Previously (227 points, 102 comments):

https://news.ycombinator.com/item?id=27417206

That was submitted to HN on June 6, 2021. A low head dam near Eden, NC would take five lives just 10 days later on June 16th:

https://www.wral.com/story/family-sues-duke-energy-over-fata...

Here's what that dam looked like from upstream:

https://d3s3k13islrvw7.cloudfront.net/original/2X/e/eb3e018c...

https://forums.paddling.com/t/looks-like-a-low-head-dam-stri...

The Toronto region has a number of low head dams that have in my memory claimed at least seven people and one dog. A number of drownings were of people who jumped in to save their dog. Usually the dog got out on its own.

Paddlers are generally aware, but one fatality was a paddler unfamiliar with the river. His daughter survived because she was in the front of the canoe and likely entered the water downstream of the peak of the boil.

If caught in a hydraulic, your best chance of survival is to immediately dive for the bottom where the current runs downstream. Good Luck!

After a number of deaths, one dam was dynamited and others have been filled in downstream.

One of Tufte's books, I suppose Envisioning Information includes a Washington Post graphic about this from forty years ago--a number of soldiers who were rafting had drowned in the hydraulic below the Little Falls pumping station dam.

This is pretty scary. These dams look so shallow and easy to run. Thanks for the PSA.

Here's a cool US Army Corps of Engineers report on the remediation of dangerous and defective dams: https://apps.dtic.mil/sti/pdfs/ADA372898.pdf.

Pages 21-24 show how they fixed the deadly "roller" at Little Falls Dam outside Washington DC, 1988-1990. Quote:

"In periods of very heavy flow, when 0.9 to 1.5 m (3 to 5 ft) of water crest the dam, a strong roller, or undertow, condition is created (Figure 9). Between 1975 and 1983, 17 people drowned in the vicinity of the dam. Rafting or canoeing near or over the dam, they were caught in the undertow and submerged. Seeking to prevent further loss of life, the USACE began looking for solutions for this problem (Davis and George 1985)."

Could these dams be made safer with something like an underwater net (in the turbulent area just below the dam) that would allow water to flow through, but would also give trapped victims an opportunity to climb up to the surface?

Nets would probably become quickly choked with debris. Im wondering about narrow concrete buttresses maybe every 3 meters or so that swimmers could grab onto

Maybe there’s a problem with people getting incapacitated by striking the buttresses? But you made a good point about nets getting blocked by debris. Maybe the nets will kill fish as well

Sounds like a bad idea. Even under ideal conditions, you'd get easily get stuck in the net or otherwise be held in place by the force of moving water.

We always called weirs drowning machines too. Even my son knows them as this from an early age, my head instinctly regards them as scary now. There's a lot of them around in the UK.