How are dams built?

(Image source: Pixabay)

Question from Oliver Johnson

Before building a dam, the river has to be diverted to keep the construction site dry. To prepare the site for this, a tunnel is built through one side of the valley to lead the river around the construction site. 

A smaller dam, called a cofferdam, is then built upstream to divert the river through this tunnel. Another cofferdam may also be built downstream to prevent the water flowing back up into the construction area. Next the site is cleared and a foundation plinth is laid, before the dam is constructed from concrete and steel.

Answered by Joanna Stass for Brain Dump in How It Works issue 123.

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