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The worst damage from floods, the loss of life and homes, is caused primarily by the sheer force of flowing water. In a flood, two feet (61 cm) of water can move with enough force to wash a car away, and 6 inches (15 cm) of water can knock you off your feet. It may seem surprising that water, even a lot of water, can pack such a wallop. After all, you can peacefully swim in the ocean without being knocked around, and that's a massive amount of moving water. And in most cases, a flowing river isn't strong enough to knock you over. So why do flood waters behave differently?
Flood
waters are more dangerous because they can apply much more pressure than
an ordinary river or a calm sea. This is due to the massive differences
in water volume that exist during many floods. In a flood, a lot of
water may collect in an area while there is hardly any water in another
area. Water is fairly heavy, so it moves very quickly to "find its own
level." The bigger the difference between water volumes across an area,
the greater the force of movement. But at a particular point, the water
doesn't look so deep, and so doesn't seem particularly dangerous --
until it's too late. Nearly half of all flood deaths result from people
attempting to drive their cars through rushing water. There is much more
water in the ocean than in a flood, but it doesn't knock us over because
it is fairly evenly distributed -- water in a calm sea isn't rushing to
find its own level.
One
of the worst flash floods in U.S. history occurred in 1976, in Big
Thompson Canyon, Colorado. In less than five hours, thunderstorms in
nearby areas dumped more rain than the region ordinarily experiences in
a year. The Big Thompson River, normally a shallow, slow-moving
waterway, abruptly transformed into an unstoppable torrent, dumping
233,000 gallons (882,000 L) of water into the canyon every second.
Thousands of campers had gathered in the canyon to celebrate the
centennial of the state of Colorado. The flood happened so quickly that
there was no time to issue a warning. When it hit, hundreds of people
were injured, and 139 were killed.
In
1966, a major storm flooded the Arno, an Italian river that runs through
the city of Florence. The small city, one of the art capitals of the
world, was overrun with water, mud and general slime. In addition to the
loss of life and the damage to buildings, there was a great deal of
damage to the city's art collection. Mud and slime covered almost
everything stored in the city's basements and ground-level rooms.
Through many years of work, scientists and art historians have been able
to restore most of the damaged artifacts to good condition. Source: http://science.howstuffworks.com |
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