一篇关于洪水的英语报告急用!

一篇关于洪水的英语报告急用!
一篇关于洪水的英语报告
急用!

一篇关于洪水的英语报告急用!
路过……贴篇给你
源自微软百科：
When it rains or snows,some of the water is retained by the soil,some is absorbed by vegetation,some evaporates,and the remainder,which reaches stream channels,is called runoff.Floods occur when soil and vegetation cannot absorb all the water; water then runs off the land in quantities that cannot be carried in stream channels or retained in natural ponds and constructed reservoirs (see Dam).About 30 percent of all precipitation is runoff,and this amount may be increased by melting snow masses.Periodic floods occur naturally on many rivers,forming an area known as the flood plain.These river floods often result from heavy rain,sometimes combined with melting snow,which causes the rivers to overflow their banks; a flood that rises and falls rapidly with little or no advance warning is called a flash flood.Flash floods usually result from intense rainfall over a relatively small area.Coastal areas are occasionally flooded by unusually high tides induced by severe winds over ocean surfaces,or by tsunamis caused by undersea earthquakes (see Earthquake).
Floods not only damage property and endanger the lives of humans and animals,but have other effects as well.Rapid runoff causes soil erosion as well as sediment deposition problems downstream.Spawning grounds for fish and other wildlife habitat are often destroyed.High-velocity currents increase flood damage; prolonged high floods delay traffic and interfere with drainage and economic use of lands.Bridge abutments,bank lines,sewer outfalls,and other structures within floodways are damaged,and navigation and hydroelectric power are often impaired.Financial losses due to floods are commonly millions of dollars each year.
The ancient Chinese built levees to raise the banks of the Huang He (Yellow River) on the supposition that the confined river would then deepen its channel to contain the maximum flow.The result,however,was a raising of the riverbed,because the sedimentary deposit of alluvial soil previously distributed over the entire flood plain during annual flooding was confined to the river bottom.In 4000 years the level of the river rose as high as 21 m (70 ft) above the surrounding plain.In 1887 one of the worst floods in recorded history occurred when the Huang He broke through the levees,killing more than a million people.Levees were constructed during the Middle Ages on the Po,Danube,Rhine,Rhône,and Volga rivers and have been supplemented in modern times by reforestation and by storage reservoirs.Levees are still in extensive use,notably on the Mississippi,where the river has been confined to a narrow channel to provide the depth necessary for navigation.Maintaining that depth has required repeated dredging of the channel,adding to the already large cost of sustaining the levee system.
Floods in the Mississippi Valley have demonstrated that levees alone do not provide sufficient protection against flooding on a large river,and other methods of flood control,including dams and floodways,are now in use on the Mississippi River (see Levee).However,flood-control measures failed to contain the great flood of the summer of 1993,one of the worst in United States history.Swelled by record spring rains,the Mississippi and Missouri rivers and many of their tributaries overflowed their banks,inundating an estimated 8 million acres by early August.The raging floodwaters also inflicted major damage on levees,dams,and floodways,ruined an additional 12 million acres of cropland,and caused over $10 billion in damage.At its height,the Missouri river crested almost 15 m (almost 50 ft) above its banks; south of Illinois and Missouri,where the riverbed is very wide and lined with levees,there was only minor damage.