Tropical Cyclones and Local Storms
Tropical cyclone representing closed low pressure system generally having a diameter of about 650 kilometres, is one of the most powerful, destructive, dangerous and deadly atmospheric storms on the planet earth. Tropical cyclones are variously called in different parts of the globe as HURRICANES in the North Atlantic Ocean (mainly in the Caribbean Sea and south eastern U.S.A.), TYPHOONS in the North Pacific Ocean (mainly in China Sea, eastern and southern coasts of China, Japan, Philippines and southeast Asia), CYCLONES in Bangladesh and eastern coastal areas of India and ‘WILLY WILLY’ in Australia.
Tropical cyclones become more disastrous natural hazards because of their high wind speed of 180 to 400 kilometres per hour, high tidal surges, high rainfall intensity (highest recorded rainfall value exceeded 2000 mm per day in Philippines) very low atmospheric pressures causing unusual rise in sea level, and their, persistence for several days or say about one week. The total cumulative effects of high velocities of wind, torrential rainfall and transgression of sea water on to the coastal land become so enormous that the cyclones cause havoc in the affected areas and thus tremendous loss of human lives and property is the ultimate result of such atmospheric deluge. The storm surge or tidal surge refers to unusual rise in sea level caused by very low atmospheric pressure and the stress of the strong gusty winds on the sea surface. These storm surges or tidal surges, when coincide with high tide, are further intensified and after intruding into the coastal land cause widespread inundation of coastal areas and great damage of human lives and property. The following case histories of a few most powerful and disastrous tropical cyclones may unravel the magnitude of destruction wrought by these natural atmospheric disturbances.
1. CYCLONIC hazards very often visit the eastern coastal areas of India and the southern coastal areas of Bangladesh. The disaster of the deadliest storm in the recorded history occurred on November 12,. 1970 in the coastal lowland of Bangladesh. This Bay of Bengal disastrous cyclone tells the magnitude of environmental hazards in respect of its killer impact on the affected people as it caused as many as 3,00,000 deaths (some sources put the figure between 300,00 and 1,000,000 deaths in Bangladesh and West Bengal of India) wherein most of the death were caused by drawing in the storm surge of oceanic water (20 feet) on the land. The official record of Bangladesh presented the total loss as death of people-200,000 missing persons-50,000 to 100,000 rendered homeless. The standing kharif crops over 1.75 million hectares were destroyed. The loss of property mounted to about 10,000 crore rupees (1000 billion rupees). The severe super cyclonic storm resulted the disruption of the supply of water and electricity. The communication system was thrown out of gear. Destruction and obstruction of roads and rails brought a grinding halt to rail and road transport which continued for weeks. Thousands of families suffered from mental agony due to separation of their kith and kins. Paradeep Port was so greatly damaged that it became unoperational for weeks. Most of the trees were uprooted. Surface and ground water was so greatly polluted due to dead bodies (both human and animal) that it became unsuitable for domestic uses and birth to the outbreak of epidemic. Though the cyclone vanished by October 31 but it left behind ugly scene of destruction and tragedy of epidemic hunger and pollution which broke the backbone of already poor inhabitants.
The northern part of the Bay of Bengal mostly the Ganga Delta Plains of West Bengal. India and Bangladesh very often suffer from frequent severe cyclonic storms and resultant storm surges (tidal waves) because of a combination of several natural conditions and phenomena such as large astronomical tides. Funneling coast configuration (spread of land from three side in arcade shape, forming the head of Bay of Bengal), low and flat terrains of coastal area and frequent occurrence of severe cyclonic storms. About 12 to 13 tropical cyclones with wind speed below 63 kilometres per hour occur every year in the northern Bay of Bengal. About 5 of these tropical cyclones become strong and severe with average wind speed of more than 63 kilometres per hour. The recurrence interval of disastrous cyclone is every five years.
2. HURRICANES very often strike the southern and the south-eastern coasts of the USA. Gulf coasts of Louisiana, Taxas, Alabama and Fonda are worst affected areas. The Galveston, Taxas (USA) disaster of September 8, 1900 tells the story of devastation caused by hurricanes in the Gulf coastal regions of the U.S.A. The terrible hurricane generated a strong storm surge (tidal wave) which raced inland and killed 6000 people mostly through drowning caused by inhumation under 10 to 15 feet (3 to 4.5m) deep water and destroyed 3000 houses. Flying planks and timbers under the force of strong gale winds also caused several deaths and damage to human structures.
It may be pointed out that Mississippi Delta Plans of the State of Louisiana (U.SA.) have the equivalence of Ganga Delta Plains of India and Bangladesh as regards the frequency and intensity of tropical cyclones but the damages mainly in the form of human causalities are far less in the former than in the latter because of more advanced and better warning systems. The Audrey Hurricane of June. 1957 struck the Louisiana Coast between New Orleans and Galveston. Though the storm was very severe as it smashed houses and floated them away, uprooted sealed concrete tombs and floated them 32 kilometres away from their resting places, but only 550 human death could be caused because of better warning systems and spontaneous response of people to the warning and predictions. In fact the water level used to rise at the rate of 1.5 feet per hour. Thus most of the people had ample time to evacuate them to safer places before the water level forced by strong storm surge could reach its peak of 8 to 12 feet (2.4 to 3.6m) above high tide water.
3. TRACY CYCLONE struck Darwin city of Australia (a coastal city of the Northern Territory) on the Christmas Day of 1974 and caused deaths of 49 persons and left 16 people missing. Thought the Tracy storm was small as the diameter of the area of gale force winds (above 63 kilometres per hour) was only about 100 kilometres but it was certainly an intense Storm as its destructive intensity was concerned. The destructiveness of the storm surge caused by this cyclone was minimized because of the fact that it occurred during a period of neap tides inspite of reduction in the intensity the Tracy cyclone damaged 80 per cent of the buildings of Darwin city having a population of 45000 people at that time (1974), caused total distuption of power supply thus plunging the entire city into darkness, put all the services of water and power supply, sanitation, transport and communication out of gear. There is little doubt that the relatively very small loss of life in Darwin (city) can be attributed to the fact that the cyclone was accompanied by a restricted storm surge effect; major damage was produced by high winds and torrential rain. Nevertheless, the problems encountered in Darwin, serve to highlight the magnitude of the tropical cyclone danger in much more densely populated areas of developing rather than developed countries (i.E. Hobbs, 1980).
4. LOCAL STORMS The severe local storms of hazardous nature include TORNADOES (which affect the southern and the eastern USA) and thunderstorms. Tornadoes, though smallest in area of all the hazardous atmospheric storm, are very deadly to human lives and property. On an average the annual toll caused by tomatoes in the U.S.A. includes damage to property worth, 100,000,000 dollars and 150 human deaths. The deadliest part of the tornandoes is the TORNADO MISSLILES (consisting of uprooted trees, their branches, roofs of buildings etc. which are carried away by dynamic force of winds) which inflict great damage to buildings, other human structures and human lives. A tornado for example at Lubbock (Texas. U.S.A.) in 1970 moved a long cylindrical fertilizer tank (3.35 m x 12.5 mm size with a weight of 11 tons) for a distance of 1.21 km. from its original place.
Two great seasons of tornado outbreak in the U.S.A. tell the awful story of ravages caused by deadly tornadoes. The occurrence of tornadoes in groups hazardous but when very hot and dry days prevail for a few weeks in continuation, environmental hazard in the form of heat waves occurs which affect the environment and human lives, plants and animals. The persistence of exceptionally cold conditions for several days causes severe snowfall. A dry season a year may not be as much disastrous as continued dry seasons for several years. The perception and concept of drought vary from place to place and from one group of people to other group based on profession and occupation. In fact, drought occurs when there is appreciable decrease in rainfall from the average normal rainfall. Floods are still very severe environmental hazards which are related to atmospheric processes. Mississippi flood plains (U.S.A.) and the Ganga Plains (India) are frequented by severe flood hazards. It may be pointed out that floods are not always hazards rather these are also boom because these bring rich fertile alluvial soils each time and thus increase agricultural productivity.