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A rescue from floods caused by Hurricane HarveyA rescue from floods caused by Hurricane HarveyFlooding occurs whenever an area that is usually dry land becomes submerged under water. Some floods happen suddenly and recede quickly. Others take days or even months to build and recede. When floods happen in an area where many people live, they can be devastating. The sheer volume of water is sometimes enough to carry away cars, trees, houses, bridges—even people. Heavy rain for a short period or light rain for many days and weeks can both result in floods. Rivers may overflow their banks when there is more water upstream than usual. But floods are often the result of catastrophic events such as a tsunami, hurricane or a collapsing dam.

Water on saturated ground draining away Water on saturated ground draining away

Conditions for flooding

Floods happen on flat or low-lying areas when rainwater falls, or snow melts more rapidly than the water can either infiltrate (soak in) or run off (flow across) the ground. This may happen if the underlying rock is impermeable—lacking pores, tiny holes that allow water through. It may also occur if the soil is frozen solid, already saturated with water (particularly where the water table is already high) and can absorb no more, or is so hard and dry that it acts like impermeable rock.

The Johnstown Flood, which took place in Pennsylvania on 31st May 1889 when the South Fork Dam collapsed after several days of heavy rainfall, took the lives of 2209 people. The disaster was the largest loss of civilian life in the United States at the time, and has since been surpassed only by two events: the 1900 Galveston hurricane and the 9/11 terrorist attacks in 2001.

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