There's a very cool project wrapping up here in Zurich where they built a relief tunnel from the sihl river underground downhill to lake Zurich to handle 500-years floods.

One weekend they let people walk the length of the tunnel from the river to the lake. It was a pretty steep slope and about 3/4 down the walls of the tunnel changed from smooth to rough. They had calculated how fast the water would be moving when it hits the rough part and then how much that would slow it down.

Then at the bottom instead of emptying directly into the lake it comes flying out of the tunnel and smashes into a giant cement wall then flows under a roadway into the lake.

Cost was $200 million. A 500-year flood would flood downtown Zurich.

https://www.zh.ch/de/planen-bauen/wasserbau/wasserbauprojekt...