The destruction caused by hurricanes in the Caribbean and Central America is a force that has shaped history and will shape the future of the region. The danger arises from a combination of factors that characterize tropical cyclonic storms: rise in sea level, violent winds, and heavy rainfall. In the Greater Caribbean Basin from 1960 through 1988 (excluding the United States and U.S. territories) hurricanes caused more than 20,000 deaths, affected 6 million people, and destroyed property worth over US$9.5 billion (OFDA, 1989). The great bulk of this harm was done to the Caribbean island countries, whose small economies are least able to withstand such impacts.
Data on hurricane damage have been collected since the discovery of the Americas, and recent statistics show that mitigation measures have made a difference since the 1930s. While the ferocity of the storms has not abated over the years, and population has increased substantially in the area, the casualty rate has decreased as a result of the incorporation of mitigation measures and the increased effectiveness of preparedness activities. This improvement in saving lives has been countered by a marked increase in property damage. This is a clear indicator that structural mitigation measures are not keeping pace with the rapid increase in development in vulnerable areas.
Hazardous Characteristics Of Hurricanes
a. Winds
b. Rainfall
c. Storm Surge
Hurricane wind speeds can reach up to 250km/h (155mph) in the wall of the hurricane, and gusts can exceed 360km/h (224mph).The destructive power of wind increases with the square of its speed. Thus, a tripling of wind speed increases destructive power by a factor of nine. Topography plays an important role: wind speed is decreased at low elevations by physical obstacles and in sheltered areas, while it is increased over exposed hill crests (Davenport, 1985; see Figure 12-6). Another contributor to destruction is the upward vertical force that accompanies hurricanes; the higher the vertical extension of a hurricane, the greater the vertical pulling effect.
Destruction is caused either by the direct impact of the wind or by flying debris. The wind itself primarily damages agricultural crops. Entire forests have been flattened by forces that pulled the tree roots from the earth. Man-made fixed structures are also vulnerable. Tall buildings can shake or even collapse. The drastic barometric pressure differences in a hurricane can make well-enclosed structures explode and the suction can lift up roofs and entire buildings. But most of the destruction, death, and injury by wind is due to flying debris (ECLAC/UNEP, 1979), the impact force of which is directly related to its mass and the square of its velocity. The damage caused by a flying car to whatever it strikes will be greater than if the wind had acted alone. Improperly fastened roof sheets or tiles are the most common projectiles. Other frequent objects are antennas, telephone poles, trees, and detached building parts.
b. Rainfall
The rains that accompany hurricanes are extremely variable and hard to predict (ECLAC/UNEP, 1979). They can be heavy and last several days or can dissipate in hours. The local topography, humidity, and the forward speed of a hurricane in the incidence of precipitation are recognized as important, but attempts to determine the direct connection have so far proved futile.
Intense rainfall causes two types of destruction. The first is from seepage of water into buildings causing structural damage; if the rain is steady and persistent, structures may simply collapse from the weight of the absorbed water. The second, more widespread and common and much more damaging, is from inland flooding, which puts at risk all valleys along with their structures and critical transportation facilities, such as roads and bridges. Chapter 8 describes flooding in more detail.