UPSC Geography Disasters NCERT Extracts - Natural Hazards and Disasters

Tsunami

• Earthquakes and volcanic eruptions that cause the sea-floor to move abruptly resulting in sudden displacement of ocean water in the form of high vertical waves are called tsunamis (harbour waves) or seismic sea waves.
• Normally, the seismic waves cause only one instantaneous vertical wave; but, after the initial disturbance, a series of after waves are created in the water that oscillate between high crest and low trough in order to restore the water level.
• The speed of wave in the ocean depends upon the depth of water. It is more in the shallow water than in the ocean deep.
• As a result of this, the impact of tsunami is less over the ocean and more near the coast where they cause large-scale devastations.
• Therefore, a ship at sea is not much affected by tsunami and it is difficult to detect a tsunami in the deeper parts of sea.
• It is so because over deep water the tsunami has very long wave-length and limited wave-height.
• Thus, a tsunami wave raises the ship only a metre or two and each rise and fall takes several minutes.
• As opposed to this, when a tsunami enters shallow water, its wave-length gets reduced and the period remains unchanged, which increases the wave height.
• Sometimes, this height can be up to 15 m or more, which causes large-scale destructions along the shores.
• Thus, these are also called Shallow Water Waves. Tsunamis are frequently observed along the Pacific ring of fire, particularly along the coast of Alaska, Japan, Philippines, and other islands of Southeast Asia, Indonesia, Malaysia, Myanmar, Sri Lanka, and India etc.

Tropical Cyclone

• Tropical cyclones are intense low-pressure areas confined to the area lying between 30°N and 30°S latitudes, in the atmosphere around which high velocity winds blow.
• Horizontally, it extends up to 500-1,000 km and vertically from surface to 12-14 km.
• A tropical cyclone or hurricane is like a heat engine that is energised by the release of latent heat on account of the condensation of moisture that the wind gathers after moving over the oceans and seas.
• There are differences of opinion among scientists about the exact mechanism of a tropical cyclone. However, some initial conditions for the emergence of a tropical cyclone are:                                                                              
• Large and continuous supply of warm and moist air that can release enormous latent heat.
• Strong Coriolis force that can prevent filling of low pressure at the centre (absencence of Coriolis force near the equator prohibits the formation of tropical cyclone between 0°-5° latitude).           
• Unstable condition through the troposphere that creates local disturbances around which a cyclone develops.
• Finally, absence of strong vertical wind wedge, which disturbs the vertical transport of latent heat

Structure of Tropical Cyclone

• Tropical cyclones are characterised by large pressure gradients.
• The centre of the cyclone is mostly a warm and low-pressure, cloudless core known as eye of the storm.
• Generally, me isobars are closely placed to each other showing high-pressure gradients.

Spatio-temporal Distribution of Tropical Cyclone in India

• Owing to its Peninsular shape surrounded by the Bay of Bengal in me east and the Arabian Sea in the west, the tropical cyclones in India also originate in these two important locations.
• Though most of the cyclones originate between 10°-15° north latitudes during the monsoon season, yet in case of the Bay of Bengal, cyclones mostly develop during the months of October and November.
• Here, they originate between 16°-2°N latitudes and to the west of 92°E. By July me place of origin of these storms shifts to around 18°N latitude and west of 90°E near the Sunderban Delta.
• The energy to the tropical cyclone comes from the latent heat released by me warm moist air.
• Hence, with the increase in distance from the sea, the force of the cyclone decreases.
• In India, the force of the cyclone decreases with increase in distance from the Bay of Bengal and the Arabian Sea.
• So, the coastal areas are often struck by severe cyclonic storms with an average velocity of 180 km/h.
• Often, this results in abnormal rise in the sea level known as Storm Surge.
• A surge is generated due to interaction of air, sea and land.
• The cyclone provides the driving force in the form of very high horizontal pressure- gradient and very strong surface winds.
• The sea water flows across the coast along with strong winds and heavy downpour.
• Rashtriya Baih Ayog (National Flood Commission) identified 40 million hectares of land as flood-prone in India.
• Assam, West Bengal and Bihar are among the high flood-prone states of India.
• However, floods also make a few positive contributions.
• Every year, floods deposit fertile silt over agricultural fields which is good for the crops.
• Majuli (Assam), the largest riverine island in the world, is the best example of good paddy crops after the annual floods in Brahmaputra. But these are insignificant benefits in comparison to the grave losses.

Disaster Management Bill, 2005

• The Disaster Management Bill, 2005, defines disaster as a catastrophe, mishap, calamity or grave occurrence affecting any area, arising from natural or man-made causes, or by accident or negligence which results in substantial loss of life or human suffering or damage to, and destruction of, environment, and is of such nature or magnitude as to be beyond the coping capacity of the community of the affected area.

Some Important Facts

• Establishment of National Institute of Disaster Management, India, Earth Summit at Rio de Janeiro, Brazil, 1993 and the World Conference on Disaster Management in May, 1994 at Yokohama, Japan.
• Hence, this issue was raised at the U.N. General Assembly in 1989 and it was finally formalised at the World Conference on Disaster Management in May, 1994 at Yokohama, Japan.
• This .was subsequently called the Yokohama Strategy and Plan of Action for a Safer World.
• All the member states of the United Nations and other states met at the World Conference on Natural Disaster Reduction in the city of Yokohama from 23-27 May, 1994.
• It acknowledged that the impact of natural disasters in terms of human and economic losses has risen in recent years, and society, in general, has become vulnerable to natural disasters.
• It also accepted that these disasters affected the poor and disadvantageous groups the worst, particularly in the developing countries, which are ill-equipped to cope with them.
• Hence, the conference adopted the Yokohama strategy as a guide to rest of the decade and beyond, to mitigate the losses due to these disasters.
• The resolution of the World Conference on Natural Disasters Reduction is as mentioned below:
• It will note that each country has the sovereign responsibility to protect its citizens from natural disasters;
• It will give priority attention to me developing countries, particularly the least developed, land-locked countries and small-island developing states;
• It will develop and strengthen national capacities and capabilities and, where appropriate, national legislation for natural and other disaster prevention, mitigation and preparedness, including the mobilisation of non-governmental organisations and participation of local communities;
• It will promote and strengthen sub-regional, regional and international cooperation in activities to prevent, reduce and mitigate natural and other disasters, with particular emphasis on:
• human and institutional capacity-building and strengthening;
• technology sharing: the collection, the dissemination and utilisation of information; and mobilisation of resource
• It also declared the decade 1990-2000 as the International Decade for Natural Disaster Reduction (IDNDR).