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In
the last section, we saw that floods occur when an atypical volume of
water collects in an area. There are a number of ways this might happen,
and there are a wide range of events that occur when it does.
Perhaps
the best known example of seasonal flooding is the annual expansion of
the Nile River in Egypt. In Ancient Egypt, monsoon rains at the source
of the river would cause the waterway to extend out a good distance
during the summer. In this case, the flooding was not a disaster, but a
godsend. The expanding waters would leave fertile silt all along the
banks of the river, making the area ideal farming land once the river
had subsided again. This is one of the main factors that allowed
civilization to thrive in the Egyptian desert. These days, the river is
blocked off by a dam upstream, which collects the summer rain and doles
it out throughout the year. This has extended the planting season so
that Egyptian farms can grow crops year-round.
Floods
may also occur when a man-made dam breaks. We build dams to modify the
flow of rivers to suit our own purposes. Basically, the dam collects the
river water in a large reservoir so that we can decide when to increase
or decrease the river's flow, rather than letting nature decide.
Engineers build dams that will stand up to any amount of water that is
likely to accumulate. Occasionally, however, more water accumulates than
the engineers predicted, and the dam structure breaks under pressure.
When this happens, a massive amount of water is released all at once,
causing a violent "wall" of water to push across the land. In 1889, such
a flood occurred in Johnstown, Pennsylvania. The townspeople were warned
that the flood was coming, but many dismissed the alert as unfounded
panic. When the rushing wall of water did hit, more than 2,000 people
were killed in only a few minutes. Source: http://science.howstuffworks.com |
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