What are the various effects of cyclone?

Severe tropical cyclones are responsible for large number of causalities and considerable damage to property and agricultural crop. The destruction is confined to the coastal districts and the maximum destruction being within 100 km from the centre of the cyclone and on the right side of the storm track.

Principal dangers from cyclones are: (i) very strong winds, (ii) torrential rain, and (iii) high storm tides.

Most casualties are caused by coastal inundation by storm surge. Maximum penetration of storm surges varies from 10 to 20 km inland from the coast. Heavy rainfall and floods come next in order of devastation. They are often responsible for much loss of life and damage to property. Death and destruction directly due to winds are relatively less. The collapse of buildings, falling trees, flying debris, electrocution, aircraft and ship accidents and disease from contaminated food and water in the post-cyclone period also contribute to loss of life and destruction of property.

Floods generated by cyclone rainfall are more destructive than winds. Rainfall of the order of 20 to 30 cm per day is common.

As mentioned, the worst danger emanates from the storm surge. In the storm centre, the ocean surface is drawn upward by 30 centimeter or so above normal due to the reduced atmospheric pressure in the centre. As the storm crosses the continental shelf and moves coast ward, the mean water level increases.

This abnormal rise in sea level caused by cyclone is known as storm surge. The surge is generated due to interaction of air, sea and land. The cyclone provides the driving force in the form of very high horizontal atmospheric pressure gradient and very strong surface winds. As a result, the sea level rises and continues to rise as cyclone moves over increasingly shallower water as it approaches coast, and reaches a maximum on the coast near the point of landfall (Point of crossing coast). Surge is maximum in the right forward sector of the cyclone and about 50-100 Km from the centre coinciding with the zone of maximum wind. Winds in this sector are from ocean to land.

Due to improvement in cyclone warning system and adequate and timely steps taken by the government and other agencies, it appears to be some stabilization (in spite of large population growth especially in coastal areas) on the loss of human lives, although loss of properties shows an appreciable increasing trend. The increase in the loss of properties is due to increased but unplanned human activities.

Engineered and non-engineered constructions along the coast also contribute to the damage suffered by property. In support of the above statements, we present some data on recent cyclones in the table 7.2 below. It may be seen that although the May 1979 and May 1990 cyclones, which occurred in the same coastal area of Andhra Pradesh and had the peak wind speeds of the same order, yet the loss of human lives in the case of the 1990 cyclone was of the same order comparison to that of 1977 cyclone but the economic losses were many times more in the 1990 cyclone.