[Note: This document was prepared by Horace H.P. Burton and Selvin DeC. Burton of the Caribbean Institute for Meteorology and Hydrology for storm surge mapping workshops held in Antigua and Barbados in November 1999.]
The impact of tropical cyclones on developing countries can be very significant in terms of the detrimental aspects. In the Caribbean this was clearly demonstrated by Gilbert in 1988, Hugo in 1989, Luis in 1995, and Georges in 1998. Although loss of lives from tropical cyclones has significantly decreased over recent years, especially in the developed countries, the loss of property has increased substantially. Reductions in fatalities are usually attributed to improvement in the tropical cyclone forecasting and warning system, while increases in property losses are attributed to accelerated property development in coastal zones.
A significant factor in the growing losses caused by meteorological hazards is the concentration of population and economic activity in coastal plains and low-lying areas that are subject to storm surges and land-borne flooding. Furthermore, the rapid growth of the tourism industry over the last decade has greatly increased the insured value of the coastal properties in the Caribbean and, associated with it, the storm related risk.
Of all the tropical cyclone damage agents, strong winds are perhaps the best understood. This is fortunate because the winds largely determine, directly or indirectly, the other agents. The low-level winds typically will be stronger on the right side of the cyclone in the Northern Hemisphere, but are highly variable both in time and space. The area of destruction in tropical cyclones vary from about 25 km in small systems to 500 km or more in large systems.
Wind damage is not simply a function of the maximum sustained surface wind, defined as the mean wind speed as measured by an anemometer over a one minute interval at an altitude of 10 meter over open terrain. Factors such as maximum wind speed in gusts, duration of high sustained wind speeds, and variations in the direction of the wind can subject different elements of a structure to different loadings with cumulative effects, and are therefore important determinants of the damage suffered by structures in a hurricane.
An important consideration in the design of structures is their response to gusts, or extreme winds. Smaller and lighter elements of a building such as roof sheeting and their support, windows, doors and cladding respond more rapidly to wind loading and are therefore more vulnerable to damage by wind gusts. Since the destructive force of wind increases with the square of its speed, it is very important to be able to accurately determine the maximum wind speed.
Two more factors need to be taken into account in assessing wind hazard. The first is the influence of topography on wind speed. This effect is well known, as evidenced by housing and urban settlements seeking shelter by locating on the leeward side of mountains and hills from prevailing winds. A wind tunnel study simulating the island of Nevis (Davenport, 1985) confirmed the shelter effect of mountains and hills, with surface wind speeds (at 10 m. elevation) dropping to less than 50% of the wind speed at 500 m. It also identified the increase in wind speed over crests in the terrain, an effect that can increase the wind speed at 500 m by up to 20%.
The last but not necessarily least factor is that a substantial part of wind damage in an urban environment can be caused by flying debris, i.e. those materials resulting from already failed structures which may become wind borne and can in turn inflict damage on surrounding properties. Buildings designed for the proper maximum wind loading may still fail due to the direct impact of debris.
Rainfall associated with tropical cyclones is both beneficial and harmful. Although the rains contribute to the water needs of the areas traversed by the cyclone, the rains are harmful when the amount is so large as to cause flooding. In 1998 Mitch was responsible for over nine thousand deaths predominantly from rain-induced flooding in portions of Central America, mainly in Honduras and Nicaragua. In Honduras as much as 900 mm of rain fell between October 25 and 31, with one station recording 466 mm on October 31 (NHC 1999).
Heavy rain from tropical cyclones is affected by the several factors, including the duration of rainfall (speed of motion), the energy/moisture supply, and topography. The longer the tropical cyclone circulation system is sustained after landfall, the more likely that torrential rains will re-develop. If the landfalling tropical cyclone stagnates or moves slowly, successive outbreaks of heavy rain over the same area may cause flooding. In addition, moisture is not only helpful in sustaining the landfalling tropical cyclone, it is also the essential condition for causing torrential rain. Topographic features such as coastal boundaries and mountains increase the upward vertical motion which increases the intensity of the tropical cyclone precipitation.
Once a tropical cyclone moves over land and begins to decay, it may produce periods of widespread rainfall which tends to cause flash flooding. Floods from tropical cyclones are dependent upon:
The storm surge - an abnormal rise of water due to a tropical cyclone - is an oceanic event responding to meteorological and other driving forces. These include the wind field, the pressure anomaly, the size and speed of motion of the system, the bottom topography near the storm's landfall point, and the astronomical tides. The most important factor, however, in determining the maximum storm surge heights is the maximum wind speed which in turn is closely related to the minimum sea-level pressure. Storm surges may be as little as 1 m or less if only a few of the factors are making their maximum contribution, and 5 m or more if all the factors are making the maximum possible contribution to the total deviation of the sea surface.
Potentially disastrous surges occur along coasts with low-lying terrain that allows inland inundation, or across inland water bodies such as bays, estuaries, lakes, and rivers leading to severe flooding. For a typical landfalling storm, the surge affects about 160 km of coastline for a period of several hours and may penetrate as much as 15 to 30 km inland. Larger storms that are moving slowly may impact considerably longer stretches of coastline.
Wind blowing across a stretch of open water creates waves on the surface of the water. The wave heights depend on a number of factors including the wind velocity, the length of time the wind has been blowing, the distance over which the wind has blown or the fetch, and upon the state of the sea at the time the wind started to blow. Some of the highest ocean waves are generated by the winds of tropical cyclones. In the Atlantic average hurricane waves are about 10 m.
The impacts of storm surge are coastal flooding, beach erosion, and the removal of beach materials among others. In critical regions with extensive building in the coastal regions, the wind wave effects are important. Waves are a constant force, which reshape coastal areas. They wear away rocky shorelines, move sand - eroding or building beach areas - and damage structures in their paths. Although inundation due the storm surge poses a damage problem, the pounding nature of the waves which accompany the storm surge is responsible for most of the damage to the structures. The effect that a wave can have on a stretch of coastline is determined not only by the strength of the wave itself, but also by the slope of the bottom offshore, presence of coral reefs or other breakwaters and the shape of the coastline. These factors can dissipate wave energy before it reaches the shore or concentrate it, significantly increasing local wave effects. When combined with increased water levels from storm related surges, waves produced during a hurricane can reach areas typically shielded from the direct effects of waves, destroying buildings and dramatically altering the existing shoreline.
Tornadoes are not normally a tropical phenomenon but are frequently spawned by hurricanes on crossing coastlines and islands. Hurricane-spawned tornadoes are much more common than was once thought. Tornadoes spawned by tropical cyclone have not generally been reported over the Caribbean. However, some patterns of damage suggest that tornadoes may actually occur more often than reported. Since tornadoes form in conjunction with strong convection, they are more likely to occur near the outer edge of the eyewall cloud or in the outer rainbands. The force of the wind and the sudden reduction in pressure are the major destructive impacts of tornadoes.
How many died and what was the damage? These are the two most frequently asked questions about tropical cyclones and rightly so. The approximately 80 tropical cyclones that occur throughout the globe each year cause billions of dollars in damage and kill about 10,000 people. However, in addition to the cost, there may be some economic benefits to be derived from tropical cyclones.
The direct or indirect costs from a cyclone can be divided into a number of broad categories, some of which are identified below.
Although we tend to focus on the losses due to a cyclone, a complete economic study must also consider the benefits. Two hurricanes lashed Mexico in 1984 and caused considerable damage to several communities. However, the rains filled reservoirs, saved crops and the economic agricultural gains more than offset the coastal losses.
In many arid regions in the tropics, a large portion of the annual rain comes from cyclones. Unfortunately, very few studies have focused on this beneficial aspect of tropical cyclones. Another possible economic benefit of a cyclone is the increase in some businesses during the recovery. In fact, outside aid may prompt a local economic boom in the affected community. However, the net economic impact on the nation is still negative.
Education and knowledge of tropical cyclones must be an essential component in disaster preparedness. Such information must be distributed through the various channels to enhance awareness of the public to the potential threat. Meteorologists should be involved in assuring that tropical cyclone information is accurate, and also play a role in dissemination through briefings, public appearances, etc. Meteorologists also have a crucial role to play in providing input into the design and implementation of tropical cyclone mitigation systems. This role involves at least three areas of national tropical cyclone preparedness planning.
The compilation and issuing of warnings usually entails the conversion of technical information into a proper format and in a meaningful language for different public and special recipients. Terminology should be well chosen to take into account the physical and human geography of the threatened areas and be conducive to evoking a rational human response to the content of the message.
Warnings should be disseminated without delay as the usefulness of promptly compiled and strategically designed user-oriented warning messages can easily be negated if not issued in a timely manner. Such delays are more critical when the lead time for implementing preparedness measures in safety is limited.
The capacity of the community and special user groups to respond effectively to natural disaster warnings is highly dependent on their interpretation of the warning, and on precisely what measures are necessary for the protection of life and property. The measure of understanding will largely be based on the awareness of the community to the inherent danger presented by the effects of the disaster. It is essential to institute awareness programmes to educate the community about the hazards of natural disasters and to train disaster management officials to interpret warnings.
Meteorologists must make significant contributions in educating the community and officials regarding tropical cyclones, the cyclone warning system, and possible mitigation measures. This education must include an understanding of the content and terms used in advisories; techniques used in identifying and monitoring areas of potential development; characteristic structure and behaviour of tropical cyclones; and methods and limitations of forecasting techniques.
Meteorological services in the region are willing to undertake the necessary educational programmes for both the community and disaster officials. Disaster managers and planners are encouraged to make uses of these opportunities.
The benefits of advances achieved in the meteorological monitoring and forecasting will not result in a lessening of losses and damage unless there is a corresponding improvement in the warning-response infrastructure. Meteorologists, disaster managers and the community all have a vital role to play in achieving this objective.
Davenport, A.G., P. N. Georgiou, and D. Surry, 1985: A Hurricane Wind Risk Study for the Eastern Caribbean, Jamaica and Belize with Special Consideration to the Influence of Topography. University of Western Ontario.
Guiney, J. L., and M. B. Lawrence, 1999: Preliminary Report - Hurricane Mitch 22 October to 05 November 1998. National Hurricane Center
Vemeiren, J. C., and C. C. Watson Jr., 1994: New Technology for Improved Storm Risk Assessment in the Caribbean. OAS
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