Hurricane Basics

Hurricane Basics 3

2.7. Hurricane Hazards. The main hazards associated with tropical cyclones and especially hurricanes are storm surge, high winds, heavy rain, and flooding, as well as tornadoes. The intensity of a hurricane is an indicator of damage potential. However, impacts are a function of where and when the storm strikes.

2.7.1. Storm Surge.

storm surge Storm surge is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall. The surge of high water topped by waves is devastating. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property. Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide (Figure 2.8.), which can increase the mean water level 15 feet or more. In addition, wind waves are superimposed on the storm tide. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. Because much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level, the danger from storm tides is tremendous. The level of surge is also determined by the slope of the continental shelf (Figure 2.9.). continental shelf A shallow slope off the coast will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf will not see as much surge inundation, although large breaking waves can still present major problems. Storm tides, waves, and currents in confined harbors severely damage ships, marinas, and pleasure boats. Wave and current action associated with the tide also causes extensive damage. Water weighs approximately 1,700 pounds per cubic yard; extended pounding by frequent waves can demolish any structure not specifically designed to withstand such forces. The currents created by the tide combine with the action of the waves to severely erode beaches and coastal highways. Many buildings withstand hurricane force winds until their foundations, undermined by erosion, are weakened and fail. In estuaries and bayous, intrusions of salt water endanger the public health and send animals, such a snakes, fleeing from flooded areas.

2.7.2. High WindsFigure 2.10. Hurricane Force Winds. . The intensity of a landfalling hurricane is expressed in terms of categories that relate wind speeds and potential damage. According to the Saffir-Simpson Hurricane Scale, a Category 1 hurricane has lighter winds compared to storms in higher categories. A Category 4 hurricane would have winds between 131 and 155 mph and, on the average, would usually be expected to cause 100 times the damage of the Category 1 storm. Depending on circumstances, less intense storms may still be strong enough to produce damage, particularly in areas that have not prepared in advance. Tropical storm-force winds are strong enough to be dangerous to those caught in them. For this reason, emergency managerís plan on having their evacuations complete and their personnel sheltered before the onset of tropical storm force winds, not hurricane force winds. Hurricane-force winds can easily destroy poorly constructed buildings and mobile homes. Debris such as signs, roofing material, and small items left outside become flying missiles in hurricanes (Figure 2.10.). Extensive damage to trees, towers, water and underground utility lines (from uprooted trees), and fallen poles cause considerable disruption. High-rise buildings are also vulnerable to hurricane-force winds, particularly at the higher levels since wind speed tends to increase with height. It is not uncommon for high-rise buildings to suffer a great deal of damage due to windows being blown out. Consequently, the areas around these buildings can be very dangerous. The strongest winds usually occur in the right side of the eyewall of the hurricane. Wind speed usually decreases significantly within 12 hours of landfall. Nonetheless, winds can stay above hurricane strength well inland.

2.7.3. Heavy Rains/Floods storm flooding Torrential Rains and Flooding. Tornadoes. . Widespread rainfall of 6 to 12 inches or more is common during landfall, frequently producing deadly and destructive floods (Figure 2.11.). Such floods have been the primary cause for tropical cyclone-related fatalities over the past 30 years. The risk from flooding depends on a number of factors: The speed of the storm, its interactions with other weather systems, the terrain it encounters, and ground saturation. Rains are generally heaviest with slower moving storms (less than 10 mph). To estimate the total rainfall in inches, one rule of thumb is to divide 100 by the forward speed of the hurricane in miles per hour (100/forward speed = estimated inches of rain). The heaviest rain usually occurs near or along the cyclone track in the period 6 hours before and 6 hours after landfall. However, storms can last for days. Occasionally hurricanes produce little rain where it is expected. Large amounts of rain can occur more than 100 miles inland where flash floods are typically the major threat. Tornadoes and high winds generally become less of a threat the farther inland a hurricane moves (although there have been several exceptions). But the heavy rains frequently continue and even intensify as the dying, but still powerful, hurricane is forced up higher terrain or merges with other storm systems in the area. tornado

2.7.4. Tornadoes. Hurricanes also produce tornadoes, which add to the hurricane's destructive power . Tornadoes are most likely to occur in the right-front quadrant of the hurricane. They are also often found elsewhere embedded in the rainbands, well away from the center of the hurricane. However, they can also occur near the eyewall. Some hurricanes seem to produce no tornadoes, while others develop multiple ones. In general, tornadoes associated with hurricanes are less intense than those that occur in the Great Plains (Table 2.3. - Fujita Tornado Intensity Scale). Nonetheless, the effects of tornadoes, added to the larger area of hurricane-force winds, can produce substantial damage. We have no way at present to predict exactly which storms will spawn tornadoes or where they will touch down. The new Doppler radar systems have greatly improved the forecaster's warning capability, but the technology usually provides lead times from only a few minutes up to about 30 minutes. Consequently, preparedness is critical. hurricane intensity scale