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close this bookIntroduction to Hazards - 1st Edition (Department of Humanitarian Affairs/United Nations Disaster Relief Office - Disaster Management Training Programme - United Nations Development Programme , 1992, 168 p.)
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close this folderGEOLOGIC HAZARDS
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close this folderCLIMATIC HAZARDS
View the documentPart 2.1: Tropical cyclones
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View the documentPart 5: Chemical and industrial accidents

Part 2.1: Tropical cyclones


This chapter is designed to enhance your knowledge of:


the physical nature and cause of tropical cyclones


hazards which can accompany a tropical cyclone such as hurricane winds, storm surges and floods

the predictability of tropical cyclones and use of warning systems


the impact of tropical cyclones on human settlements and infrastructure and measures that can be taken to reduce and prevent damage and casualties.

Introduction


The World Meteorological Organization (WMO) uses the generic term "tropical cyclone" to cover weather systems in which winds exceed "gale-force" (minimum of 34 knots or 63 kph). Tropical cyclones are rotating, intense low-pressure systems of tropical oceanic origin. "Hurricane-force" (63 knots or 117 kph) winds mark the most severe type of tropical storm. They are called hurricanes in the Caribbean, the United States, Central America and parts of the Pacific; typhoons in the Northwest Pacific and East Asia; severe cyclonic storms in the Bay of Bengal and severe tropical cyclones in the South Indian, South Pacific and Australian waters. To facilitate identification and tracking, the storms are generally given alternating masculine and feminine names, or numbers which identify the year and annual sequence.

Tropical cyclones are the most destructive of seasonally recurring rapid onset natural hazards. Between 80 and 100 tropical cyclones occur around the world each year. Devastation by violent winds, torrential rainfall and accompanying phenomena including storm surges and floods can lead to massive community disruption. In the last decade, the official death toll in individual tropical cyclones reached 140,000 (Bangladesh, 1991) and damages approaching US$ 10 billion in Hurricanes Gilbert (1988) and Hugo (1989).

TROPICAL CYCLONE HAZARD DATA SHEET

Selected cyclones of the 1980s, Losses in US$ million

Year

Location

Name

Deaths

Losses

Insured Losses

1980

United-States

Hur. Allen

235

1,500

58

1983

Fiji

Cyclone Oscar

7

85

43

1984

South Africa

Cyc. Domoina

109

90

29

1984

Philippines

Typ. Ike/June

1,000

220


1985

Bangladesh

Cyclone

11,000



1987

Vanuatu

Typhoon Uma

50

200

140

1987

Philippines

Typhoon Nina

650

27


1989

Thailand/India

Typhoon Gay

1,000

456


Number of persons affected by declared tropical cyclone disasters from 1980-89: 24,110,139

Sources: Nature and Resources, Vol. 27, No. 1, 1991, and OFDA, 1990

Causal phenomena

The development cycle of tropical cyclones may be divided into three stages: formation and initial development, full maturity, and modification or decay. Depending on their tracks over the warm tropical seas and proximity to land, they may last from less than 24 hours to more than three weeks, (the average duration is about six days). Their tracks are naturally erratic, but initially move generally westward, then progressively poleward into higher latitudes where they may make landfall or into an easterly direction as they lose their cyclonic structure.

Formation and initial development stage

Four atmospheric and oceanic conditions are necessary for development of a cyclonic storm:

1. A warm sea temperature in excess of 26 degrees centigrade, to a depth of 60 m, which provides abundant water vapor in the air by evaporation.

2. High relative humidity (degree to which the air is saturated by water vapor) of the atmosphere to a height of about 7000 m facilitates condensation of water vapor into water droplets and clouds, releases heat energy and induces drop in pressure.

3. Atmospheric instability (an above average decrease of temperature with altitude) encourages considerable vertical cumulus cloud convection when condensation of rising air occurs.

4. A location of at least 4-5 latitude degrees from the Equator allows the influence of the forces due to the earth's rotation (Coriolis force) to take effect in inducing cyclonic wind circulations around low pressure centers.


Figure 2.2.1 - Formation of a tropical cyclone

The atmosphere can usually organize itself into a tropical cyclone in two to four days and is characterized by increasing thunderstorms and rain squalls at sea. Meteorologists can monitor these processes with weather satellites and radar as far as 400 km away from the storm. The existence of favorable conditions for cyclone development determine the cyclone season for each monitoring center. In the Indian/South Asian region the season is divided into two periods, from April to early June and from October to early December. In the Caribbean and United States, tropical storms and hurricanes reach their peak strengths in middle to late summer. In the NW Pacific-South China Sea typhoons are most frequent in the months of July to November, but have been known to occur in each month of the year. In the Southern Hemisphere, the cyclone season extends from November to April/ May but occasionally cyclones do occur in other months in lower latitudes.

Q. What are the four conditions required for the formulation of a cyclonic storm?

A. ____________________________________________________________
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______________________________________________________________
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ANSWER

1. Warm sea temperature
2. High relative humidity
3. Atmospheric instability
4. Location 4-5 degrees latitude from the equator

Mature tropical cyclones

As viewed by weather satellites and radar imagery, the main physical feature of a mature tropical cyclone is a spiral pattern of highly turbulent giant cumulus thundercloud bands. These bands spiral inwards and form a dense highly active central cloud core which wraps around a relatively calm and cloud-free "eye". The eye typically has a diameter of from 20-60 km of light winds and looks like a black hole or dot surrounded by white clouds.

In contrast to the light wind conditions in the eye, the turbulent cloud formations extending outwards from the eye accompany winds of up to 250 kph, sufficient to destroy or severely damage most non-engineered structures in the affected communities. These strong winds are caused by a horizontal temperature gradient which exists between the warm core of the cyclone (up to 10 degrees centigrade higher than the external environment) and the surrounding areas, and results in a correspondingly high pressure gradient.

The weakening stage of a tropical cyclone

When the cyclone hits land, especially over mountainous or hilly terrain, widespread riverine and flash flooding may last for weeks.

A tropical cyclone begins to weaken in terms of its central low pressure, internal warm core and extremely high winds as soon as its sources of warm moist air begins to ebb or are abruptly cut off. This would occur during landfall, by movement into higher latitudes, or through influence of another low pressure system. The weakening of a cyclone does not mean the danger to life and property is over. When the cyclone hits land, especially over mountainous or hilly terrain, widespread riverine and flash flooding may last for weeks. Or, the energy from a weakening tropical cyclone may be reorganized into a less concentrated but more extensive storm system causing widespread violent weather.


A tropical cyclone in the Bay of Bengal on May 8, 1990 as recorded on weather radar in Madras, India. The dark "eye" of the storm can be plainly seen.

Nature and Resources, Vol. 27, No. 1, 1991.

General characteristics

Tropical cyclones are characterized by their destructive winds, storm surges and exceptional level of rainfall which may cause flooding.

Destructive winds - The strong winds generated by a tropical cyclone circulate clockwise in the Southern Hemisphere and counter-clockwise in the Northern Hemisphere, while spiraling inwards and increasing toward the cyclone center. Wind speeds progressively increase toward the core.


Figure

As the eye arrives, winds fall off 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.

A scale classifying the intensity of the storms, the Beaufort scale, estimates the wind velocity by observations of the effects of winds on the ocean surface and familiar objects. Both the United States (Saffir-Simpson Potential Hurricane Damage Scale) and Australia (Cyclone Severity Categories) use country-specific scales which estimate potential property damage in five categories. The Philippines recently increased its typhoon warning signal numbers from 3 ranges of wind speeds to 4 in order to take into account the lower standards of building structures and regional variations.


Storm Surges - The storm surge, defined as the rise in sea level above the normally predicted astronomical tide, is frequently a key or overriding factor in a tropical storm disaster. As the cyclone approaches the coast, the friction of strong on-shore winds on the sea surface, in combination with the "suction effect" of reduced atmospheric pressure, can pile up the sea water along a coastline near a cyclone's landfall well above the predicted tide level for that time. In cyclones of moderate intensity the effect is generally limited to several meters, but in the case of exceptionally intense cyclones, storm surges of up to eight meters can result. The major factors include:

a) A fall in the atmospheric pressure over the sea surface. In the center of a tropical storm the atmospheric pressure is much less than outside. The maximum rise in mean sea level due to this effect is about one meter.

b) The effect of the wind. As the winds strengthen they will exert force on the surface of the water causing waves, swells and storm surges.

c) The influence of the sea bed. As the storm approaches a coastline, especially if the sea bed slopes gradually, friction at the sea bottom will interfere with the return water currents and the wind will pile up water along the shore. This combination of strong winds and gently sloping sea bed can result in very strong storm surges, reaching as high as eight meters.

d) A funneling effect. A semi-enclosed bay in the path of a storm surge permits the storm's winds to trap high water there for extended periods.

e) The angle and speed at which the storm approaches the coast. In general, the greater the forward speed and the more nearly perpendicular the track is to the coast, the higher the surge will be, but these two conditions do not have to exist for a severe storm surge to occur.

f) The tides. Tides in some countries show variation in height with the seasons as well as the time of day. If a storm surge coincides with the high tide and/or the maximum seasonal tide, the effect can be devastating.

Of the countries experiencing cyclonic storms, those most vulnerable to storm surges are those with low lying land along the dosed and semi-enclosed bays facing the ocean. These countries include Bangladesh, China, India, Japan, Mexico, the United States and Australia. Prevailing on shore winds and low pressures due to winter depressions in non-tropical latitudes, as in countries bordering the North Sea, are also subject to storm surges which require substantial mitigation measures, such as dikes.

Exceptional rainfall occurrences - The world's highest rainfall totals over one or two days have occurred during tropical cyclones. The highest 12 and 24 hour totals, 135 cm and 188 cm have both occurred during cyclones at La Reunion Island in the SW Indian ocean. 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 even dry catchment areas and rapid runoff may explosively flood the usual water courses and create new ones.

The relationship between cyclone strength and rainfall is often not proportional. For instance, if the atmosphere over land is dry, a strong cyclone may decay quickly and rainfall may be quite limited. On the other hand, if the atmosphere is already saturated and the terrain flooded over large areas, a weak to moderate strength cyclone may weaken only slowly and rainfall will persist. In recent times catastrophic cyclone riverine and flash flood events have increased due to the incidence of heavy rainfall on heavily deforested hill slopes. Landslides and small rivers jammed with floating logs and debris have quickly swamped villages and inhabited flood plains resulting in thousands of casualties. A relatively weak typhoon at Ormoc in the central Philippines drowned 6000 persons in such circumstances in 1991 in barely 30 minutes.

Q. Aside from strong winds, what other hazards are associated with cyclonic storms?

A. ____________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________


ANSWER

Flooding, of all types, flash floods, riverine flooding, and storm surge.

Predictability

Tropical cyclones form in all the oceans of the world except the South Atlantic and South Pacific east of 140 degrees W longitude. Nearly one quarter form between 5 and 10 degrees latitude of the equator and two thirds between 10 and 20 degrees latitude. It is rare for a tropical cyclone to form south of 20-22 degrees latitude in the Southern Hemisphere, however, they occasionally form as far north as 30-32 degrees in the more extensive warmer water of the Northern Hemisphere.

The locations, frequencies and intensities of tropical cyclones are well known from historical observations and, more recently, from routine satellite monitoring. Tropical cyclones do not follow the same track except coincidently over short distances. Some follow linear paths, others recurve in a symmetrical manner, others accelerate or slow down and become "quasi-stationary" for a time. For this reason it is often difficult to predict when, where and if the storm will hit land, especially islands. In general, the difficulty in forecasting increases from the low to higher latitudes while the margin of error in determining the cyclone center decreases as landfall approaches depending on the availability of radar detection.

Special warning and preparedness strategies for evacuation from off shore facilities or closure of industrial plants must be worked out relating the costs and benefits of the decision against the uncertainties of precision in the forecasts. For general community purposes which require a minimum 12 hours of preparedness time, the imprecision in forecasting the location of landfall within 24 hours should be generally tolerable, bearing in mind that highly adverse cyclonic weather usually commences about 6 hours prior to the landfall of the cyclone.

Regrettably, progress in reducing forecasting errors, has remained slow in the last two decades despite huge investment in monitoring systems. However, substantial progress has been made in the organization of warning and dissemination systems particularly through regional cooperation. The activities of national meteorological services are coordinated at the international level by the World Meteorological Organization (WMO). Forecasts and warnings are prepared within the framework of the WMO's World Weather Watch Program (WWW). Under this program, meteorological observational data provided nationally, data from satellites and information provided by the regional centers are exchanged around the world. Specialized products are being provided by centers designated under the WWW and the associated Tropical Cyclone Program (TCP) in the form of guidance material to assist in detecting, monitoring and forecasting cyclones.

The WWW system includes 8,500 land stations, 5,500 merchant ships, aircraft, special ocean weather ships, automatic weather stations and meteorological satellites. A tropical cyclone is first identified and then followed from satellite pictures, A complex Global Telecommunications System relays the observations. Ultimately, however, the responsibility for providing forecasts and warnings to the local population regarding tropical cyclones and the associated winds, rains and storm surges, falls upon the national services. Unfortunately, many of the less developed countries, where most deaths from tropical cyclones occur, do not possess state of the art warning systems, nor can many of them take proper advantage of the warnings sent to them.


Figure 2.2.2 - The annual average global occurrence (numbers and percentages) of tropical cyclones reaching storm and hurricane force, and their regions of genesis. Cyclones form only where areas affected by subsequent winds and flooding are shown hatched (after Gray, 1975). The names of WMO regional cyclone bodies and the basins covered by their programmes are also indicated.

Factors contributing to vulnerability

Human settlements located in exposed, low lying coastal areas will be vulnerable to the direct effects of the cyclone such as wind, rain and storm surges.

Human settlements located in exposed, low lying coastal areas will be vulnerable to the direct effects of the cyclone such as wind, rain and storm surges. Settlements in adjacent areas will be vulnerable to floods and mudslides or landslides from the resultant heavy rains. The death rate is higher where communications systems are poor and warning systems are inadequate.

The quality of structures will determine resistance to the effects of the cyclone. Those most vulnerable are lightweight structures with wood frames, older buildings with weakened walls, and houses made of unreinforced concrete block. Infrastructural elements particularly at risk are telephone and telegraph poles, fishing boats and other maritime industries.


The cumulative effect of all damage will be to impede information gathering and transport networks, Hurricane damage in Nicaragua, 1988.

UNDRO NEWS, May/June 1989.


Downed power lines between South Africa and Maputo after March 26, 1985 cyclonic storm.

UNDRO NEWS, March/April 1985, p.9.

Typical adverse effects

Physical damage

Structures will be damaged or destroyed by wind force, through collapse from pressure differentials, by flooding, storm surge and landslides. Standing crops may be lost to floods, storm surges, and sea water salinity. Salt from storm surges may also be deposited on agricultural lands and increase ground water salinity. Fruit, nut or lumber trees may be damaged or destroyed by winds, flood or storm surges. Plantation type crops such as banana and coconut are extremely vulnerable. Erosion could occur from flooding and storm surges. Additional items subject to severe damage include overhead powerlines, bridges, culverts and drainage systems, jetties and retaining walls, embankments and coastal dikes, general lack of weatherproofing of buildings, huge losses to building work in progress, scaffolding, marinas, and roofs of most structures. Falling trees, wind-driven rain and flying debris cause considerable damage.

Casualties and public health

Their are relatively few casualties due to the high winds associated with cyclonic storms. Storm surges may cause many deaths but usually few injuries among the survivors. Due to flooding and possible contamination of water supplies, malaria and other viruses may be prevalent several weeks after the flooding.

Water supplies

Open wells and other ground water supplies may be temporarily contaminated by flood waters and storm surges. They be contaminated by pathogenic (disease producing) organisms only if bodies of people or animals are lying in the sources or sewage is swept in. Normal water sources may be unavailable for several days.

Crops and food supplies

The combination of high winds and heavy rains, even without flooding, can ruin standing crops and tree plantations. Food stocks may be lost or contaminated if the stores/structures in which they were held have been destroyed or inundated. It is possible that food shortages will occur until the next harvest. It is also possible that tree and food crops may be blown down or damaged and must be harvested prematurely.

Communications and logistics

Communications may be severely disrupted as telephone lines, radio antennas and satellite disks are brought down, usually by wind. Roads and railroad lines may be blocked by fallen trees or debris and aircraft movements will be curtailed for at least 12 to 24 hours after the storm. Modes of transportation such as trucks, carts and small boats may be damaged by wind or flooding. The cumulative effect of all damage will be to impede information gathering and transport networks.

Q. From your own experience, what has been the greatest loss to communities caused by tropical cyclones? Do your answers agree with the typical losses described above?

A. ____________________________________________________________
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______________________________________________________________
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Possible risk reduction measures

Risk assessment

The evaluation of risk for a tropical cyclone is a relatively straightforward process. A hazard map should be prepared which illustrates the areas vulnerable to a tropical cyclone in any given year. The following information may be used to estimate the probability of storms of cyclones of various intensities which may strike different sections of the country.

1) Analysis of climatological records to determine how often tropical cyclones have struck, their intensities and locations.

2) History of wind strengths, frequencies, height and location of storm surges, and frequencies of flooding.

3) Information about tropical cyclone occurrences in the past 50-100 years over the ocean adjoining the country.

Land use control

Land use planning for disaster prevention and mitigation is designed to control land use so that least critical activities can be placed in most vulnerable areas. Sensitive issues must be addressed regarding existing conditions, such as cultural patterns of ownership, characteristics of the local economy and population pressures. Population growth and land shortages have pushed the poor further into marginal lands. Squatters may settle in floodplains to be dose to urban centers where they seek jobs and services. Land use regulations in these cases would have to be integrated with other social and economic policies.

Policies regarding future development may regulate land use and enforce building codes for areas vulnerable to the effects of tropical cyclones such as wind, flooding and storm surges. For example, in coastal areas, regulations can stipulate maximum building height, type of land use and occupant density of buildings. Another option entails purchase of vulnerable areas by government for use as parks, sports facilities, wildlife preserves, or open grazing land.

Flood plain management

All three major types of flooding (flash, river and coastal floods) may result from a tropical cyclone. Therefore, a master plan for flood plain management must be enacted. (See chapter on floods for additional information.)

Reducing vulnerability of structures and infrastructure

Building regulations establish minimum standards of design, construction and materials that strengthen structures to avoid collapse. The majority of homes in developing countries, however, receive no engineering input and are made of locally available materials. In these cases it would be more sensible to provide performance standards, recommendations for construction or improvements to existing construction, as follows:


Improvement of a building sites by raising the ground level to protect against flood and storm surges.


Low cost housing may be strengthened to resist wind and flood damage. Houses subjected to intense winds are literally pulled apart by the wind moving around and over the building. In preventing this effect, the construction materials are often not as important as the manner in which they are used.


New buildings should be designed to be wind and water resistant.


Infrastructure should be inspected prior to the cyclone season and strengthened against wind and floods. Communications lines should be located away from the coastal areas or installed underground.


Buildings or silos used to store food supplies must be protected against the winds and water.


Protective river embankments, levees and coastal dikes should be regularly inspected for breaches and erosion, and opportunities taken to plan mangroves to reduce breaking wave energy.

Improving vegetation cover

An important concern in management of the watershed lies in improvement of the vegetation to increase the water infiltration capacity of the soil. The roots of trees and other plants keep the soil intact, preventing erosion while slowing runoff to prevent or lessen flooding. The use of trees planted in rows may also act as a windbreak near houses and compounds, or may be planted around towns. Reforestation and conservation are very cost effective in mitigation of floods and other disasters. (See chapters on deforestation and environmental degradation.)

Q. What are some mitigation measures that might lessen the loss of the type you answered as being most severe in the previous question?

A. ____________________________________________________________
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______________________________________________________________
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Specific preparedness measures

An integrated warning/response system

Specific preparedness measures to counter the impact of tropical cyclones may be classified into two categories:

1. Those of a long term or seasonal nature which need to be planned, funded, implemented and operationally tested and coordinated by means of simulation exercises well before a seasonal threat commences. Among these are pre-season coordination meetings at headquarters, district and local levels at which operational contingency plans are reviewed and amended, training and community preparedness programs conducted and maintenance inspections made of all facilities and services which constitute community lifelines. Critical supplies may have to be stockpiled.

2. Those of a short-term nature which relate to a state of readiness to act once a contemporary cyclone threat is announced. Among these are domestic, vocational and animal husbandry arrangements to safeguard the survival, property assets and livelihoods of individual families and communities. Crops may have to harvested when a warning is issued to prevent their complete loss. Or boats may need to be moved to more protected shelters.

An integrated warning/response system consists of five sets of tasks:


technical arrangements for monitoring the lifecycle of tropical cyclones, with supporting research to aid in successful tracking


conversion of this technical information into timely weather forecasts and warnings to the public


technical and organizational arrangements with the media and other communication networks to disseminate of warnings and recommended actions and obtain feedback or acknowledgment of messages


public education through community awareness programs about the hazards of tropical cyclones and training for cyclone disaster management officials


frequent evaluations to identify deficiencies in the plan

Public Warning system

The three main objectives in a tropical cyclone warning are:

1. To ALERT the people to the danger by announcing the existence of a threat due to a cyclone.

2. To IDENTIFY THE AREAS where people will be actively threatened by the cyclone and where communities should monitor further warning announcements, and

3. To CALL THE PEOPLE TO ACTION by recommending specific preparedness activities which may be part of an integrated warning/ response plan to protect vulnerable resources.


Warning phases

It is the responsibility of each country to determine its public warning procedures, although generally WMO has assisted in coordinating regional arrangements. A suggested warning schedule is as follows:

1. 12 hour media announcements of the existence and expected motion of a tropical cyclone which has possibly already been named but is unlikely to be a threat for 48-72 hours. This will encourage preliminary community readiness.

2. 6 hour "Alert" or "Watch" bulletins when a threat is expected but wind strengths of gale force will not commence before 36-48 hours. The exact landfall location is not yet available. At this stage, counter-disaster emergency offices will begin to implement plans with a long lead time. Cautionary advice should be offered to distant fishing fleets concerning coastal areas coming under increasing threat (Note: the speed of approach of a cyclone is likely to equal or exceed the return speed of fishing craft).

3. 3 hourly or 6 hourly full "warnings" are issued when gales are likely and landfall may occur within 24-36 hours. The strength of the cyclone and weather details are available. Depending on the expected angle of approach of a cyclone to the shoreline the locations warned of destructive impact may be narrowed down to coastal sectors within about 300-400 km on either side of expected landfall. Specific advice to communities regarding evacuation to shelters or emergency camps, closure of public facilities, or use of them for services. Small craft and fisherman operating in near-coastal waters are warned to return to harbor as roughening of seas occur 12-18 hours ahead of landfall.

4. Hourly warnings. If the cyclone comes within radar range, frequently about 18 hours ahead of landfall, warnings may be issued on an hourly basis. Important decisions must occur earlier than 12-18 hours. Attention is given to the eye of the cyclone to determine the velocity of the winds and the likely location of storm surges, heavy rain and flash flooding. General flooding warnings should be issued. All preparedness measures should be finalized six to eight hours before forecasted landfall.

5. Post landfall forecasts and warnings. The warnings decrease in frequency as the cyclone moves inland and weakens. Winds will usually moderate within 6-12 hours to a low danger level. Flooding may be promoted by heavy rain which may continue for several days and possible rise in river systems due to storm surges onshore winds. Evacuation may be necessary upon very short notice.


Figure

Q. What are the three main objectives of a cyclone warning system?

A. ____________________________________________________________
______________________________________________________________
______________________________________________________________


______________________________________________________________
______________________________________________________________
______________________________________________________________


ANSWER

1. Alert the population to the hazard threat.
2. Define the Area that is threatened.
3. Call those in the warning area to Action by explaining preparedness measures that should be taken by the public.

Evacuation plan

An essential component of disaster preparedness is an evacuation plan. The plan would specify: a) areas to be evacuated and time required for the operation; b) areas/buildings to be used as shelters; c) assembly points to be used in transferring people to places of safety. The plan must be tested prior to a real event.


East Sonadia Cyclone Shelter, Cox's Bazaar, Bangladesh.

Training and community participation

Systematic methods must be employed to inform people about the threat of a disaster. Public awareness programs must explain some of the very basic issues to clarify the causes of the hazard, the disaster impact, and the ways in which the local population is vulnerable. Explanations of the warning system and evacuation plans must be puplicized. There are several methods of promoting public information and education:

1) Public dissemination of information through mass media, poster campaigns, town councils and village meetings. These campaigns should intensify as the tropical cyclone storm season approaches, and should encourage the public to share responsibility for preparedness measures with the government officials.

2) Education programs, designed for different age levels may be offered in schools and universities, perhaps as part of a science curriculum.

3) Training programs should be offered for officials who will play a part in disaster mitigation, preparedness and post disaster assistance. Training programs are also essential for medical personnel to deal with the specific types of casualties brought on by a tropical cyclone.

4) Community based training which emphasizes post disaster activities such as practicing the evacuation plan, team efforts to search and rescue, flood fighting measures such as filling and stacking sandbags.

Typical post-disaster assistance needs

The initial response by local authorities includes:


evacuation

emergency shelter

search and rescue

medical assistance

provision of short term food and water

water purification

epidemiological surveillance

provision of temporary lodging

reopening roads

reestablishing communications networks and contact with remote areas

brush and debris clearance

disaster assessment

provision of seeds for replanting.


CASE STUDY

Cyclone Disaster Vulnerability in Bangladesh1

1 Source: Sevenhuysen, G.P., "Report of Cyclone Disaster Response in Bangladesh. " Disaster Research Unit, The University of Manitoba, June 25, 1991

the climatic conditions required for cyclone formation are common in the Bay of Bengal and 16% of cyclones reach the coast of Bangladesh, usually during May to October. An early cyclone formed in late April of 1991, with lower pressures than other recorded cyclones, resulting in stronger winds and greater increases in sea level, due to its prematurity.

Two days before landfall, warnings were issued to the public. Red Crescent Society volunteers brought the warnings to the village level. In response to the warnings, about 3 million people shifted to safer places. People were told to bury needed supplies for use after the storm.

The cyclone hit with great destructive force, with gusts up to 223 kph, able to lift a 100 mm concrete roof slab. The cyclone bought torrential rains for five hours, and then a storm surge struck the coast and islands both north and south of Chittagong. On the morning of 30 April, the storm surge submerged parts of the mainland and seven densely populated islands, where it remained standing for another 24-48 hours. Wind and rain continued for five days, constraining rescue efforts, particularly for the island populations.

The cyclone affected more than 4.5 million people in 16 districts, killing 131,539. Casualties were mostly attributable to the storm surge and 40-50% of the population died in the unprotected islands. On islands protected by embankments, 30-40% died and on the mainland, 20-30%. Considerable damage was inflicted on infrastructure and production and eventually a severe negative impact on domestic growth. Immediately after the cyclone, UNDP/UNDRO undertook a study of actual disaster management activities for use in the Disaster Management Training Programme (DMTP). The following conclusions were reached regarding the vulnerability of the Bangladesh coastal areas:

1. Protective embankments may have saved some lives but many are minimally effective. Because of population pressures, farmers plant them with root crops and need river pools to grow rice which conflicts with objectives of shrimp producers who require sea pools.

2. The women are very vulnerable to disaster in a male-dominated society where they are dependent on men for disaster preparedness and recovery resources.

3. Infrastructure such as communications, health and service facilities were very vulnerable and had not been assessed for disaster preparedness in recent years. Interruptions in communications severely disrupted relief operations.

4. The number of cyclone shelters were totally inadequate and poorly maintained.

5. The possible severity and effect of the cyclone were not understood either by the urban or the rural population. Some did not prioritize saving their lives against protecting their possessions and livelihood from robbery or squatting. Secondly, past false alarms and no means to redress the loss of possessions, should they disappear after the warnings ceased, increased the possibility that these warnings went unheeded. People living on the islands would have had to travel long distances to reach high ground and any two story buildings were privately owned.

Q. 1) What particular features of the Bangladesh coast increase the vulnerability of the population?

2) What socioeconomic factors increase the vulnerability of the population ?

A. ____________________________________________________________
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______________________________________________________________
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ANSWER

1. The probability of occurrence of tropical cyclones is very high. Many islands support very large populations who must travel some distance to reach higher ground. The storm surge accompanying the cyclone inundated the islands making rescue attempts difficult or impossible. The severe rainfall which normally accompanies tropical storms lasted a very long time, necessitating safe refuge for that time.

2. The extreme insecurity brought on by population pressure and poverty makes people willing to risk their lives to save their property and livelihoods. Population pressures on land force mitigation measures, such as embankments, to be used for other purposes and thus reduces their effectiveness. Education was lacking regarding the risks accompanying a cyclone, such as storm surges and torrential rains. Women, and thus children, are at great risk, if they have no resources to protect


References

A Global View of Tropical Cyclones, RL Elsberry, ED, Proceedings, International Workshop on Tropical Cyclones, Bangkok, Thailand, 1985, Chicago University press, 1987.

Carter, T. Michael, Probability of Hurricane/Tropical Storm Conditions: A User's Guide for Local Decision Makers, US Department of Commerce, National Oceanic and Atmospheric Administration, June, 1983.

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