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What causes flooding?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. Technically, a flood occurs whenever water in excess of the expected amount comes into an area. Complex weather patterns can increase rainfall, which leads to flooding. A river naturally accommodates a certain volume - - it has been formed over millennia to handle that amount of water. If an unexpected rush of water arrives in the waterway, the water exceeds the walls of the channel and the surrounding area floods. Another way rainfall can cause flooding is if precipitation falls faster than the ground can absorb it. Both earthquakes and strong winds can create tsunamis (huge waves in the ocean). A tsunami can crash over a coastal area and flood it. Floods can also be caused if dams or levees, which are intended to control the flow of excess water, break or fail.

    There are three key factors that determine the severity of a flood: water volume, time span and the space the water has to fill. How much rain or water is present relative to what the area can usually handle? This factor is affected by the span of time over which the water enters the system. For example, a city may receive its usual annual rainfall over the course of a year with no problem. However, if this same amount of rainfall occurs over a two-day period, it will cause massive flooding. The severity of a flood is also directly affected by where the water has to go. Areas with greater absorbency, like forests and fields, can handle more rain. Developed areas, which are usually covered in asphalt or concrete, can't absorb much water. In a paved area, water can collect in a quick and dangerous way - - especially if the area lacks an adequate drainage infrastructure (such as a system of ditches). Also, built-up areas are more likely to force the floodwater into buildings, rather than allowing water to flow into wide-open spaces.

    The climate can also affect flooding. When land temperatures rise and remain higher than ocean temperatures (which usually happens in the summer), wind currents tend to blow from the ocean toward the land. This allows more water vapor to collect over land, causing increased rains. Increased rains, of course, can lead to flooding. This system, in which water vapor in the air moves from the cool ocean to the warm land, is called the monsoon wind system.

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