A. To explain the mechanical functioning of barrages
B. To discuss some possible ecological effects of building barrages
C. To discuss the effects of ocean tides on coastal ecosystems
D. To describe ways to increase biological productivity of estuaries
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NARRATOR:Listen to part of a lecture in an environmental science class.
FEMALE PROFESSOR:Now, there's 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.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.
Let me give you an example of the kind of thing I'm talking about.There's currently a lot of interest in harnessing the power of the oceans—of the ocean tides, that is...the movement of huge amounts of water, which causes the water level of oceans to rise and fall.The idea is that if we can harness that tidal energy, it'd be a great, clean, renewable energy source.One place where this tidal energy can be harnessed is at a shallow body of water, such as an estuary.
Now, can anyone tell us what an estuary is? Yes, Ted.
MALE STUDENT:An estuary's where a river enters the ocean... the fresh water meets the ocean water.Sometimes it's covered in water; sometimes it's not...
FEMALE PROFESSOR:Some parts of the estuary—as the tides go in and out... but other parts are always submerged.Now, estuaries are an ideal place to try to capture energy from changes in tides because, well, there's an exceptionally large difference between the water level at high tide and at low tide.All that movement of water generates a lot of energy.And one way to harness that energy is by building a structure called a barrage there.
A barrage is basically a large low dam that's built across an estuary.When the tides go in and out, the moving water flows through tunnels in the barrage.So you have huge amounts of water trying to flow through these relatively small tunnels...and that turns turbines that generate electricity.
Now, these estuaries are important because of their high level of biological productivity.They're home to lots of birds, fish, and other marine life.So when you propose to construct a barrage, you have lots of issues to consider.For example, it would change the existing water levels in the estuary...since a lot of water is getting held up by the barrage, the incoming tides won't go as high... but they wouldn't be as low during low tide, either.This might help prevent flooding, but it would also affect the mud flats—those areas of mud that normally are exposed when the tide recedes.
MALE STUDENT:But don't lots of birds rely on exposed mud flats for food? I mean, don't they eat tiny animals that live in the mud?And what about those tiny animals?What would happen to them if the mud flats were endangered?
FEMALE PROFESSOR:So you're seeing some of the potential problems with a barrage.But consider this...Right now the water in an estuary is very cloudy.The tidal currents are constantly churning up the sediments that rivers deposit in the estuary.But a barrage would reduce the tidal currents, so a lot of that sediment would settle to the floor of the estuary; it wouldn't get stirred up so much.The water would be clearer... allowing more sunlight to reach deeper into the water... which might lead to more food for birds and other animals...attracting new wildlife to the area.So it's a complicated environmental picture.
MALE STUDENT:Have they tried this anywhere... built a barrage?
FEMALE PROFESSOR:Yes, there are several in operation.There's one in France.They have to be careful there about how they turn on the turbines, because they create currents and waves that can affect boats.But I haven't read about any major ecological problems.In fact, the fishing is supposed to have improved...plus there's even more bird life.
But some of the barrages we're considering now would be much larger than that one.There's been one proposed for the estuary of Great Britain's Severn River...one of the largest estuaries in the world.It would be 16 kilometers long... just to give you an idea, the barrage in France is less than a kilometer.Outside of environmental concerns, such a project would be hugely expensive.
And that's another argument against barrages—well, such large ones anyway.Critics say that it would be better to use that money for something else, such as improving the energy efficiency of buildings.You could make a lot of buildings more efficient with all that money, and that would reduce the need for electricity.
旁白：请听环境科学课上的部分内容。
教授：这些天人们对利用可再生能源发电的兴趣越来越浓，对吧？从发展风电场到开发源自地下的地热能。在考虑一个新项目时，重要的是要考虑到发展这种能源的成本和收益。
我给你们举个例子解释一下我在说的事情吧。目前很多人对利用海洋中的能源兴趣颇多，利用海洋潮汐，也就是会导致海洋的水平面上升下降的大量水体的运动。想法是如果我们能利用潮汐能，它会成为一种很棒的可再生清洁能源。这种潮汐能可以得到利用的一个地方就是在较浅的水域中，比如入海口。
有人能告诉我们入海口是什么吗？好的，Ted。
学生：入海口是一条河流入海的地方……淡水与海水相遇。有时候它会被覆盖在水中，有时候不会。
教授：当潮汐进进出出时，入海口的一些部分是显露在外的，但是其他部分总是沉没在水中。入海口是捕捉潮汐变化中产生的能量的一个理想地点，因为满潮和低潮的水平面之间有特别大的差异。水体的所有运动会产生很多能量。而利用这些能量的一个方法就是在那里建造一个叫做拦河坝的建筑物。
拦河坝基本上就是建在入海口上的一个很大的低坝。当潮汐进进出出时，移动的水会流过拦河坝里的隧道。所以会有巨大量的水流试图流过这些相对较小的隧道，而这推动了发电的涡轮。
这些入海口之所以很重要是因为它们高水平的生物生产力。它们是很多鸟类、鱼类和其他海洋生物的家园。所以当你提出要建造一个拦河坝时，你有很多问题需要考虑。比如它会改变入海口现有的水位...因为很多水被拦河坝拦住了，流入的潮汐水位就没有那么高了，但它们在低潮时也不会降到那么低。这也许有助于防洪，但它也会影响泥滩，退潮后，那些泥滩一般是暴露在外的。
学生：但是不是很多鸟类都依赖那些暴露在外的泥滩进食吗？我是说它们不是以生活在泥里的微小动物为食吗？那些微小动物呢？如果泥滩受到了危害它们会怎么样呢？
教授：所以你看到了拦河坝带来的一些潜在问题。但是你想啊...现在入海口的水非常浑浊。潮汐的水流不断地搅动河水沉积在入海口的沉淀物。但是拦河坝会减少这些潮汐的流量，所以很多那种沉淀物会沉入入海口底部，不会被那么频繁地搅动起来。水质会更清澈，更多的阳光得以照入水底更深的地方，这可能会给鸟类和其他动物带来更多食物，吸引新的野生动植物来到这个区域。所以这是一个很复杂的环境情况。
学生：他们试过在哪里建造一个拦河坝吗？
教授：有，有好几个正在使用中。法国就有一个。在如何打开涡轮机方面他们必须多加小心，因为它们会制造出影响船只的水流和波浪。但是我还没读到过任何大型的生态问题。事实上，渔业状况应该改善了...而且鸟类更多了。
但是我们现在在考虑的一些拦河坝会比那一个大得多。英国塞文河的入海口提议建立一个拦河坝，这是世界上最大的入海口之一。它将会有16千米长，为了让你们有个对比，法国的那个拦河坝还不到一千米长。除去一些环境方面的担忧，这样的项目会非常昂贵。
而这又是一个反对建设拦河坝的观点，至少是反对那种大型的拦河坝。批评家说把这笔钱用在其他事情上会更好，比如说提高建筑物的能源效率。有了那么些钱能让很多建筑物变得更节能，而那会减少电力的需求。