E A V Y R A I N A N D
F L A S H F L O O D S
Floods are the deadliest natural disasters, killing more people in the United States per year than tornadoes, hurricanes, windstorms or lightning. In recent years, on average more than one hundred people per year have died in the U.S. during floods, especially flash floods. Sixty percent of those killed died trapped in vehicles or while trying to escape from automobiles stalled in rising water. As little as two feet of water can float a car or truck, possible sweeping it to deeper water or overturning it. Water moving at 4 miles per hour, a brisk walking pace, pushes on anything in its path with a force of 66 pounds per square foot. At double that speed the force increases dramatically to 264 pounds per square foot.
Floods happen when water breaks free of its banks or boundaries and drenches land that is usually dry. Because seventy-five percent of the Earth is covered by water in the form of oceans, lakes, rivers, streams, rainforests, swamps, glaciers, and springs, floods are common and expected occurrences. But throughout history, humans have chosen to live near bodies of water for their considerable benefits: the fertile soil left when the waters recede, easy access to shipping and trade, fishing, cooking, bathing, washing clothes, plentiful drinking water, and the aesthetic value of the waters. But there are drawbacks to living by a river or lake or ocean: the land onto which their waters spill is called a floodplain, and people who live on floodplains must, as the name implies, be prepared for the eventuality of a flood. Even when dams, dikes, and seawalls have been built to reduce the severity of a flood, the water contained in one place often floods the land upstream or downstream. When floods descend on populated areas that are insufficiently prepared, they become natural disasters. The severity and lethality of floods in such places is dependent on weather, the grounds ability to soak up moisture, and the people and buildings in the area of the floodplain.
Floods originate in the atmosphere, where the unequal heating of Earths surface by the Sun causes air and moisture to circulate. Though the quantity of water on the planet remains constant, the amount of water in any given location is always in flux due to this process, called the water cycle. (See Marshall Shepherds presentation of the water cycle in Live From The Storm program 1, and Activity 1.3 in the Teachers Guide.) The Sun heats water until the liquid becomes a gas, water vapor. This vapor then mixes with tiny salt, soil and smoke particles and, when the temperature cools, condenses on the particles into liquid water drops, which, in turn, create a cloud. (See Activity 2.1 in the Teachers Guide, Creating Clouds.) In the daily course of the water cycle, 4,200 billion gallons of water (out of the 40,000 billion gallons which exist in the Earths atmosphere) fall to the ground as precipitation, 2,800 billion gallons evaporate into the air, and 1,400 gallons soak into the ground or run back into bodies of water, continuing the cycle.
Particularly heavy rains can result in abnormally high water levels; heavy snowfall can cause damage in the spring when rising temperatures cause snowmelt. The ground in a particular area may become saturated, causing unabsorbed water to pool, when a warm or cold front stalls over it, dumping a large amount of precipitation, or when a quickly moving storm keeps passing over the same area in a phenomenon called "training," drenching the land below.
Saturationand floodingis also dependent on the grounds ability to accept copious amounts of moisture. Precipitation sinks into the ground into plant roots and through layers of soil and porous rock, filling underground holes until this groundwater hits solid rock and cant sink any further. The soil and roots become saturated until they cant absorb any more moisture, and so the underground water table moves the only way it can, up, to the surface where it forms puddles and other bodies of water. In places where there is a thick layer of soil and porous rock between the surface and the solid rock, more moisture can be absorbed. The opposite is true for rocky areas like mountains or canyons where there is little to no soil. When no more water can be absorbed, the excess runs off into oceans and other bodies of water, and their water levels increase.
Soil that is parched and hardened can accept less moisture than soil that is already wet, and dryness of the Earth is a factor in flash floods in the Western U.S.a seemingly small amount of precipitation can trigger a flood if it is not absorbed quickly. Floods usually occur in the Eastern U.S., on the other hand, because of the huge amounts of precipitation which saturate the moist ground until the soil can hold no more water. The Great Flood of 1997 was occasioned by the melting of the 100 inches of snow dumped by 8 blizzards in the area around Grand Forks, North Dakota, during the previous winter.
River floods often build gradually and can be measured by scientists, who pay attention to rain and snowfall and calculate the rising water levels and ground saturation to make a prediction about when and if a river will overflow its banks. If a season has been especially rainy or snowy, conditions are ripe for this largest and most severe type of flood. Being able to predict what will happen means that people are usually prepared, but river floods are still extremely dangerous because the power of the water builds as the water levels rise. Pressure builds, especially at the bottom of the river, and as the water rushes along the riverbed, whirlpools may form. The river can act like a cannon, spitting out tree trunks and other objects at high speeds. The raging water carries with it the debris it has collected along the way: uprooted trees, mud, rocks, even human refuse like old car parts or machinery.
Depending on the severity of the flood, the polluted river will cover enormous tracts of land before its pent-up energy is expended, dragging along whatever gets in its way. During the Great Flood of 1927, the enormous Mississippi Riverwhich, including tributaries, touches 31 statesoverwhelmed its banks after months of heavy rain. Floodwater covered 26,000 square miles from April to June, killing close to a thousand, but a million more received advanced warning and evacuated their homes in time.
The Mississippi River and its tributaries were severely flooded again in 1993, after heavy rains fell in the river basinthe land outlying a riverfrom April through July. Parts of Iowa, Illinois, Minnesota, Nebraska, North and South Dakota and Wisconsin, got twice as much rain as usual. From June 1 to August 1, rain fell in some places at 4 to 5 times the average amount. As a result, more than 100 rivers overflowed their banks.
"Ice-jam floods" are a colder-weather version of river floods, because they are caused by melting snow and river ice. During the winter, ice forms and thickens on a river, and there is usually snowfall on top of that. Come spring, the temperatures rise and rain falls, melting the snow and breaking apart the melting ice into huge chunks, which are carried downstream on the current of the river. If these ice blocks reach a point too narrow for them to pass throughlike a bridge, dam, or a bend in the riverthey jam together, forming a sort of dam themselves so that the water behind the ice wall is forced to spill over the banks.
Unlike most river floods, flash floods happen suddenly. Because they can occur anywherefrom mountains to desertsand at any time, they are difficult to predict. They can be triggered by huge storms dropping a lot of precipitation in a relatively short time, like when a storm stalls or repeatedly passes over the same area. In June, 1998, a months worth of rain drenched Princeville, Oregon, in a single day. As a result, the Ochoco Creek quickly overwhelmed its banks and swept through the town, home to 5,400 people. 400 houses were damaged; others were destroyed.
The ground saturation level can make the situation better or worse; in the latter case, a flash flood can reach its height in a matter of minutes. In July, 1967, 3,000 people were celebrating the Colorado State centennial in the Thompson River Canyon, in the Colorado Rocky Mountains. An unexpected 3-hour thunderstorm poured 12 inches onto the rocky, non-absorbent surface, causing a wall of water 19 feet high to rush along the canyon with enough strength to move boulders up to 10 feet wide. 139 people were killed and 6 were never found. Many of those who died were trapped in their cars; signs posted in the canyon today warn visitors to abandon their vehicles and climb to higher ground at the very first signs of a flash flood.
Dam failures can also cause flash floods. On May 31, 1889, the South Fork Dam, built less than half a century earlier about 20 miles upriver from Johnstown, Pennsylvania, gave way after an overnight rainstorm. A wall of water 70 feet high in places crashed down the Appalachian Mountains, carrying with it the trees, houses, rocks, people and animals it had swept up along the way. Floodwaters 23 feet high and half a mile wide swept over the town, killing 2,209 people in one of the deadliest flash floods in history.
Coastal floods also happen quickly, but since they are usually caused by storms such as hurricanes, they are easier to predict than flash floods. People who live in coastal flood zonesany place touching an oceanare used to the threat of elevated water levels and waves and have prepared accordingly. In California, seawalls have been built to protect people and their homes as well as prevent beach erosion. Hurricanes are almost always accompanied by coastal flooding, as high winds and waves batter shore communities and drag debris out to sea. In the Eastern U.S., storms in the fall, winter, and early springoften called Noreasters for the rain, wind, and snow they bring from the northeastare also notorious triggers for floods along the coasts of states from Florida up to Maine.
A less predictable form of coastal flooding is caused by tsunamis, huge waves induced by seismic or volcanic activity deep in the ocean, or by meteorites or nuclear explosions. Tsunamis are as rare but occur most often near Alaska, Hawaii, and parts of the northwest coast of the U.S.
For more information on how to be safe in the event of a flood, see the Federal Emergency Management Agencys weather site for kids at www.fema.gov/kids, and the Weather Channel Safety home page at www.weather.com/safeside.