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...

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wolttam 2 hours ago | parent [-]

That IS very cool. I can’t help but appreciate the insight and forethought that goes into the decision to construct such massive waterworks projects.

	▲	BoxOfRain an hour ago \| parent \| next [-]
		The cross-border system fed from the Elan Valley in the UK is like this, Wales is famously very rainy due to its hills and the Victorians exploited this by damming the Elan Valley and gravity-feeding the water from the Cambrian Mountains all the way to Birmingham which was a rapidly growing industrial centre. There's no pumps, it was all calculated to drop 170 feet over 73 miles! It's a really stunning place to visit as well, mid Wales in general is so pretty and the dams themselves are ornamented in a way nobody would bother with today. Of course the Victorians got away with things that would be unthinkable now, when Liverpool did the same approach of flooding Welsh valleys to water English cities in the mid 20th century it was very controversial because the scheme flooded a majority Welsh-speaking village and the language was in a fairly steep decline then; nowadays there's a lot more protections for Welsh.
	▲	cyanydeez 5 minutes ago \| parent \| prev [-]
		our facility builds features to only 25 year floods, and that appears to be typical in the US.