Types of dams: construction, sizes, uses
A dam is fairly simple in principle however as they have gotten bigger they have also gotten much more complicated to build. Some dams that are being built these days are huge like the Three Gorges Dam in China. That being said most dams are built the same way regardless of their size.
Dams are largely built for one of two reasons, to store water for use as hydroelectricity or for use as drinking water. A dam will create a reservoir behind it in either case so the difference is mainly about what you do with the water. There are a few other reasons that you would build a dam for example if you needed to divert the flow of a river but in most of these cases they are called dikes or levees and not dams. A dam is generally used to refer to a structure that creates a reservoir of water.
There are two commonly used designs for dams; the first of these is called an arch dam. This is pretty much what you would think that it would be, it is a dam that is shaped like an arch. The reason that you would design a dam like that is because an arch is very strong. Dams have to be able to stand up to the force of a flowing river, this takes a very strong structure.
The other option for building a dam is called a gravity dam. This is really just a dam that is designed to be so big and heavy that the water is not able to move the weight. These dams usually have a very wide bottom with a narrower top on order to make sure that the dam can't tip over. Gravity dams are generally considered to be safer than arch dams but they are more expensive to build.
When it comes to the materials for building a dam by far the most common is concrete. You will find dams that are made out of rocks or dirt that are dumped into the river but this is really only effective for small dams. Almost all of the larger dams are built with concrete. There has been some experimentation with building steel dams but they are not widely used.
The actual process of building a dam can be very complicated or quite simple. In some cases all that is required is to through enough rocks and dirt into the river to restrict the flow and allow a reservoir to form. However for the larger concrete dams it will be necessary to divert the river before you can start building the dam. In some cases this may require building tunnels to allow the water to go around the construction site. Once the river has been diverted the actual construction process is similar to any other large concrete structure.