How do you build a hydro dam?

Answer 1

Very carefully

More seriously, there are two main types of hydrolectric installations: Impoundment, and run-of-river (there's also electricity generation from the tides, waves, etc, but we'll leave those aside for now).

The impoundment types are probably what you're thinking of when you say 'hydro dam'. These are large dams which create a reservoir upstream. The water in the reservoir is then forced to flow through penstocks within the dam which direct the flow to where it can power turbines which are in turn used to generate electrcity. The dams themselves are usually concrete, while the turbines are usually metal.

The run-of-river type involves diverting water from a river and once again using it to turn turbines. The turbines are again usually metal. They can be housed in a building made of just about any material, and some of the older ones are quite pretty stone buildings etc.

Answer 2

You are most likely talking about how hydroelectric dams are constructed in China and around the world. First you must direct the current in the river by making a makeshift dam in front of the dam you are constructing and a dam behind the dam you are constructing. Then you create a tunnel to change the flow. Since you block the river from going to the dam construction site, it is dry and ready to be built. First you put a slab of concrete for the generators to sit. Next you put in the generators and turbines so you can create electricity. Then you build the structure with concrete. You also build intake pipes and pipes that go back into the river so the turbines can function properly. You also need to build an overflow system. The overflow system will have gates to open when there is excess water. You might also need to build a canal if it is a busy river full of boats. Finally you fill up the tunnel and blow up the makeshift dams.