A few days ago I posted to LinkedIn and it turns out that my post went onto the new Ecohydraulics LinkedIn group there. My post generated some interest and animal response, so I thought I would make a blog post here to go into the issue in more depth.
I just happen to be in Des Moines, Iowa this week and I walked on an impressive pedestrian bridge over an undulating weir on the Des Moines River right downtown. Standing right over the thing, I saw debris trapped in the drowned jump at the toe of the structure, so I watched it for a little while. I saw that the drowned jump kept the debris within an individual “horseshoe” with no chance to get out. I have no idea if they engineered it considering this issue. But anyway, I posted a selfie of my above the structure to LinkedIn and here we are.
I just checked online and found the article below. It says that at least 15 people have died at the Center Street Dam and the Scott Street Dam on this river, so yeah, these things here are killers. They are apex predators for humans.
Let me take a step back and talk about my background in this topic for context, so you can understand why it concerns me. I have a lot of experience on the issue about the risks of hydraulic jumps and I have researched them, weirs, waterfalls, etc as part of my career.
I grew up along the Potomac River in Washington, DC and spent my youth through my college years doing a lot of whitewater kayaking, including in class V rapids. At 14, I had the experience of being trapped in a really big hydraulic jump. The shock of it was the danger my kayak and lifejacket posed, because they held me up above the water, but not above the aerated water, so I was kept at the worst spot with not enough air to breath but not enough water to get a grip to get out. Luckily, the river gods had their fun and eventually decided to let me go 😉 As an aside, I do know the wisdom for a swimmer to take off the lifejacket and try to swim under the jump, but I also have the experience of being in the “washing machine” of 3D aerated flow at which point you are so disoriented that don’t know which way is up, so that theory has its limitations to put in practice. I know someone who has done it successfully, but that was one of the best extreme kayakers in the world at the time. But I digress…
Around the same time as my experience, there was a highly publicized tragedy in 1984 at Brookmont Dam (aka Little Falls) on the Potomac River, which had a submerged jump that was totally inescapable. Highly athletic soldiers and navy seamen were no match for it, and many others have paid the ultimate price there. I provide some links to the news article about it below.
Eventually, around 2000, this dam’s hydraulic jump was broken up with rock to improve fish passage and end the deaths.
As a professor in my early career from 1998-2010 I did a fair amount of scientific research on the fluid mechanics of hydraulic jumps and waterfalls. Most recently I published a paper exploring drowning hazards in real whitewater rivers in 2017. We figured some pretty interesting things out through all of that work. One line of the research addressed horseshoe waterfalls using flume experiments in a large flume pulling water from the Mississippi River. Although the danger of this form was not a focus of the work, had I failed into the flume during the downed-jump runs when I was standing over it on the carriage, I would have died.  It was during that time that I learned about labyrinth weirs and their benefits. I don’t know if people have researched the drowning risk of labyrinth weirs or not.
Turning to fish passage, I have to declare that I am not a specialist for fish passage per se. I took the fish passage workshop at ISE2024 in Quebec last month and learned a lot from those folks. Obviously, a lot of our skills in ecohydraulics apply to that topic, but I just haven’t done that due to that not being a major source of funding in my work. Thus, I do not know the pros and cons of labyrinth weirs for fish passage.
It’s weird but it seems that after watching my home river’s drowning machine get overhauled then I just assumed this lesson was being applied everywhere, but it seems not. I’m pretty surprised.
So that’s my story. Let me know what you think.
-Greg
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