How do they build dams on rivers if there’s nothing stopping the water from flowing while they are building?
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They divert the river to other channels then dig whatever they are going to dig. Once the area is ready, they redirect the river back to the area.
For some of the larger dam projects the dig out a bypass trench or tunnel around where the want to build the dam.
Cofferdams are structures built to keep water out of a working area.
They also will drill tunnels that may be used as spillways(overflow) infrastructure first and then divert the river through there while working.
The Hoover dam built the tunnels first to divert the river, then cofferdams to keep the area clear, then used some of the diversion tunnels for emergency spillways.
https://youtu.be/URC125wpMS4 from an engineering YouTube channel for a quick 10min *dive* into the topic.
Before the dam was built, they will create an artificial river to divert the water to bypass the dam.
Once the dam is finished, they will close back the diversion and it will take from few days to few month to fill the water into the dam.
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