A flood is the accumulation of too much water in too little time in an area where the land is normally dry. Floods are extremely common all over the world (they are the most common and widespread natural disaster execpt for fire), and are caused by a variety of things, depending on the type of flood that they are. In some equatorial countries (Bangladesh for example), the monsoon rains can cause bad flooding because the rain comes down fairly steadily for a long period of time. The water level in a certain area can rise extremely fast or slow, but generally, they develop over several days. Floods are most common near water, downstream from a dam, or in land that has a low elevation. There are several types of floods, these are: regional floods, flash floods, ice-jam floods, storm surge floods, and debris landslide and mudflow floods.
Regional floods usually occur seasonally; rain from the winter and spring added to the melting snow fill the river basins with more water than they can hold and the banks overflow. If the ground is still frozen, less water can be absorbed into the soil, which increases the runoff. This is exactly what happened in March of 1936 in a New England flood where more than one hundred and fifty people were killed, and the total property damage was three hundred million dollars. Long periods of time with excessive rain can also help to create a regional flood because all the rain saturates the soil so that any more that is accumulated runs into streams and rivers, and overflows the banks. Slow-moving, low pressure or frontal storm systems like hurricanes that are dying off, and tropical storms can help create a regional flood.
Flashfloods can occur with very little, or no warning, and they can easily become their strongest in minutes. Large amounts of rain falling on a particular area in a short period of time is the main factor than triggers a flashflood, but characteristics like surface conditions, topography, and the slope of the land can make a flashflood more likely in a certain area. Urban areas have a larger risk than other places because streets, roofs, and parking lots are a good place for water to runoff and gather. Mountainous areas also have a larger risk because the ground has a steep slope, and runoff water can go into a narrow canyon, and then the canynon can overflow. If you are standing below a flashflood when it starts, it will look like a wall of water is desending upon you. The waves of water can become up to ten or twenty feet tall, and usually a bunch of debris will be swept along with it. Flashfloods can be extremely deadly because of the sudden rise in water level, and the velocity of the water is so great, that a pretty small amount can easily knock a person off their feet and make it impossible for them to get back up. In Rapid City, South Dakota in 1972, only fifteen inches of rain over five hours from thunderstorms that were moving slowly caused a flashflood that killed two hundred thirty seven people.
Ice jam floods occur on rivers that are at least partially frozen. If the water level rises, the ice will break up and then pile together in shallow places to block other things that might be coming downstream, like logs. The combination of the ice, logs, and other debris will block the river and keep the water from flowing by. Eventually the natural dam will break and the water behind it will be released. With all this water being released at the same time, the flood becomes a flashflood with large ice chunks in it that can badly hurt people or other objects. During the spring break-up of the Yukon River in Alaska in 1992, ice jams caused severe flooding.
Storm surge flooding is caused by water pushed up on land that is usually dry by a storm surge from a storm. Dam and Levee (An embankment raised to prevent a river from overflowing)failure floods occur if more water piles up behind the dam or levee than the structure is built to hold. The water will spill over the dam, and most likly, the force of the water will break the dam, letting all the water inside pour out. In this case, the flood becomes a flash flood. Debris and landslide floods are created when debris, mud, rocks and maybe logs pile up over a river and form a temporary dam. As the water gathers behind the debris, a flood begins, and when the dam breaks and the water is released, it becomes a flashflood.
The first picture is of a flood where the water has risen to the roof of a house, courtesy of http://www.ccrh.org/comm/sand/images/flood.jpg The second picture is of a flashflood, courtesy of http://www.mckerracher.org/images/flashflood.jpg