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Floods: Flash Floods
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Floods usually are local, short-lived events that can happen suddenly, sometimes with little or no warning. They usually are caused by intense storms that produce more runoff than an area can store or a stream can carry within its normal channel. Rivers can ... flood when dams fail, when ice jams or landslides temporarily block a channel, or when snow melts rapidly. In a broader sense, normally dry lands can be flooded by high lake levels, by high tides, or by waves driven ashore by strong winds. Small streams, particularly in the Southwest, are subject to flash floods (very rapid increases in runoff), which may last from a few minutes to a few hours. On larger streams, floods usually last from several hours to a few days.
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Floods are among the most frequent and costly natural disasters in terms of human hardship and economic loss. (Floods) Peoples’ lack of education about flooding preparation and the instability of predicting floods has led to a $3.1 billion annual cost for the United States alone (Floods) and more than 1.6 million deaths worldwide in one year. (Confronting Natural Disasters: An International Decade for Natural Hazard Reduction) Excluding droughts, almost ninety per cent of damages relating to natural disasters is caused directly or indirectly by floods. Seventy-five per cent of the time that a President of the United States declares an area a disaster, it is due to flooding. Floods and flash floods kill more people in this country than any other natural disaster. The damage and death toll seem to be rising.
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Floods can occur at any time, but weather patterns have a strong influence on when and where floods happen. Cyclones, or storms that bring moisture inland from the ocean, can cause floods in the winter and early spring in the western United States. Thunderstorms are relatively small but intense storms that can cause flash floods in smaller streams in late summer and fall in the Southwest. Frontal storms form at the front of large, moist air masses moving across the country and can cause floods in the northern and eastern parts of the United States during the winter and spring. Hurricanes are intense tropical storms that can cause floods in the Southeast during the late summer and fall.
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Generally speaking there are two types of floods, 1) where water slowly rises and spills over the banks of a stream or river and 2) flash floods. Floods can occur at anytime of year, but particular seasonal weather patterns are more conducive to the creation of floods than others in different geographic regions. In the United States, cyclonic storms roaring off the ocean and into the Pacific coast states during the winter and early spring can cause flooding. In the southwest, summer and fall thunderstorms release torrents of water that rush down dry stream beds or arroyos as flash floods. Flooding can occur in the north central states during the winter as rain fall or snow melt runs off the frozen ground surface, or ice jams rivers causing them to flood. Flooding in the mid portion of the United States tends to occur in spring and summer as polar front cyclones march across the North American continent.
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Periodic floods occur naturally on many rivers, forming an area known as the flood plain. These river floods usually result from heavy rain, sometimes combined with melting snow, which causes the rivers to overflow their banks. A flood that rises and falls rapidly with little or no advance warning is called a flash flood. Flash floods usually result from intense rainfall over a relatively small area, as happened in 2007 with the Sudan floods. Coastal areas are occasionally flooded by high tides caused by severe winds on ocean surfaces, or by tsunami waves caused by undersea earthquakes. There are often many causes for a flood.
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Debris or landslide floods are created by the accumulation of debris, mud, rocks, and (or) logs in a channel, which form a temporary dam. Flooding occurs upstream as water becomes stored behind the temporary dam and then becomes a flash flood as the dam is breached and rapidly washes away. Landslides can create large waves on lakes or embayments and can be deadly. Mudflow floods can occur when volcanic activity rapidly melts mountain snow and glaciers, and the water mixed with mud and debris moves rapidly downslope. These mudflow events are ... called lahars and, after the eruption of Mt. St. Helens in 1980, caused significant damage downstream along the Toutle and Cowlitz Rivers in southwest Washington.
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