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  3. Floods Water is one of the most useful things on Earth. We drink it, bathe in it, clean with it and use it to cook food. Mos

Floods Water is one of the most useful things on Earth. We drink it, bathe in it, clean with it and use it to cook food. Mos

admin2009-04-23  58
问题                                              Floods
    Water is one of the most useful things on Earth. We drink it, bathe in it, clean with it and use it to cook food. Most of the time, it is completely beneficial to the humans. But in large enough quantities, the very same stuff we use to wash a toothbrush can overturn cars, destroy houses and even kill. Flooding has claimed millions of lives in the last hundred years alone, more than any other weather phenomenon.
How water exists on our planet
    The total amount of water on Earth has remained fairly constant for millions of years (though its distribution has varied considerably in that time). Every day, a very small amount of water is lost high in the atmosphere, where intense ultraviolet rays can break a water molecule apart, but new water is also emitted from the inner part of the Earth, by volcanic activity. The amount of water that is created and the amount that is lost are pretty much equal.
    At any one time, this volume of water is in many, different forms. It can be liquid, as in oceans, rivers and rain; solid, as in the glaciers of the North and South Poles; or gaseous, as in the invisible water vapor in the air. Water changes from state to state as it is moved around the planet by wind currents.
Wind currents
    Wind currents are generated by the heating activity of the sun. The sun shines more on the area around Earth’s equator than it does on areas farther north and south, causing a heat discrepancy over the surface of the globe. In warmer regions, hot air rises up into the atmosphere, pulling cooler air into the unoccupied space. In cooler regions, cold air sinks, pushing/driving warmer air into the unoccupied space. The rotation of the Earth breaks this cycle up, so there are several, smaller air-current cycles all along the globe.
    Overall, wind currents in the atmosphere are fairly consistent. At any particular time of year, currents tend to move in a certain way across the globe. Consequently, specific locations generally experience the same sort of weather conditions year to year. But on a day-to-day basis, the weather is not so predictable. Wind currents and precipitation(降水) are affected by many factors, chiefly geography and neighboring weather conditions. A huge number of factors combine in an infinite variety of ways, producing all sorts of weather. Occasionally, these factors interact in such a way that an atypical volume of liquid water collects in one area. For example, conditions occasionally cause the formation of a hurricane, which dumps a large quantity of rain wherever it goes. If a hurricane lingers over a region, or multiple hurricanes happen to move through the area, the land receives much more precipitation than normal.
Waterways
    Since waterways are formed slowly over time, their size is proportionate to the amount of water that normally accumulates in that area. When there is suddenly a much greater volume of water, the normal waterways overflow, and the water spreads out over the surrounding land, At its most basic level, this is what a flood is—an abnormal accumulation of water in an area of land.
Sources of floods
    As has been mentioned above, 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.
    -Rainstorms
    The sort of flooding that most people are familiar with occurs when an unusually large number of rainstorms hit an area in a fairly short period of time. In this case, the rivers and streams that divert the water to the ocean are simply overwhelmed. The varying temperatures of different seasons lead to different weather patterns. In the winter, for example, the air over the ocean might be warmer than the air over the land, causing the wind flow to move from the land out to sea. But in the summer, the air over the land heats up, becoming warmer than the air over the ocean. This causes the wind current to reverse, so that more water from the ocean is picked up and carried over land. This monsoon(季风) wind system can cause a period of intense rain that is completely out of step with the climate the rest of the year. In some areas, this flooding may be worsened by excess water from melting snow.
    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(淤泥) 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 lets it out throughout the year. This has extended the planting season so that Egyptian farms can grow crops year-round.
    -Unusual tidal activity
    Another common source of flooding is unusual tidal activity that extends the reach of the ocean farther inland than normal. This might be caused by particular wind patterns that push the ocean water in an unusual direction. It can also be caused by tsunamis(海啸), large waves in the ocean triggered by a shift in the Earth’s crust.
    -Break of man-made dams
    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.
Severity of a flood
    The severity of a flood depends not only on the amount of water that accumulates in a period of time, but also on the land’s ability to deal with this water. As we’ve seen, one element of this is the size of rivers and streams in an area. But an equally important factor is the land’s absorbency. When it rains, soil acts as a sort of sponge. When the land is saturated—that is, has soaked up all the water it can—any more water that accumulates must flow as runoff.
    Some materials become saturated much more quickly than others. To see how this works, just take a bucket of water outside and try wetting various surfaces. Soil in the middle of the forest is an excellent sponge. You could dump several buckets of water on it and it would soak the water right up. Rock is not so absorbent—it doesn’t seem to soak up any water at all. Hard clay falls somewhere in between. Generally, soil that has been tilled for crops is less absorbent than uncultivated land, so farm areas may be more likely to experience flooding than natural areas.
选项 A、Y
B、N
C、NG
答案A
解析 第1段最后一句：Hooding has claimed millions of lives in the last hundred years alone，more than any other weather phenomenon．题目中的destructive概括了原文中的claimed millions of lives，而原文中的more than any other…其实表示的是最高级的概念，题目与原文意思一致，因此本题答案为Y。
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