Tropical Cyclones: Characteristics and Destructive Power of Tropical Cyclones !
Tropical cyclones are characterised by destructive winds, storm surges and exceptional levels of rainfall, which may cause flooding. Wind speeds up-to 200 km/h, rainfall of up-to 50 cm/day for several consecutive days and storm surges of 7 m high are not uncommon.
A mature cyclone releases energy equivalent to that of 100 hydrogen bombs. Cyclone is a heat engine whose heater is the oceanic water. The released heat after condensation converts into kinetic energy for the cyclone. Yet it is described as a poor heat engine since it converts only 3% of the heat generated into kinetic energy.
The strong winds that blow counter-clockwise in the Northern Hemisphere, while spiralling inward and increasing toward the cyclone centre are highly destructive. Wind speeds progressively increase towards the core. As the eye arrives, winds decrease to become almost calm but rise again just as quickly as the eye passes and are replaced by hurricane force winds from a direction nearly the reverse of those previously blowing.
Wind speeds greater than 120 kmph are characteristic of severe cyclonic storm with a core of hurricane winds. All physical structures are vulnerable to the extreme pressures exerted by winds and thus collapse or are damaged.
Wind speeds of cyclonic storms have been incorporated into the building code for coastal area. However, the postdamage surveys done by the scientists from the Structural Engineering Research Centre at Chennai and IIT Chennai have revealed that structures are subjected to pressure which far exceed the basic pressures computed in the building code.
A micro-wave tower designed to withstand a wind velocity of 250 kph was razed to the ground in the Kavali cyclone of 1989 and another one in the cyclone of 1990 and lately at Ravulaperam in November 1996. The typical damages to buildings are due to failure of roofing systems, windows, doors, uprooting of trees, blowing away of the thatched huts, etc.
Damages due to wind are not confined to the coastal areas only. Damage can occur well in the interior. For example, a number of structures around Vijayawada (which is about 100 km from the coast) were damaged in a cyclone.
The wind hazard map in Figure 8.21 shows that almost the whole east coast of India from Tamil Nadu to West Bengal is vulnerable to wind hazard. The coasts of Kachchh and Kathiawar in Gujarat are also highly vulnerable to wind hazard. Such wind hazards are mostly associated with tropical cyclones. High wind velocity of 50 m/s is not uncommon in these coastal regions.
One of the peculiar characteristics and having a very high damage potential is storm surge. Storm surges are an abnormal rise of seawater due to tropical cyclone and is greatly amplified where the coastal water is shallow, in the estuarian region and where the shape of the coast is like a funnel.
The major factors include:
i. A fall in the atmospheric pressure over the sea surface
ii. The effect of the wind
iii. The influence of the sea bed
iv. A funnelling effect
v. The angle and speed at which the storm approaches the cost
vi. The tides
The coastal areas of North Bay of Bengal satisfy most of the above mentioned criteria and the storm surge gets enormously amplified there. Due to several favourable factors in these areas, the world’s highest storm surge of 41 feet (over 13 metre) was reported from the area in 1876 near Bakerganj. Cyclonic storms are sometimes accompanied by tidal waves with heights of five metres and sometimes hit 20 km inland with wind speed of 150 kmph.
The low-pressure area or “eye” of the cyclone allows the sea level to rise. The high-speed winds surrounding the “eye” drive more water over this rise. The sloping bed of the sea and contours off the shoreline add further to the height. A further contribution to the height of the storm surge is added if the cyclone arrives at high-tide time.
Storm surges are not waves though they may look like them. A storm surge is a mass of water, which will submerge everything in its path, till it recedes back into the sea. It moves at the same speed as that of the cyclone. It travels upto the point where the ground height is equal to the height of the surge.
The period of submergence of the land may be as long as 45 minutes or more depending on the depth it reaches inland. Map in Figure 8.22 shows that the entire coast of India is influenced by storm surges caused by cyclones and tides.
As the leading edge of the surge crashes against the coastline and the water continues to travel inland there will be surface waves created which criss-cross each other and carry much under water turbulence. The destruction caused by the surge is tremendous. Houses are the worst affected.
First the speed of the surge places great stress on the walls. The turbulence and currents created destroy the foundation of the structure. The debris like uprooted trees, fences and parts of broken houses act as battering rams, which cause further damage.
The sand and gravel carried by the fast moving currents at the bottom of the surge can cause sand papering action of the foundations. The huge volume of water can cause such pressure differences that the house “floats” and once the house is lifted from the foundations water enters the structure and causes collapse of the building.
Damages occur to every kind of assets built above the ground level due to the above characteristics and crops get affected very badly.
The world’s highest rainfall spread over one or two days has occurred during tropical cyclones. The very high specific humidity condenses into exceptionally large raindrops and giant cumulus clouds, resulting in high precipitation rates. When a cyclone makes landfall, the rain rapidly saturates the catchment areas and the rapid runoff may extensively flood the usual water sources or create new ones.
In a mature cyclone, rainfall over land commences even when the cyclone is 400-500 km away from the coastline. In the November 1996 Andhra Pradesh cyclone, which was a small core storm about 60 to 70 km in diameter, the rain commenced in Kakinada when it was about 80 to 100 km away from the coast gradually escalating in intensity along with rise in wind speed.
Rainfall is generally very heavy and spread over a large area thus leading to excessive amount of water, which leads to flooding. The size of drops, in a rainfall, increases with increase in rainfall intensity. Soil erosion results on a large scale as raindrops strike the ground with energies substantially greater than those in ordinary rainfall. The heavy rains waterlog the ground and cause softening of the earth due to soaking. This contributes to weakening of tank embankments, the leaning over of utility poles or collapse of pole type structures.