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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.)
close this folderCLIMATIC HAZARDS
View the documentPart 2.1: Tropical cyclones
View the documentPart 2.2: Floods
View the documentPart 2.3: Drought

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.


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).


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






Insured Losses



Hur. Allen






Cyclone Oscar





South Africa

Cyc. Domoina






Typ. Ike/June









Typhoon Uma






Typhoon Nina





Typhoon Gay



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. ____________________________________________________________



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.


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. ____________________________________________________________


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


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. ____________________________________________________________


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. ____________________________________________________________


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.


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

A. ____________________________________________________________



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:


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.


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. ____________________________________________________________



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


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.

Disaster Management Center, Natural Hazards: Causes and Effects, University of Wisconsin Board of Regents, 1986.

Economic and Social Commission for Asia and the Pacific, the World Meteorological Organization and the League of Red Cross Societies, Guidelines for Disaster Prevention and Preparedness in Tropical Cyclone Areas, Geneva/Bangkok, 1977.

Primer on Natural Hazard Management in Integrated Regional Development Planning, Department of Regional Development and Environment Executive Secretariat for Economic and Social Affairs Organization of American States, Washington, DC, 1991

Tropical Cyclone Warning Systems, WMO Technical Document - No. 394, tropical Cyclone Programmed Report No TCP-26, Geneva, 1990.

Typhoon Committee Operational Manual, WMO Technical Document, No 196, TCP Report No TCP-23, Geneva, 1987.

UNDRO, Mitigating Natural Disasters, Phenomena, Effects and Options, United Nations, New York, 1990.

UNDRO News, "Toward More Effective Early Warning Systems," July/August, 1990, p.14-15.

Vickers, Donat O., "Tropical Cyclones", Nature and Resources, Vol. 27, No. 1,1991, p. 31-36.

WMO TCP Project 12, Human Response to Tropical Cyclone Warnings and Their Content, Geneva 1981.

Part 2.2: Floods

This chapter of the module aims to improve your understanding of:

the causes of floods and factors which intensify their effects

impacts of floods on human settlements

flood control, prevention and preparedness measures

flood forecasting and warning systems


Throughout history people have been attracted to the fertile lands of the floodplains where their lives have been made easier by virtue of close proximity to sources of food and water. Ironically, the same river or stream that provides sustenance to the surrounding population, also renders these populations vulnerable to disaster by periodic flooding. Floods can arise from abnormally heavy precipitation, dam failures, rapid snow melts, river blockages or even burst water mains. Flood disasters are second only to droughts in the total number of people affected world wide.


Number killed by declared flood disasters, 1980-89:16.108
Number affected: 279,330,901 (OFDA, 1990)

Selected severe flood disasters




Losses In US$ million






Peru, Ecuador







































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

Causal phenomena

Types of floods

Flash floods - These are usually defined as floods which occur within six hours of the beginning of heavy rainfall, and are usually associated with towering cumulus clouds, severe thunderstorms, tropical cyclones or during the passage of cold weather fronts. This type of flooding requires rapid localized warnings and immediate response by affected communities if damage is to be mitigated. Flash floods are normally a result of runoff from a torrential downpour, particularly if the catchment slope is unable to absorb and hold a significant part of the water. Other causes of flash floods include dam failure or sudden breakup of ice jams or other river obstructions. Flash floods are potential threats particularly where the terrain is steep, surface runoff is high, water flows through narrow canyons and where severe rainstorms are likely.

Truck carried away by flooding

Mass Media Production Centre, Manila, UNDRO News, Sep/Oct 1984

Figure 2.2.1 - Flooding and its causes

Natural Hazards, Disaster Management Center 1989.

River floods - River floods are usually caused by precipitation over large catchment areas or by melting of the winter's accumulation of snow or sometimes by both. The floods take place in river systems with tributaries that may drain large geographic areas and encompass many independent river basins. In contrast to flash floods, river floods normally build up slowly, are often seasonal and may continue for days or weeks. Factors governing the amount of flooding include ground conditions (the amount of moisture in the soil, vegetation cover, depth of snow, cover by impervious urban surfaces such as concrete) and size of the catchment basin. In some larger semi-arid countries, such as Australia, inland flooding of dry or stagnant rivers may occur many weeks after the onset of heavy coastal monsoon or cyclonic rain has directed river flows many hundreds of km inland, and in the complete absence of any sign of disturbed weather.

Historical records of flooding of towns on the main river flood plains prove unreliable for flood protection purposes due to the varying source of the contributing river tributaries.

Coastal floods - Some flooding is associated with tropical cyclones (also called hurricanes and typhoons). Catastrophic flooding from rainwater is often aggravated by wind-induced storm surges along the coast. Salt water may flood the land by one or a combination of effects from high tides, storm surges or tsunamis. (See the chapters on tsunamis and tropical cyclones for more information.) As in river floods, intense rain falling over a large geographic area will produce extreme flooding in coastal river basins.

Q. Is your community or country susceptible to flooding? What types?

A. ____________________________________________________________


How do humans contribute to flooding?

Floods are naturally occurring hazards. They become disasters when human settlements occupy the floodplain.

Floods are naturally occurring hazards. They become disasters when human settlements occupy the floodplain. Population pressure is now so great that the risks associated with floods have been accommodated because of the greater need for a place to live. In the United States, for example, billions of dollars have been spent on flood protection programs since 1936. In spite of this the annual flood hazard has become greater because people have moved to and constructed upon flood plains faster than the engineers can design better flood protection.

Increase in population combined with poor resource management have resulted in new types of flooding. Conversion of forests in the catchment area to pasture and arable land means that less water is held in the upper reaches of the catchment basin, and the increased runoff water flows rapidly to the plains, with the effect of more frequent, unexpected and severe flooding.

Another type of flood becoming more common is urban flash flooding. Buildings and roads cover the land preventing infiltration so that rainwater runs over the impervious surfaces forming artificial streams. Inattention to maintenance of drainage systems, especially after long dry spells when dust, debris and overgrown vegetation have blocked natural water flow, can accentuate the degree of flash flooding.

General characteristics

Floods may be measured and analyzed by the following criteria:

Depth of water - Building foundations and vegetation will have different degrees of tolerance to being inundated with water.

Duration - Damage or degree of damage to structures, infrastructure and vegetation is often related to the length of time of inundation by water.

Velocity - Dangerously high velocities of flow may create erosive forces and hydrodynamic pressure which may destroy or weaken foundation supports. These may occur on the floodplains or in the main river channel.

Rate of rise - Estimation of the rate of rise and discharge of a river is important as a basis for flood warnings, evacuation plans, and zoning regulations.

Frequency of occurrence - The cumulative effects and frequency of occurrence measured over a long period of time will determine what types of construction or agricultural activities should take place on the floodplain.

Seasonality - Flooding during a growing season may completely destroy crops while cold weather floods from snow melts may seriously affect the functioning of a community.


Flooding, resulting from precipitation or snow melt in the catchment system, or from upstream flooding, is predictable from about 12 hours to as much as several weeks ahead.

Riverine flood forecasting estimates river level stage, discharge, time of occurrence, and duration of flooding, especially of peak discharge at specific points along river systems. Flooding, resulting from precipitation or snow melt in the catchment system, or from upstream flooding, is predictable from about 12 hours to as much as several weeks ahead. Forecasts issued to the public result from regular monitoring of the river heights and rainfall observations. Hash flood warnings, however, are dependent solely on meteorological forecasts and a knowledge of local geographical conditions. The very short lead time for the development of flash floods does not permit useful monitoring of actual river levels for warning purposes.

For comparison with previous flood events, and conversion to warning information, assessment of the following elements should be included: flood frequency analysis, topographic mapping and height contouring around river systems with estimates of water holding capacity of the catchment area, precipitation and snow melt records, soil filtration capacity, and (if in a coastal area) tidal records, storm frequency, topography, coastal geography and breakwater characteristics.

An effective means of monitoring floodplains is through remote sensing techniques such as Landsat. The images produced by the satellites can be interpreted and used to map flooded and flood-prone areas. Other efforts to improve forecasting are being implemented by UN organizations such as the World Meteorological Organization using World Weather Watch and Global Data Processing Systems. These systems are strategic when flood conditions exist across international boundaries. The great majority of river and flash floods forecasts, however, depend on observations made by national weather services for activation of flood alert warnings.


At notable risk in flood plain settlements are buildings made of earth or with soluble mortar, buildings with shallow foundations or non-resistant to water force and inundation. Infrastructural elements at particular risk include utilities such as sewer systems, power and water supplies, machinery and electronics belonging to industry and communications. Of great concern are food stocks and standing crops, confined livestock, irreplaceable cultural artifacts, and fishing boats and other maritime industries.

Other factors affecting vulnerability are lack of adequate refuge sites above flood levels and accessible routes for reaching those sites. Similarly, lack of public information about escape routes and other appropriate response activities renders communities more vulnerable.

Typical adverse effects

Physical damage

Structures are damaged by a) force of impact of flood waters on structures b) floating away on rising waters c) becoming inundated d) collapsing due to undercutting by scouring or erosion and e) damage by water-borne debris.

Damage is likely to be much greater in valleys than in open, low-lying areas. Flash floods often sweep away everything in their paths. In coastal areas, storm surges are destructive both on their inward travel and again on the outward return to the sea. Mud, oil and other pollutants carried by the water are deposited and ruin crops and building contents. Saturation of soils may cause landslides or ground failure.

Casualties and public health

Major floods may result in large numbers of deaths from drowning, particularly among the young and weak but generally inflict few serious but non-fatal injuries requiring hospital treatment.

Currents of moving or turbulent water can knock down and drown people and animals in relatively shallow depths. Major floods may result in large numbers of deaths from drowning, particularly among the young and weak but generally inflict few serious but non-fatal injuries requiring hospital treatment. Slow flooding causes relatively few direct deaths or injuries, but often increases occurrences of snake bites.

Endemic disease will continue in flooded areas, but there is little evidence of floods directly causing any large scale additional health problems apart from diarrhea, malaria and other viral outbreaks eight to ten weeks following the flood.

Water supplies

Open wells and other groundwater supplies may be contaminated temporarily by debris carried by flood waters or salt water brought in by storm surges. They will, however, only be contaminated by pathogenic organisms if bodies of people or animals are caught in the sources or if sewage is swept in. Normal sources of water may not be available for several days.

Application of remote sensing data to flood prone areas:

Honduras coastal plain - In September 1974, the coast of Honduras was flooded by Hurricane Fifi. The government of Honduras requested assistance from OAS/DRDE for mapping of the floodplain for use in an integrated development study to rebuild the coastal areas. Using Landsat data sheets, pre-flood and post-flood imagery was overlapped to show the areas susceptible to flooding and where prevention and mitigation measures and land use zoning would be necessary.

Pilcomayo River Floodplain: Recurrent flooding along the Pilcomayo River in southwestern Paraguay, prompted the Paraguayan Government to draw up a hazard map of the floodplain. Several different maps such as those showing desertification and soils were combined with Landsat maps to draft floodplain boundaries and hazard zones, using overlay mapping. The maps showed the changes in the river channel over time and served to demonstrate the highly dynamic nature of the floodplain, indicating continuous need for floodplain monitoring. Further analysis was used to assess flood hazard vulnerability.

Crops and food supplies

An entire harvest may be lost together with animal fodder resulting in long-term food shortages. Food stocks may be lost by submersion of crop storage facilities resulting in immediate food shortages. Grains will quickly spoil if saturated with water even for a short time.

Most agricultural losses result from the inundation of crops. Susceptibility to inundation depends on the type of crop and duration of flooding. Some crops, such as taro are quickly killed by relatively small amounts of flood water. Others may be able to resist submersion but may die eventually if large amounts of standing water stagnate as in the 1988 Bangladesh flood.

Large numbers of animals, including draught animals, may be lost if they are not moved to safety. This may reduce the availability of milk and other animal products and services, such as preparation of the land for planting. These losses, in addition to possible loss of farm implements and seed stocks, may hinder future planting efforts.

Floods bring mixed results in terms of their effects on the soil. In some cases, land may be rendered infertile for several years following a flood due to erosion of the topsoil or by salt permeation in the case of coastal floods. Heavy silting may either have adverse effects or may significantly increase the fertility of the soil.

In coastal areas where fish provide a source of protein, boats and fishing equipment may be lost or damaged.

On the positive side, floods may flush out pollutants in the waterways.

Other positive effects include preserving of wetlands, recharging groundwater, and maintaining the river ecosystems by providing breeding, nesting, and feeding areas for fish, birds and wildlife.

Possible risk reduction measures

The majority of deaths and much of the destruction created by floods can be prevented by mitigation and preparedness measures.

The majority of deaths and much of the destruction created by floods can be prevented by mitigation and preparedness measures. The first step involves identifying vulnerable elements by preparation of a flood hazard map and then integrating that information into a plan for preparedness and development. A strategy might combine regulation of land in the floodplains with flood control measures. Planners may seek contribution from a variety of disciplines to assess risk, the level of acceptable risk, and viability of proposed activities. Information and assistance may be obtained from different sources ranging from international agencies to the community level.

Mapping of the floodplain - Floods are normally described in terms of statistical frequency using the 100 year flood plain event parameters for flood mitigation programs. The 100 year flood plain describes an area subject to a 1% probability of a certain size flood in a given year. Depending on the degree of acceptable risk that is selected for an evaluation, other frequencies may be chosen such as 5,20,50, or 500 year floodplains.

The basic map is combined with other maps and data to form a complete picture of the floodplain. Other inputs include frequency analysis, inundation maps, flood frequency and damage reports, slope maps and other related maps such as land use, vegetation, population density and infrastructural maps. In some developing countries, obtaining extensive long term information may be difficult. Remote sensing techniques provide an alternative to traditional techniques of floodplain mapping and can be equally or more cost effective as they allow estimates of data otherwise requiring labor intensive collection methods, as in hydrology studies over extensive areas.

Multiple hazard mapping - Floods often cause, occur in conjunction with, or result from other hazards. A multiple hazard map, known as a composite, synthesized or overlay map, serves to highlight areas vulnerable to more than one hazard. It is an excellent tool for designing a multiple hazard mitigation and emergency plan. It may, however, not be adequate for site-specific, hazard specific engineered activities.

Land use regulations ensure that flood risks are not made worse by ill-conceived new land uses.

Land use control - The purpose of land use regulations is to reduce danger to life, property and development when high waters inundate the floodplains or the coastal areas. Land use regulations ensure that flood risks are not made worse by ill-conceived new land uses. Of particular concern are regions of urban expansion. The following elements should be addressed.

1. Reduction of densities: In flood prone areas, the number of casualties is directly related to the population densities of the neighborhood at risk. If an area is still in the planning stages, regulation of densities may be built into the plan. For areas already settled, especially squatter settlements, regulation of density can be a sensitive issue and would have to address the socioeconomic implications of resettlement. Unfortunately, many situations exist where dense unplanned settlements are located on floodplains. Planners must incorporate measures to improve sites and reduce vulnerability.

2. Prohibiting specific functions: No major development should be permitted in areas found to be subject to flooding once every 10 years on average. Areas of high risk can be used for functions with a lower risk potential such as, nature reserves, sports facilities and parks. Functions with high damage potential such as a hospital are permitted in safe areas only.

3. Relocation of elements that block the floodway: In addition to the obvious danger of being washed away, buildings blocking the floodway may cause damage by trapping floodwaters which then overflow into formerly flood free zones.

4. Regulation of building materials: In certain zones wooden buildings and other light structures should be avoided. In some cases, mud houses are permitted only if flood protection measures have been taken.

5. Provision of escape route: Neighborhoods should have clear escape routes and provision of refuge areas on higher ground.

Figure 2.2.2 - Schematic flood plain regulation map

Figure 2.2.3 - Schematic flood plain regulation map

Natural Hazards, Disaster Management Center, 1989.

Other preventative strategies include:

the acquisition of floodplain land by developmental agencies, perhaps by swaps that provide alternatives for building sites

establishment of incentives (loans or subsidies, tax breaks) to encourage future development on safer sites using safer methods of construction

diversification of agricultural production such as planting flood resistant crops or adjusting the planting season; establishing cash and food reserves

reforestation, range management and animal grazing controls to increase absorption (see chapters on deforestation, desertification)

construction of raised areas or buildings for use as refuge if evacuation is not possible.

Flood control

For developing countries with intensively utilized floodplains, considerable political influence in conjunction with the cooperation of the communities may have to be employed.

As mentioned above, land use controls will be of limited use in already developed floodplains. Yet, changes must be implemented to reduce a community's vulnerability to flood damage. For developing countries with intensively utilized floodplains, considerable political influence in conjunction with the cooperation of the communities may have to be employed. The most commonly used options are:

Existing channel improvements - Deepening and widening the river bed are methods to increase capacity and thus reduce the area of the floodplain.

Diversion and relief channel construction - New channel construction may be a feasible alternative relative to the cost of moving a settlement. Several options exist for channel construction such as open grass-lined channels, concrete or rock lined channels. Great care must be exercised in the design and construction of diversion channels because of the possible environmental impacts and necessary safety features. Costs may be prohibitive for such highly engineered works.

Dikes and dams - These facilities are capable of storing flood water and releasing them at manageable rates. Again, careful engineering is required to anticipate maximum flood levels. If exceeded, the damage caused may be much higher than if the facility had not been built. Dams and other retention facilities may give the public a false sense of security if not properly engineered and constructed.

Flood-proofing - Individual property owners may reduce the risk of damage by strengthening buildings to a) resist the water's force and b) retain integrity when inundated with water. Newly constructed buildings should have foundations which are not susceptible to scouring

Protection against erosion - An important element of flood defense is protection against erosion. Streambeds should be stabilized with stone masonry or vegetation especially near bridges.

Site improvement - The elevation of sites can be an effective option for individual or group dwellings.

Q. What are some possible risk reduction measures which may be used in regard to flood hazards?

A. ____________________________________________________________



Some of the possible risk reduction measures which can be used in relation to flood hazards are: preparation of floodplain mapping, multiple hazard mapping and associated land use controls, physical flood control structures (existing river channel improvement, diversion channels, and dikes and dams), individual site improvements, and individual structural modifications.

Specific preparedness measures

Flood forecasting and warning systems

Case studies in some countries have shown that flood forecasts and warnings can reduce damage between 6 and 40 percent (WMO). Flood detection systems which provide the basis for flood forecasts, warning and preparedness systems range from inexpensive networks involving volunteers who observe rainfall and stream stages to sophisticated networks of gauges and computerized models. A system known as ALERT (Automated Local Evaluation in Real Time) has demonstrated a cost effective capability to protect life and property through participation by local level agencies. Field stations designed as complete modules are maintained locally.

Whatever the method for warning the public, communication systems must be must be well planned. Evacuation procedures should be practiced on a regular basis. Ways to disseminate warnings include radio, television, warning sirens or bells, public address systems, and at village level by bicycle and on foot. National warning systems are more vulnerable to failure because of problems disseminating warnings locally in a clear manner which reach the target community in time for action to take place. National warning systems are more effective in warning urban populations. Rural systems require respected local leaders to issue clear instructions and prior arrangements for protecting assets and reaching evacuation sites.

Figure 2.2.4 - Schematic of flood warning system

Community participation

Inhabitants of flood prone areas usually have a number of traditional methods at their disposal for coping with floods. In countries such as Bangladesh where a great number of people and area of land is vulnerable to flooding, governments would be hard pressed to provide complete coverage with even simply engineered mitigation measures. Some aspects of flood planning and response can be managed at the village level and upgraded with outside assistance. These are:

issuing warnings at the local level

participating in flood fighting by organizing work parties to repair embankments or clear debris from drainage areas, pile sandbags and stockpile needed materials

facilitating agricultural recovery

planning emergency supplies of food and clean drinking water

identifying traditional mitigation and preparedness measures and determining their effectiveness.

Programs to promote public awareness of the flood hazard may contain the following components:

Explanations of the function of floodplains, location of local floodplains and drainage patterns.

Identification of flood hazards and warning signs

Encouraging individuals to floodproof their possessions and develop personal escape plans

Awareness of community evacuation plans and warning systems, and appropriate post-disaster activities

Encourage personal responsibility for flood prevention/mitigation in day to day living practices. This would include use of proper farming practices, prevention of deforestation and maintenance of drainageways.

Andrew Maskrey, Disaster Mitigation, A Community Based Approach.

Master plan

The basic guide that provides local officials and developers or landowners with information about the floodplain is called the master plan. The master plan should contain land use control regulations and a public information program. Steps to be taken in developing a master plan are as follows:

1) Obtain an accurate mapping of the area.

2) Develop the hydrology for several frequencies of flood occurrence including the 100 year.

3) Delineate the floodplains for the flood frequencies using existing channel and floodplain conditions.

4) Estimate flood damages for various frequencies and develop flood damage frequency curves and average annual damages.

5) Conduct a review of all possible flood damage reduction alternatives such as dams, channels etc.

6) Prepare preliminary designs and cost estimates for the remaining alternatives and delineate residual floodplains for the frequencies being used.

7) Determine residual flood damages for each alternative.

8) Complete a cost benefit analysis for each alternative.

9) Review each alternative for other factors such as political considerations, multiple use opportunities and environmental factors.

10) Select an alternative or combination acceptable to each affected jurisdiction.

11) Publish a master plan report with documentation of the above process.

The ABC$D of Your Next Flood - A public information publication of the Australian Bureau of Meteorology.

Typical post disaster needs

The initial response to flooding by local authorities should include:

search and rescue

medical assistance disaster assessment

short term food and water provision

water purification

epidemiological surveillance

temporary shelter

The secondary response should include:

repair or reconstruction

reestablish or create employment

assist with recovery of agriculture through loans, distribution of farm equipment and tools and animals

assist with recovery of small businesses and fisheries


The Relief operation following the 1986 floods of Lake Titicaca in Puno District, Peru

Natural disasters are frequent in Peru due to its rugged topography. The area surrounding Lake Titicaca has been subjected throughout recorded history to regular periods of drought and flooding. The region of Puno is situated in the southeast and shares a border with Bolivia. It is part of the Altiplano Andino where heavy rain falls in the summer months and intense drought occurs in the winter. Observations carried out by the government planning office indicated a trend toward heavy rains since 1983 in this area. Puno is one of the country's poorest regions. The population is very dense near the lake where Andean farmers depend on the rich soils for agriculture and livestock.

Precipitation beginning in November 1985 was much higher than average causing a gradual rise in Lake Titicaca's water level. In early 1986, the lake began to overflow its basin, resulting in extensive flooding along lake shores and three tributary rivers. Villages and crops were slowly inundated as waters encroached upon the countryside. In March of 1986, a locally compiled survey numbered the victims at 240,000 persons, or about 20% of the population. Included in the flooded land was more than 38% of the planted land and more than 80% of the total arable land. Although the urban areas were severely affected, the consequences of the floods more heavily impacted on the rural population, where earthen houses soon gave way to the advancing waters.

The Government of Peru, which had not realized the extent of damage until the March report, sought international assistance and received support from the United Nations and voluntary organizations. The government then developed an emergency relief plan to be administered by the National Civil Defense. Prior to this time, no local or government action had been taken to control the situation. The priorities of the plan included:

1. Provision of immediate care for rural and urban populations by evacuating inundated areas and resettling populations and animals in temporary settlements.

2. Provision of acceptable shelters, blankets or clothing, food and water and health surveillance.

3. Rehabilitation of damaged communication lines.

The occurrence of heavy precipitation seemed to have been viewed by both central government and local authorities as normal, despite dome evidence of heavy rain trends, until the extent of damage became critical. This may have resulted from response patterns of previous years when local residents organized themselves to deal with the flooding and no pressing need existed for government assistance.

As there was no preparedness plan, there was no immediate response. When the plan was elaborated it seemed logical and coherent but was difficult to implement for the following reasons:

1. The coordination between national authorities and local representatives was inadequate. The staff members were not prepared and were disorganized.

2. No preparations existed for reception of international aid, including clearance of goods. The roles of the relief organizations, including international and national organizations (both governmental and nongovernmental) in the management of relief assistance were not clearly defined.

3. The population had been evacuating itself prior to government intervention, looking with difficulty for safe refuge. This exodus produced conflicts.

The relief process was impaired by lack of preparedness. The rehabilitation process, however, may prove to be even more problematic. Valuable farming land has been lost to the lake and may not be recovered for some time.

Options include continuing the traditional lifestyle and accepting the occasional disaster, resettling part of the Puno population permanently, or developing flood control technologies and safe agricultural practices.

Q. What preparedness measures may have prevented the problems confronted in the emergency relief operations in Puno?

A. ____________________________________________________________



The preparation of a flood emergency plan, including the roles of all involved organizations, and in anticipation of international intervention. The preparation of a flood evacuation plan including designated shelters. Community participation in flood planning and response. Establishment of a flood forecasting and warning system and network to communicate local conditions to central agencies. Studies of flood plain areas and preparation of a master plan to determine potential

Discuss the perspectives of these three groups on the Lake Titicaca flooding disaster: the Government of Peru; the people of Puno affected by flooding; the relief organizations.

Discuss the pros and cons of the three rehabilitation options.

A. ____________________________________________________________



"Automated Local Evaluation in Real Time (ALERT): A Cooperative Flood Warning Service for Your Community", National Weather Service, Western Region, Box 11188 Federal Building, Salt Lake City, Utah 84147, USA.

Askew, Arthur, "Learning to Live With Floods", in Nature and Resources, Vol. 27, No. 1, 1991, p.4-8.

Cuny, Frederick C., "Living with Floods: Alternatives for Riverine Flood Mitigation", in Development: from vulnerability to resilience, p. 62-73.

Disaster Management Center, Natural Hazards: Causes and Effects, University of Wisconsin Board of Regents, 1986.

Facing Geologic and Hydrologic Hazards, W.W. Hays, editor, U.S. Government Printing Office, Washington, D.C., 11981.

Khan, H.R., "Floods and Flood Preparedness", Disaster Management Course, Asian Disaster Preparedness Center, Asian Institute of Technology, Bangkok, Thailand.

Natural Disaster Reduction: How Meteorological and Hydrological Services Can Help, WMO - No. 722, Geneva, 1989.

Sztorch, L., and V. Gicquel, J.C. Desenclos, "The Relief Operation in Puno District, Peru, after the 1986 Floods of Lake Titicaca," in Disasters, Vol. 13, No. 1,1989, p. 30-40.

UNDRO, Mitigating Natural Disasters, Phenomena, Effects and Options, United Nations, New York, 1991.

UNDRO NEWS, "Unprecedented Hoods Devastate Bangladesh", Sept/Oct. 1988.

Part 2.3: Drought

This chapter of the module will enhance your understanding of:

the meaning of the term drought

the causal phenomena leading to drought

the effects of drought on affected communities

the factors affecting vulnerability to drought

drought preparedness and mitigation measures

typical assistance needs


Of all the natural disasters, droughts have the greatest potential economic impact and can affect the largest number of people. Earthquakes and cyclones are potentially of enormous physical intensity but are of short duration and geographically limited. The death toll from such disasters can be very high if urban or densely populated rural areas are affected. By contrast droughts affect large geographical areas - often covering whole countries or parts of continents - they may last for months and in some cases several years. They invariably have a direct and significant impact on food production and the overall economy.


Number of persons killed in drought/food shortage declared disasters from 1980-89: 404,139

Number of persons affected: 101,074,389 (OFDA, 1990)

Selected recent drought disasters




Number affected



no report





5-7 million











no report




no report





1.9 million



no report

2.3-3.8 million



no report


Source: OFDA, 1987-90

A general working definition of drought might be a temporary reduction in water or moisture availability significantly below the normal or expected amount for a specified period.

The key components of such a definition are:

a) that the reduction is temporary (if the reduction were permanent then terms such as "dry" and "arid" would be more appropriate)

b) that the reduction is significant

c) that the reduction is defined in relation to a "norm"

d) that the period taken as the basis for the "norm" is specified.

All societies tend to stabilize their socio-economic systems around what is perceived to be the normal rainfall and this perception is heavily weighted towards recent experience.

How the "norm" is measured is of critical importance. Components c) and d) therefore require more detailed clarification. The "norm" may be defined either:

1) technically - for instance, a reduction of water availability might qualify as a "drought" when it falls below 80% of the average availability over the preceding 20 years. However, within naturally fluctuating climate and weather systems the period selected as the basis for estimating the average may be significantly misleading (see figure 2.3.1), or

2) culturally - in terms of the level of water availability around which society has taken to be the "norm". All societies tend to stabilize their socio-economic systems around what is perceived to be the normal rainfall and this perception is heavily weighted towards recent experience. Thus, after a run of perhaps ten years with above average rainfall a society may have become used to the wetter state and perceive the first year of average rainfall as a drought.

Figure 2.3.1 - Schematic presentation of fluctuations occurring on different time scales

Drought types

There are three types of drought: meteorological, hydrological and agricultural. The first two types describe physical events; the third describes the particular impact of the first two types on an area of human activity - agricultural production. It is necessary to carefully distinguish between these types and to clarify their relationships to one another.

Meteorological drought involves a reduction in rainfall for a specified period (day, month, season, year) below a specified amount - usually defined as some proportion of the long term average for the specified time period. Its definition involves only precipitation statistics Care needs to be taken in utilizing and aggregating rainfall data.

Hydrological drought involves a reduction in water resources (streamflows, lake levels, groundwater, underground aquifers) below a specified level for a given period of time. Its definition involves data on availability and offtake rates in relation to the normal operations of the system (domestic, industrial, irrigated agricultural) being supplied.

Agricultural drought is the impact of meteorological and hydrological droughts on this particular area of human activity. Crops have particular temperature, moisture and nutrient requirements during their growth cycle in order to achieve optimum growth. If moisture availability (or indeed any other required input) falls below the optimum amount during the growth cycle, then crop growth will be impaired and yields reduced. Because of the complexity of the relationships involved, agricultural drought is difficult to measure. A fall in yields may be due to insufficient moisture but it may also stem from, or be exacerbated by, such factors as the unavailability of fertilizers, lack of weeding, the presence of pests and crop diseases, the lack of labor at critical periods in the growth cycle, unattractive crop prices, etc. Because of all these other factors capable of affecting yields, it is incorrect to assume that meteorological drought and agricultural drought are synonymous. For the scope of this module we will use the term drought to mean agricultural drought induced by meteorological and hydrological drought.

Q. What are the differences between meteorological, hydrological, and agricultural drought?

A. ____________________________________________________________



Meteorological drought is a reduction in rainfall, hydro-logical drought is a reduction of water resources including lakes, streams, underground aquifers, and groundwater. Agricultural drought is the result of both hydrological and meteorological drought on agricultural practices.

Parched agricultural land

FAO, UNDRO NEWS, May/June, 1989.

Causal phenomena

When discussing the causes of drought it is helpful to distinguish between short drought "episodes" lasting 1-3 years and long dry "regimes" of predominantly subnormal rainfall spanning about a decade or more, which may include several intense drought episodes.

The proximate or immediate cause of rainfall deficit may be due to one or more factors such as an absence of available moisture in the atmosphere; large scale subsidence (downward movement of air within the atmosphere) which suppresses convective activity; and the absence or non-arrival of rain-bearing systems. Changes in such factors involve changes in weather systems ranging from local to regional to global. While it may be possible to indicate the immediate cause of a meteorological drought occurring in any particular location, it is often not possible to indicate the underlying cause.

While it may be possible to indicate the immediate cause of a meteorological drought occurring in any particular location, it is often not possible to indicate the underlying cause.

Short term episodes can often be linked (or "teleconnected") to global-scale fluctuations in the atmosphere and oceans elsewhere in the world. Thus, the El Niouthern Oscillation (ENSO) phenomenon, which involves the periodic invasion of warm surface water into the normally colder waters off the Pacific coast of South America, affects the levels of rainfall in many different parts of the world, including south-eastern Africa. However, understanding the process which causes the invasion of the warmer currents is presently incomplete.

Many causes of long dry "regimes" have been postulated. Among the local level causes are human-induced changes resulting from vegetation loss due to overgrazing and deforestation either in the general vicinity or "upwind" of the area along the line of the prevailing, moisture carrying winds. Such changes may recur and perpetuate the drought conditions.

On a larger spatial scale the link between sea surface temperatures and rainfall has been suggested as a possible cause of the long dry regimes. Thus it has been suggested that the fact that the southern Atlantic has been consistently warmer than the northern Atlantic since around 1970 is related to the predominantly dry period in the Sahel since the mid-1960s.

However, one of the main problems with the theories involving human induced change is that of distinguishing human induced change from natural long term fluctuations. For instance, there would seem to be a fluctuation in rainfall in the western Sahel on a time scale of around 50 years with the predominantly dry period since the mid-1960s being part of such a cycle. However, reliable rainfall data series for the Sahel and many other parts of the world are available only for the last 80-90 years, and this is far too short a period to support the assertion that there is such a rainfall cycle for the area. The World Meteorological Organization believes that five or six such cycles are necessary to confidently predict trends.


Whatever the period, the warning time allows for a response by governments to mitigate the impacts of the drought before they become significant.

Of the main natural disasters, droughts are unique in terms of the length of time between the first indications from, for example, rainfall monitoring, that a drought is developing and the point at which it begins to impact significantly upon the population of the affected area. The length of such "warning time" varies significantly between different societies. In many countries the warning time is in the order of several months. In those countries with a high proportion of landless laborers dependent upon crop-related employment the warning time may be much less, perhaps a matter of a few weeks. Whatever the period, the warning time allows for a response by governments to mitigate the impacts of the drought before they become significant. In countries where there is a lead time in the order of months, it is generally possible to mobilize relief assistance, including food aid, from the international community. Thus, by virtue of modern meteorological monitoring and telecommunication systems it has become perfectly possible to prevent excess mortality resulting from food shortages caused by drought alone. Droughts may continue to be a contributory cause of famines but other factors such as conflict and international politics are now invariably responsible for propelling a situation of economic hardship caused by drought into a famine.

During the last two decades, many developing countries have, with donor encouragement and assistance, invested in and improved the coordination of their existing systems for reporting on meteorological, agricultural, crop marketing and such indicators as nutritional status so that they are better able to provide "early warning" information. In most countries such systems have involved the creation of "Early Warning Units" located in an appropriate department of Central Government into which the Meteorological Departments, Agricultural Extension and Statistics Departments feed information. In addition to these national systems there are other "Early Warning Systems" functioning at different levels. Thus, regional groupings such as CILSS (Comitermanent Inter-Etats de Lutte Contre la Secheresse dans le Sahel or Permanent Inter-State Committee for Drought Control in the Sahel) in West Africa, SADCC (Southern African Development Coordinating Conference) have established Regional Early Warning Systems which combine the output of the national systems with information from other sources, such as remote sensing information from satellites.

FAO GIEWS is an important source of information for donor organizations. Within countries there may also be local level "Early Warning Systems". Often these are funded by NGOs and usually, but not always, feed into the government-run National Early Warning Systems.

Factors contributing to vulnerability

In the discussion of factors affecting vulnerability to drought we are referring to agricultural drought. The following factors, then, are those contributing to overall vulnerability to agricultural drought.

The proportion of production which is irrigated

The correlation between rainfall and yields is clearly weaker in irrigated rather than rainfed areas. The extent to which this is the case, however, will be determined by the importance of local rainfall in the irrigated water supply and whether all or only part of the crops' moisture requirements are normally met through irrigation.

Early warning systems

Water pumped from the Niger River for irrigation.

UNDP/Ruth Massey. World Development, Sept. 1989.

The moisture retention capacity of the soil

Different soil types have different capacities to "hold" or retain moisture. For instance the water retention capacity of sandy soils is generally significantly lower than that of clay soils. Soils therefore represent potential short term "stores" of water. In areas where soil moisture retention capacities are high, crop growth may not be affected by prolonged dry periods (as much as 20 days) and some moisture may actually be held over from one wet season to another.

In contrast, in areas where retention capacity is low, dry periods of only a week may result in reduced yields and any moisture present in the soil at the end of one wet season will not be held over to the start of the next. In many arid and semi-arid areas of the tropics the predominant soil types are sandy. To attain optimum crop growth such areas need frequent and evenly spaced rains throughout the growing season.

Timeliness of the rain

Deficiencies in moisture supply at critical stages during the growth cycle (e.g. germination, flowering, etc.) can significantly reduce yields. Consequently the distribution or timeliness of the rainfall during the growing season is potentially as important as the overall amount of rain.

The adaptive behavior of farmers

In the face of an intermittent start to a wet season, some farmers may respond with repeated replantings of the same crop variety to take account of the rains when (and if) they finally start, while others may replant using other varieties. Some farmers may not have seed reserves of their own or be in a position to purchase replacement seeds for the first failed planting, in which case these farmers may experience a crop failure while other farmers in the same area may enjoy a satisfactory harvest.

The effects of drought

L. AnstrUNDRO News, Jan/Feb. 1985

Typical adverse effects

Typical adverse effects of agricultural drought include the following:

Reduced income for farmers

Reduced employment opportunities for agricultural laborers

Reduced demand within the economy generally

Increased defaulting on loans in the rural sector (central and commercial banks)

Reduced government revenues and foreign exchange earnings as a result of a decline in agricultural exports

Increased price of staple foods

Increased inflation rate within the economy

Inability of certain groups within the population to afford increased food prices results in their:

- switch to cheaper and sometimes less preferred foods
- reduction in overall food intake
- borrowing/taking loans in order to maintain food intake
- selling their assets to raise funds
- engaging in alternative income earning activities locally
- migrating in search of employment opportunities elsewhere
- migration to where relief food is being distributed

Increased stress and morbidity due to migration journey

Reduced food intake leads to deterioration of nutritional status and reduction in ability to resist infection

Drying-up of water sources leads to reduction in water quality, the need to travel further to collect water and possibly to migrate to better water sources, all of which are likely to increase levels of morbidity

Increased competition for access to remaining water sources may lead to increased incidence of local disputes/conflict

Social costs caused by migration, e.g. break-up of communities and families

Q. What are the basic factors contributing to vulnerability from drought?

A. ____________________________________________________________



The basic factors contributing to the vulnerability to drought are:

- the proportion of agricultural production which is irrigated
- the moisture retention capacity of the soil
- the timeliness of the rain

- the adaptive behavior of the farmers

Possible risk reduction measures

To reduce the risk of transitory food insecurity due to drought, it is necessary to protect people's access to food through

1) ensuring the availability of food in the affected area and
2) protecting the entitlements of all groups within the affected population.

Some of the principle measures for maintaining food security during agricultural droughts are:

price stabilization

food subsidies

employment creation programs

general food distributions

supplementary feeding programs

special programs for livestock and pastoralist populations

complementary water programs

complementary health programs

Preparedness measures

Preparedness for drought requires that additional response capacity be sustained within the system.

Preparedness for drought requires that additional response capacity be sustained within the system. Where governance is poor such capacity is unlikely to be maintained between episodes of transitory food insecurity due to drought.

Several UN agencies are involved in responding to transitory food insecurity and famines. The greater part of resources channelled through the UN in response to such situations are via WFP (food aid), UNHCR (all forms of assistance for refugees) and UNICEF (non-food relief, but especially health and water supply with a focus on women and children as priority groups).

The proliferation and increasingly important role of NGOs has a number of implications. Where the government or UN has established effective coordination mechanisms and the number of NGO programs in the country in response to the emergency is limited, coordination amongst the NGO community may be adequate. Where government and UN coordination mechanisms are weak and there is a significant increase in the number of NGOs working in the country, significant coordination problems may arise. While in the short run the use of NGOs may be necessary in the interests of ensuring a prompt and effective response to the population in the affected area, care should be taken to ensure that the use of NGOs does not prevent the longer run efforts to strengthen the capacity of government agencies.

Care should be taken to ensure that the use of NGOs does not, prevent the longer run efforts to strengthen the capacity of government agencies.

Typical post disaster assistance needs

The affected population must be assisted to replace assets lost during the period of temporary food insecurity and, where this is realistic, to reestablish their livelihoods. The severity of this food insecurity episode will determine the nature and scale of the rehabilitation requirements. Thus, if migration to camps and significantly increased mortality has occurred, then a comprehensive rehabilitation program will be required. This may involve health care, counseling, assisting the migrants back to their homes and material support to re-establish their homes and productive activities. Such provisions may include seeds, tools, cooking utensils, blankets, and support until households are capable of supporting themselves. If the impact of the temporary food insecurity episode has not been severe and most households have not been obliged to sell productive assets (e.g. consume seed stocks and breeding livestock) then a rehabilitation program may not be required.

Rehabilitation needs should therefore be carefully assessed and interventions tailored to the particular situation.



Rehabilitation With Rice

During the drought years of the early 1980s, Songhai farmers, living near Timbuktu in Mali, became destitute. The Niger River which had annually overflowed to flood the farmers' lowland fields failed. Dust storms raged, wells dried up, and the river reached the lowest levels ever recorded. The farmers became victims of the long dry spell that forced them to relinquish their traditional way of life and to seek jobs in local towns for the survival of their families.

In 1984, the United Nations Capital Development Fund (UNCDF) assisted 13,000 inhabitants of the villages of Timbuktu and Gao to grow a new type of irrigated rice in the desert with water pumped from the Niger River. Pipes hundreds of meters long were installed and a revolving fund established to purchase seeds, pesticides, fuel and spare parts for the pump. The project was assisted by three UN volunteers using a new high yielding hybrid rice crossed from the traditional floating rice to the much higher yielding Asian irrigated rice.

When the project began to produce 1,800 kilos of rice per hectare, 15 more villages formed similar associations to those at Gao and Timbuktu. The new irrigation system allowed two crops per year instead of the traditional annual crop.

The project, however, was not trouble free. The farmers were promised an incentive of 5 kilos of maize for each tree planted as a shelter crop for the rice. Although beneficial to both the environment and the crop, the trees were not watered and died after the local authorities failed to deliver the maize as promised.

Bookkeeping also proved to be a problem. The Songhai do their accounting in their heads, a practice which is difficult to monitor, especially when accounting expands from the repayment of simple debts between farmers to balance sheets for large cooperatives. On the whole, however, the development of the communities involved has attested to the success of the project. Many villagers also attended literacy classes and other training.

Q. What are the strong points of this project? What are the weak points? If another drought occurs, what effects may it have on the irrigation system?

A. ____________________________________________________________


Soeters, R. 1988. "Pitfalls with weight-for-height measurements in surveys of acute malnutrition". Tropical Doctor 16(4), 174-176.

Wilhite, D.A. and Glantz, M.H. 1985. "Understanding the Drought Phenomenon: The Role of Definitions". Water International 10,111-120.

World Bank. 1986. Poverty and Hunger: Issues and options for food security in developing countries. Washington D.C.

WHO (World Health Organization). 1985. "Energy and Protein Requirements" Report of a Joint FAO/WHO/UNU Expert Consultation, Technical Report Series 724, Geneva.

WMO (World Meteorological Organization). 1983. "Report of the Expert Group Meeting on the Climate Situation and Drought in Africa". Geneva: WCP-61.