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第一段
1 .Listen to part of a lecture in an environmental science class.
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第二段
1 .Now, there is growing interest these days in generating electricity from renewable energy sources, right? From developing wind farms to tapping into an underground source of geothermal energy.
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2 .And when you're considering a new project, it's important to look at the costs as well as benefits of developing that energy source.
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3 .Let me give you an example of the kind of thing I am talking about.
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第三段
1 .There is currently a lot of interest in harnessing the power of the oceans,
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2 .of the ocean tides, that is,
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3 .the movement of huge amounts of water which causes the water level of oceans to rise and fall.
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4 .The idea is that if we can harness that tidal energy,
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5 .it'd be a great clean renewable energy source.
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第四段
1 .One place where this tidal energy can be harnessed is at a shallow body of water, such as an estuary.
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第五段
1 .Now, can anyone tell us what an estuary is? Yes, Ted.
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第六段
1 .An estuary is where a river enters the ocean.
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2 .The fresh water meets the ocean water.
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3 .Sometimes it is covered in water. Sometimes it is not.
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第七段
1 .Some parts of the estuary, as the tides go in and out, but other parts are always submerged.
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第八段
1 .Now, estuaries are an ideal place to try to capture energy from changes in tides because,
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2 .well, there is an exceptionally large difference between the water level at high tide and at low tide.
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3 .All that movement of water generates a lot of energy.
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4 .And one way to harness that energy is by building a structure called a barrage there.
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第九段
1 .A barrage is basically a large low dam that's built across an estuary.
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2 .When the tides go in and out, the moving water flows through tunnels in the barrage.
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3 .So you have huge amounts of water trying to flow through these relatively small tunnels and that turns turbines that generate electricity.
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第十段
1 .Now, these estuaries are important because of their high level of biological productivity.
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2 .They are home to lots of birds, fish and other marine life.
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3 .So when you propose to construct a barrage, you have lots of issues to consider.
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第十一段
1 .For example, it would change the existing water levels in the estuary.
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2 .Since a lot of water is getting held up by the barrage, the incoming tides won¡¯t go as high,
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3 .but they wouldn't be as low during low tide, either.
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4 .This might help prevent flooding. But it would also affect the mudflats, those areas of mud that normally are exposed when the tide recedes.
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第十二段
1 .But don't lots of birds rely on exposed mudflats for food?
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第十三段
1 .I mean, don't they eat tiny animals that live in the mud?
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2 .And what about those tiny animals?
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3 .What would happen to them if the mudflats were endangered?
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第十四段
1 .So you are seeing some of the potential problems with the barrage.
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2 .But consider this, right now, the water in an estuary is very cloudy.
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3 .The tidal currents are constantly churning up the sediments that rivers deposit in the estuary.
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4 .But a barrage would reduce the tidal currents. So a lot of that sediment would settle to the floor of the estuary.
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5 .It wouldn't get stirred up so much.
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6 .The water would be clearer,
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7 .allowing more sunlight to reach deeper into the water,
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8 .which might lead to more food for birds and other animals,
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9 .attracting new wildlife to the area.
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10 .So it;s a complicated environmental picture.
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第十五段
1 .Have they tried this anywhere? Built a barrage?
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第十六段
1 .Yes, there are several in operation. There is one in France.
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第十七段
1 .Now, they have to be careful there about how they turn on the turbines because they create currents and waves that can affect boats.
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第十八段
1 .But I haven't read about any major ecological problems.
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2 .In fact, the fishing is supposed to have improved.
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3 .Plus there is even more bird life.
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第一十九段
1 .But some of the barrages we're considering now would be much larger than that one.
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2 .There has been one proposed for the estuary of Great Britain's Severn River, one of the largest estuaries in the world.
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3 .It would be 16 kilometers long.
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4 .Just to give you an idea, the barrage in France is less than a kilometer.
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第二十段
1 .Outside of environmental concerns, such a project would be hugely expensive and that's another argument against barrages,
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2 .well, such large ones anyway.
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3 .Critics say that it would better to use that money for something else,
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4 .such as improving the energy efficiency of buildings.
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5 .You could make a lot of buildings more efficient with all that money and that would reduce the need for electricity.
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