Cyclone and Fire

1. Cyclone and Fire Dr. Shweta Goyal Assistant Professor Thapar University

2. What are Cyclones? • "Cyclone" is an intense whirl in the atmosphere with very strong winds circulating around it in anti-clockwise direction in the Northern Hemisphere and in clockwise direction in the Southern Hemisphere. • Word "Cyclone" is derived from the Greek, word "Cyclos" meaning the coils of a snake. • To Henri Peddington, the tropical storms in the Bay of Bengal and in the Arabian Sea appeared like the coiled serpents of the sea and he named these storms as "Cyclones". • known as Hurricane in the Atlantic and Eastem Pacific, Typhoon in Western Pacific, Willy-Willies in Australian sea, Baguis in the Philippines.

3. Cyclones are intense low pressure areas - from the centre of which pressure increases outwards- The amount of the pressure drop in the centre and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds. 

4. Criteria followed to classify cyclones • As adopted by Meteorological Department of India 1 knot - 1.85 km per hour

5. Mechanism of cyclones

7. Mechanism of cyclones • A full-grown cyclone is a violent whirl in the atmosphere 150 to 1000 km across, 10 to 15 km high. • The central calm region of the storm is called the "Eye". The diameter of the eye varies between 30 and 50 km and is a region free of clouds and has light winds. • Around this calm and clear eye, there is the "Wall Cloud Region" of the storm about 5O km in extent, where the gale winds, thick clouds with torrential rain, thunder and lightning prevail. • Away from the "Wall Cloud Region", the wind speed gradually decreases. • The gales give rise to a confused sea with waves as high as 20 metres, swells that travel a thousand miles. Torrential rains, occasional thunder and lightning flashes - join these • Through these churned chaotic sea and atmosphere, the cyclone moves 300 to 500 km, in a day to hit or skirt along a coast, bringing with it strome surges

8. Mechanism of cyclones • Once the cyclones reach higher latitudes they often change their direction and move north and then north-east (south and south east hemisphere). The process is known as recurreature. • Before it recurves, the speed decreases and the system remains stationary for a day or so. • When two cyclones exist near to each other, they inter-act and move anti-clockwise with respect to each other. • In the Atlantic, tracks often execute a parabola. • In India, when cyclones recur they get broken up over the Himalayas and their further eastward movement ceases.

9. Naming of cyclone • Cyclones derive their names through a systematic procedure laid down by the World Meteorological Organisation (WMO) and the United Nations Economic and Social Commission for Asia and the Pacific (ESCAP). • naming of cyclones began in September 2004 • Eight north Indian Ocean countries - Bangladesh, India, the Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand - have prepared a list of 64 names. • Since 2004, the eight countries have faced 20 cyclones. The countries take turns in naming the cyclones. The last six were: Nargis (Pakistan), Rashmi (Sri Lanka), Khai-Muk (Thailand), Nisha (Bangladesh), Bijli (India) and Aila (Maldives).

10. Cyclone Map of India

11. The principal dangers of a cyclone • Gales and strong winds • damage installations, dwellings, communication systems, trees., etc. resulting in loss of life and property. • Torrential rain • may cause river floods • Storm surges or high tidal waves • A storm surge is an abnormal rise of sea level near the coast caused by a severe tropical cyclone • as a result, sea water inundates low lying areas of coastal regions drowning human beings and live- stock, eroding beaches and embankments, destroying vegetation and reducing soil fertility.

12. What is Storm Surge?

14. How high will the Storm Surge be? • Every cyclone that affects the coast produces a storm surge. But not all storm surges rise to dangerous levels. The height of the surge depends on: • The intensity of the cyclone - as the winds increase, the sea water is piled higher and the waves on top of the surge are taller. • The forward speed of the cyclone - the faster the cyclone crosses the coast, the more quickly the surge builds up and the more powerfully it strikes. • The angle at which the cyclone crosses the coast - local zones of enhanced surge in areas such as narrow inlets and bays. • The shape of the sea floor - the surge builds up more strongly if the slope of the sea bed at the coast is shallow. • Past history indicates that loss of life is significant when surge magnitude is 3 metres or more and catastrophic when 5 metres and above

15. Surge prone coasts of India • Vulnerability to storm surges is not uniform along Indian coasts. • east coast of India are most vulnerable to high surges • i) North Orissa, and West Bengal coasts. • ii) Andhra Pradesh coast between Ongole and Machilipatnam. • iii) Tamil Nadu coast, south of Nagapatnam. • The West coast of India is less vulnerable to storm surges • i) Maharashtra coast, north of Harnai and adjoining south Gujarat coast and the coastal belt around the Gulf   of  Bombay. • ii) The coastal belt around the Gulf of Kutch.

16. Cyclone Accounts • The oldest and the worst cyclone on record is that of October 1737: hit Calcutta and took a toll of 3,00,000 lives in the deltaic region. It was accompanied by a 12 metre high surge. • Midnapore Cyclone of October 1942 was accompanied by gale wind speed of 225 kmph • Rameswaram Cyclone of 17th to 24th December 1964 wiped out Dhanuskodi in Rameswaran Island from the map. A passenger train which left Rameswaram Road station near about the midnight of 22nd was washed off by the storm surges sometimes later, nearly all passengers traveling in the train meeting water graves.

17. Cyclone Accounts • Bangla Desh Cyclone of 8-13 November 1970 which crossed Bangla Desh coast in the night of 12th was one of the worst in recent times, with storm surges of 4 to 5 metres height at the time of high tides, and with 25 cm of rain in the areas, the inundation took toll of about 3,00,000 people. • Andhra Cyclone of 14-20 November 1977 that crossed coast near Nizampatnam in the evening of 19th, took a toll of about 10,000 lives.

18. How to avoid the catastrophe? • Effective Cyclone Disaster Prevention and Mitigation Plan requires: • A Cyclone Forecast - and Warning Service. • Rapid dissemination of warnings to the Government Agencies, Marine interests like the Ports, Fisheries and Shipping and to General Public. • Organisations to construct Cyclone Shelters in the cyclone-prone areas and ready machinery for evacuation of people to safer areas. • Community preparedness at all levels to meet the exigencies.

19. Cyclone warning • "Two Stage Warning Scheme“ • The first stage warning known as the "Cyclone Alert" is issued 48 hours in advance of the expected commencement of the adverse weather over the coastal areas. • The second stage warning known as the "Cyclone Warning" is issued 24 hours in advance. • Both cyclone "Alert" and "Warning" messages are passed to the AIR stations for repeated broadcast.

20. Vulnerable Communities • vulnerability of a human settlement to a cyclone is determined by its siting, the probability that a cyclone will occur, and the degree to which its structures can be damaged by it. • most vulnerable to cyclones are • Light weight structures with wood frames, especially older buildings where wood has deteriorated and weakened the walls • Houses made of unreinforced or poorly-constructed concrete block • Buildings in low- lying coastal areas or river floodplains • Furthermore, the degree of exposure of land and buildings will affect the velocity of the cyclone wind at ground level, • Certain settlement patterns may create a "funnel effect" that increases the wind speed between buildings, leading to even greater damage.

21. How High Winds Damage Buildings • Contrary to popular belief, few houses are blown over. Instead, they are pulled apart by winds moving swiftly around and over the building. This lowers the pressure on the outside and creates suction on the walls and roof, effectively causing the equivalent of an explosion. • Whether or not a building will be able to resist the effects of wind is dependent not so much upon the materials that are used but the manner in which they are used. • common belief: that heavier buildings, such as those made of concrete block, are safer. • Truth: well-built and properly-engineered masonry house offers a better margin of safety than other types of buildings

22. Catastrophic Failures • Foundations • The uplift forces from cyclone winds can sometimes pull buildings completely out of the ground. • In contrast to designing for gravity loads, the lighter the building the larger (or heavier) the foundation needs to be in cycloneresistant design.

23. Catastrophic Failures • Steel Frames • A common misconception: the loss of cladding relieves the loads from building frameworks. • Truth: the wind loads on the structural frame increases substantially with the loss of cladding • Usual weakness in steel frames is in the connections. Thus economising on minor items (bolts) has led to the overall failure of the major items (columns, beams and rafters)

24. Catastrophic Failures • Masonry Houses • usually regarded as being safe in cyclones. • There are countless examples where the loss of roofs has triggered the total destruction of un-reinforced masonry walls.

25. Catastrophic Failures • Timber Houses • The key to safe construction of timber houses is the connection details. • The inherent vulnerability of light-weight timber houses coupled with poor connections is a dangerous combination which has often led to disaster.

26. Catastrophic Failures • Reinforced Concrete Frames • The design of reinforced concrete frames is usually controlled by the seismic hazard. • If seismic design is not done, wind analysis must be performed • ignoring this, can lead to disaster.

27. Component Failures • Roof Sheeting • This is perhaps the commonest area of failure in cyclones. • The causes are usually inadequate fastening devices, inadequate sheet thickness and insufficient frequencies of fasteners in the known areas of greater wind suction.

28. Catastrophic Failures • Roof Tiles • These were thought to have low vulnerability in storms but past cyclones have exposed the problem of unsatisfactory installation practices.

29. Catastrophic Failures • Rafters • Of particular interest in recent cyclones was the longitudinal splitting of rafters with the top halves disappearing and leaving the bottom halves in place. • The splitting would propagate from holes drilled horizontally through the rafters to receive holding-down straps.

30. Catastrophic Failures • Windows and Doors • After roof sheeting, these are the components most frequently damaged in cyclones. • Of course, glass would always be vulnerable to flying objects. • The other area of vulnerability for windows and doors is the hardware latches, bolts and hinges. • Walls • Cantilevered parapets are most at risk. • But so far walls braced by ring beams and columns have remained safe.

31. Damaging Effects of Cyclone on Houses • Due to the high wind pressure and improper connection of the house to the footings it can be blown away.

32. Damaging Effects of Cyclone on Houses Roofing materials not anchored can be blown away

33. Damaging Effects of Cyclone on Houses Light weight verandah roofs are more susceptible to damage due to high wind speed.

34. Damaging Effects of Cyclone on Houses • When cyclones are accompanied with heavy rain for a long duration, the buildings can be damaged due to flooding also.

35. Design Wind Speed and Pressures • The basic wind speed is reduced or enchanced for design of buildings and structures due to following factors: • The risk level of the structure measured in terms of adopted return period and life of structures. • Terrain roughness determined by the surrounding buildings or trees and, height and size of the structure. • Local topography like hills, valleys, cliffs, or ridges, etc.

36. On which elements the wind pressure effect must be considered? • It is considered on various elements depends on • Aerodynamics of flow around buildings. • The windward vertical faces being subjected to pressure. • The leeward and lateral faces getting suction effects and • The sloping roofs getting pressures or suction effects depending on the slope.

37. typical effects of openings in the walls • Wind generating opening on the windward side during a cyclone will increase the pressure on the internal surfaces • may be sufficient to cause the roof to blow off and the walls to explode.

38. typical effects of openings in the walls Windward face of the building collapses under pressure of wind force

39. typical effects of openings in the walls • During a cyclone an opening may suddenly occur on the windward side of the house. • The internal pressure which builds up as a result may be relieved by providing a corresponding opening on the leeward side.

40. If the building is not securely tied to its foundations, and the walls cannot resist push/pull forces, the house tends to collapse starting from the roof with the building leaning in the direction of the wind.

41. Failure of the Wall: Wind forces on the walls of the house may produce failure. Wind striking a building produces pressure which pushes against the building, on the windward side, and suction which pulls the building on the leeward side and the roof.

42. Overturning is another problem for light structures. • This occurs when the weight of the house is insufficient to resist the tendency the house to be blown over.

43. When choosing a site for your house, consider the following • Though cyclonic storms always approach from the direction of the sea towards the coast, the wind velocity and direction relative to a building remain random due to the rotating motion of the high velocity winds. • In non-cyclonic region where the predominant strong wind direction is well established, the area behind a mound or a hillock should be preferred • Similarly a row of trees planted upwind will act as a shield. • The influence of such a shield will be over a limited distance, only for 8 – 10 times the height of the trees. • A tree broken close to the house may damage the house also hence distance of tree from the house may be kept 1.5 times the height of the tree.

44. When choosing a site for your house, consider the following No shielding from high wind due to absence of barriers Shielding from high wind by permeable barriers such as strong trees

45. When choosing a site for your house, consider the following • In hilly regions, • construction along ridges should be avoided since they experience an increase of wind velocity • whereas valley experiences lower speeds in general

46. When choosing a site for your house, consider the following • In cyclonic regions close to the coast, a site above the likely inundation level should be chosen. In case of non availability of high level natural ground, construction should be done on stilts with no masonry or cross bracings up to maximum surge level, or on raised earthen mounds to avoid flooding/inundation but knee bracing may be used.

47. Shape of the House • Shape is the most important single factor in determining the performance of buildings in cyclones. • Simple, compact, symmetrical shapes are best. • The square plan is better than the rectangle since it allows high winds to go around them. • The rectangle is better than the L-shaped plan. • This is not to say that all buildings must be square. But it is to say that one must be aware of the implications of design decisions and take appropriate action to counter negative features. • The best shape to resist high winds is a square.

48. Shape of the House If other shapes are desired, efforts should be made to strengthen the corners. If longer shapes are used, they must be designed to withstand the forces of the wind. Most houses are rectangular and the best layout is when the length is not more than three (3) times the width.

49. In case of construction of group of buildings, a cluster arrangement can be followed in preference to row type.

50. Roofs • Lightweight flat roofs are easily blown off in high winds. • In order to lessen the effect of the uplifting forces on the roof, the roof Pitch should not be less than 22º. • Hip roofs are best