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

从古早就已有人建造拱坝，但从来没有这么模。

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河床上，形成了一道严密围堰。

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水全部抽干，堤坝抢修工人们就入现场行堤坝修复作业了。

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，狭窄黑峡谷最适合盖重力拱坝。

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" 重建" 拱坝与常方法建临时围堰相比不仅省时、省工，而且还省钱。