Arch dams have been built since ancient times, but never on such a massive scale.

从古早就已经有造拱坝，但从来没有这么大的规模。

First, the arch is filled with air and floated to the desired location.

首先将这个拱坝注入空气使它能够漂到指定的位置。

Not only is installing REHAB easier and faster than building a conventional temporary dam, it is also less wasteful.

设置一道" 重" 拱坝与常规的方法临时围堰相比不仅省时、省工，而且还省钱。

But once local governments agreed, the Bureau determined that Black Canyon's narrow gorge was best suited for an arch-gravity dam.

但，地方政府答应之后，垦务局便决定，狭窄的黑峡谷最适合盖重力拱坝。

That done, a second set of pumps evacuates the gap between the arch and the PLUG, allowing workers access to the site.

这些工作完成后，再调用一组水泵将拱坝与堵塞间的水全部抽干，堤坝抢修工们就可以进入现场进行堤坝的修复作业了。

Then, once it is in place, it is partly flooded and allowed to sink to the riverbed around the breach, making a tight seal.

一旦到位，就向拱坝内灌水使它沉到裂口周围的河床上，形成了一道严密的围堰。