Natural and Man Made Hazards |
The 74 to 160 mile per hour winds of a hurricane can extend inland for hundreds of miles. Hurricanes can spawn tornadoes, which add to the destructiveness of the storm. Floods and flash floods generated by torrential rains also cause damage and loss of life. Following a hurricane, inland streams and rivers can flood and trigger landslides. Even more dangerous than the high winds of a hurricane is the storm surge-a dome of ocean water that can be 20 feet at its peak and 50 to 100 miles wide. The surge can devastate coastal communities as it sweeps ashore. Nine out of 10 hurricane fatalities are attributable to the storm surge.
Coastal communities deciding how strong their structures should be need to consider the strength of hurricane winds and the pressure they generate. As winds increase, pressure against objects is added at a disproportionate rate. Pressure against a wall mounts with the square of windspeed so that a threefold increase in windspeed gives a nine-fold increase in pressure. Thus, a 25 mph wind causes about 1.6 pounds of pressure per square foot. A four by eight sheet of plywood will be pushed by a weight of 50 pounds. In 75 mph winds, that force becomes 450 pounds, and in 125 mph winds, it becomes 1,250 pounds. For some structures, this force is enough to cause failure. These winds will weaken after landfall due to loss of warm-water energy source; and the encountering of great friction over land.
Heavy rains and ocean waters brought ashore by strong winds can cause flooding in excess of 50 cm (20 in) over a 24-hour period. The runoff systems in many cities are unable to handle such an increase in water because of the gentle topography in many of the coastal areas where hurricanes occur. Hurricanes are capable of producing copious amounts of flash flooding rainfall. During landfall, a hurricane rainfall of 10 to 15 inches or more is common. If the storm is large and moving slowly-less than 10 mph-the rainfall amounts from a well-organized storm are likely to be even more excessive. To get a generic estimate of the rainfall amount (in inches) that can be expected, divide the storm's forward motion by 100, i.e. Forward Speed/100 = estimated inches of rain. Rainfall and Flooding fact: Tropical Storm Claudette (1979) brought 45 inches of rain to an area near Alvin, Texas, contributing to more than $600 million in damage.
The heaviest rain usually occurs along the coastline, but sometimes there is a secondary maximum further inland. This heavy rain usually occurs slightly to the right of the cyclone track and usually occurs between 6 hours before and 6 hours after landfall. The amount of rain depends on the size of the cyclone, the forward speed of the cyclone and whether it interacts with a cold front. Interaction with a cold front will not only produce more tornadoes but more rainfall as well.
Note: Buying flood insurance is the best thing you can do to protect your home, your business, family, and financial security.Storm surge is an abnormal increase in the ocean's level, sometimes in excess of several meters high and miles wide. Storm surges can come ashore up to five hours before the storm and destroy low-elevation coastal areas. It is especially damaging when the storm surge occurs during high tide and consequently is often responsible for most hurricane-related deaths. 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. Storm surge can range from 4 to 6 feet for a minimal hurricane to greater than 20 feet for the stronger ones. 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, even more so than the high winds.
Hurricanes also produce tornadoes, which add to the hurricane's destructive power. Typically, the more intense a hurricane is, the greater the tornado threat. When a hurricane brings its winds inland, the fast-moving air hits terrain and structures, causing a frictional convergence which enhances lifting. Frictional convergence may be at least a contributing factor to tornado formation in hurricanes. The greatest concentration of tornadoes occurs in the right front quadrant of the hurricane. A number of theories exist about their origin, but in the case of Hurricane Andrew, severe damage was inflicted by small spin-up vortices that developed in regions of strong wind-shear found in the hurricane's the eye wall. The strong damaging winds of the hurricane frequently cover the smaller tornado paths, making the separation of their damaging effects very difficult.
Tropical cyclone spawned tornadoes facts
