone and electricity poles, cables, light elements of structures – roofs, signboards, hoardings, coconut crowns, fishing boats and large trees.

Occurrence of Cyclone:
West Bengal has two Cyclone seasons – pre-monsoon and post-monsoon cyclone during April-May and Nov-Dec, respectively. Pre-monsoon cyclone, which causes wide spread hailstorm and it is traditionally called n the state as Kalbaishaki. Paschim Medinipur, Purba Medinipur, South 24 Parganas, North 24 Parganas, Howrah, Hooghly, part of Nadia, Burdwan and Bankura are located in very high damage risk zone (V=50m/s) with respect to Cyclone; whereas a major part of Nadia, Burdwan, Bankura, Murshidabad, Malda, Uttar and Dakshin Dinajpur, Jalpaiguri, Cooch Behar, Darjeeling, part of Purulia encounter with high damage risk zone (V=47m/s). Major portion of Purulia falls under moderate damage risk zone (V=29m/s) with respect to cyclone.

Typical Effects:
First, in a sudden, brief onslaught, high winds cause major damage to infrastructure and housing, in particular fragile constructions. They are generally followed by heavy rains and floods, and in flat coastal areas by storm surge riding on tidal waves and inundating the land over long distances of even upto 15km inland.

Physical damage -Structures will be damaged or destroyed by the wind force, flooding and storm surge. Light pitched roofs of most structures especially the ones fitted on to industrial buildings will suffer severe damage.

Casualties and Public Health – Caused by flooding and flying elements, contamination of water supplies may lead to viral outbreaks, diarrhea and malaria.

Water supplies – Ground and piped water supply may get contaminated by flood waters.

Crops and Food Supplies – High winds and rains will ruin the standing crop and food stock lying in low lying areas. Plantation type crops such as banana and coconut are extremely vulnerable. Salt from the sea water maybe deposited on the agricultural land and increase ht salinity. The loss of the crop may lead to acute food shortage.

Communication –Severe disruption in the communication links as the wind may bring down the electricity and communication towers, electricity and telephone poles, telephone lines, antennas and satellite disk and broadcasting services. Transport lines (road and rail) may be curtailed. Relief materials may not reach the affected site.

Following Table provides the information for damaging cyclones in West Bengal from 1900 A.D.

Special Nature of the Problem
Though the frequency of Tropical Cyclones in the North Indian Ocean (NIO) are the least in the world (7% of the global total), their impact on the coasts bordering the North Bay of Bengal (North of 150 N latitude) in West Bengal are extremely disastrous. The problem can be fathomed from the fact that during the past two and a half centuries, 20 out of 23 major cyclone disasters (with human loss of life 10,000 or more and not considering the damages) in the world have occurred over the Indian Subcontinent (India and Bangladesh). One of the major reasons for this is the serious storm tide problem in these coasts. A tropical cyclone of specific intensity when it strikes the east coast of India and Bangladesh, usually produces a higher storm surge compared to that when such a cyclone strikes elsewhere in the world. This is because of the special nature of the coastline, the shallow coastal ocean topography and the characteristics of tide in the North Bay of Bengal region.
International Classifications of Storms

Low Pressure Systems Wind Speed in Kms. Per Hour
Depression 36 – 55
Deep Depression 56 – 66
Cyclonic Storm 67 – 96
Severe Cyclonic Storm 96 – 117
Severe Cyclonic Storm with core of Hurricane > 117

Vulnerability to Cyclones:
Cyclones are natural events, which cannot be prevented. What actually makes these hazards turn in to disasters is the vulnerability of the people and their means of livelihood and the fragility of infrastructure. The Indian Sub-continent is the worst affected part in the world as far as loss of lives is concerned though more severe cyclones do occur in other parts of the world and financial losses are much more elsewhere. This could primarily be attributed to the special nature of the problem discussed above and the vulnerability of the people. High population density, comparatively better employment opportunities and economic compulsions force people to occupy, areas, which are susceptible to cyclones, saline ingress and flooding. Traditional coping mechanisms have been the mainstay for these people to counter hazards, but during major disasters these coping mechanisms are found wanting. Though communities have a natural tendency to face hazards by joining hands, they usually fail to generate the desired synergy because of unsystematic and ad hoc approaches. On many occasions people are not even aware of the risks involved. The frequent disasters nullify the development of several years and turn the clock back for these vulnerable families.
Number of Cyclones Crossing West Bengal Coast during 1891- 2000

Station Coastal Districts No. of Cyclonic storms
24 - Parganas (North & South) 35
Midnapur (East & West) 34

Historical records of 3 most devastating cyclones, which formed in the Bay of Bengal and made landfall on the coast of West Bengal

1. 7-12 October,1737 Super Cyclone* Crossed West Bengal coast over Sunderbans
Surge height: 12 m
Loss of life: 300,000
2. 2-5 October, 1864 Very Severe Cyclonic Storm Cross the coast near Contai, West Bengal Surge Height: The wave in many places rose to 9 m The Maximum height of the waves reached 12 m. At Sagar Island it was 5 m above land level. At Diamond Harbour, the wave was 3 m
Loss and Damage: People Killed=50,000 (mostly due to drowning), and 30,000 (due to diseases as a result of inundation)
3. 14-16 October, 1942 Severe Cyclonic Storm Crossed West Bengal coast near Contai Surge Height: 3-5 m
Loss and Damage: People Killed=19,000, Cattle heads killed=60,000
4. 23-26 May, 2009 Severe Cyclonic Storm Crossed close to the east of Sagar Island
Surge Height: 3-5 m
Loss and Damage: People Killed=137, Cattle heads Killed= 50,000


Impact on the Coastal Eco-System:
“Coastal ecosystem” includes estuaries and coastal waters and lands located at the lower end of drainage basins, where streams and river systems meet the sea and are mixed by tides. The coastal ecosystem includes saline, brackish (mixed saline and fresh) and fresh waters, as well as coastlines and the adjacent lands. All these water and landforms interact as integrated ecological units. Shore-lands, dunes, sandbars, offshore islands, headlands, and freshwater wetlands within estuarine drainages are included in the definition since these interrelated features are crucial to coastal fish and wildlife and their habitats. Mangroves are located all along estuarine areas, deltas, tidal creeks, mud flats, salt marshes and extend over 4871 sq. km (about 7% of world’s mangrove areas). Impact of global warming- induced sea level rise due to thermal expansion is more pronounced in the Bay of Bengal due to the shallowness of the waters. The entire coastal ecosystem in general and the eastern coast in particular are highly vulnerable due to flat and low terrain, high population density, over exploitation of natural resources, high rate of environmental degradation on account of pollution and non-sustainable development. On many occasions, the livelihood requirements of people are detrimental to maintaining the delicate balance of the fragile coastal ecosystem. Degradation of the eco-system not only affects the environment adversely, but also makes the people living in the coastal areas more vulnerable.
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