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The Causes of Floods
By Sarah Anderson
Overall, the water cycle is a balanced system. Water flowing into one part of the cycle, such as streams, is balanced by water flowing back to the ocean. Sometimes the amount flowing in to one area is greater than the capacity of the system to hold it within natural confines. The result is a flood.
Combinations of factors, along with exceptional precipitation, can also lead to flooding. For example, heavy snow melts, water saturated ground, unusually high tides, and drainage modifications when combined with heavy rain can lead to flooding.
Areas along coastlines become subject to flooding as a result of heavy storms, tsunamis, hurricanes, and unusually high tides. In addition, long term processes like subsidence and rising sea level as a result of global warming can lead to the encroachment of the sea onto the land.
Dam & Levee Failures
Whether a dam is constructed by humans or nature or both, it may fail with the sudden release of water into the downstream drainage, leading to a flood. Levee systems are designed to prevent flooding; however, levees can fail and lead to catastrophic flooding.
Floods will occur when a river overflows its banks. Floods happen when soil and vegetation cannot absorb all the water. In these situations water runs off the land in quantities that cannot be carried in stream channels.
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.
Flash floods are floods that rise and fall rapidly, often with limited or no advance warning. Flash floods are usually the result of intense rainfall over a relatively small area and timeframe.