The Most Dangerous Dams

Dams serve a wide variety of purposes from
hydropower to flood control to storage of water for municipal and industrials uses. But when a dam’s useful purpose fades away,
the structure itself still remains. Dams come in all shapes and sizes, but contrary
to what you might think, the most dangerous dams are often the smallest. Hey I’m Grady and this is Practical Engineering. Today we’re talking about the dangers of
low-head dams. This video is sponsored by Squarespace. Visit
to get a free trial, and use code PracticalEngineering to get an additional 10% off. More on that later. A low head dam, sometimes simply called a
weir, is a small structure that impounds a small amount of water and spans the width
of river or stream. Usually made from concrete, the purpose of
a low head dams is to raise the water level upstream on a river. This can assist with navigation of the channel
by boats, create a drop for generating hydropower, and make water available at intakes for water
supply and irrigation. Thousands of these structures have been constructed
over the years to take advantage of natural watercourses and rivers. The heyday of low head dam construction was
actually in the 1800s when mills and factories often relied on waterpower to drive grinding
wheels and other equipment. This was at a time when moving water was the
most consistent source of power available in large quantities before widespread adoption
of electricity. Most of these old mills and factories are
long gone, and the ones that still survive certainly don’t depend on water for power
anymore. That means many property owners are forced
to maintain these old structures that no longer have any practical use. Or more commonly and much worse, these dams
are abandoned by their owner and gradually fall into disrepair. In the U.S., dam safety regulations focus
primarily on the possibility of a dam breaching and causing a flood wave downstream. But, because low head dams are relatively
short, a breach poses minimal danger, so most states don’t keep track of these small structures. And, especially if they’ve been abandoned,
it can be difficult to enforce maintenance requirements on the owners. But, even though they pose little danger in
the event of a breach, low head dams create a public safety issue that has caused more
fatalities in the U.S. than all dam failures in the past 20 years. To understand why, we first need to know a
little bit about open channel hydraulics. If you haven’t seen my video about hydraulic
jumps, I’ll summarize it here. Go back and check out that video if you want
to learn more. Open channel flow – that’s flow not confined
within a pipe – has a very important property related to its velocity that governs its behavior. Slow, tranquil flowing water is called subcritical
because waves propagate faster than the flow velocity. Fast moving water is supercritical because
waves move slower than the flow velocity. Any time a supercritical flow encounters subcritical
flow, an interesting phenomenon called a hydraulic jump is formed. Low head dams almost always have subcritical
flow upstream. The flow is deep, slow, and tranquil as it
makes its way to the dam. But as the flow passes over the weir, it picks
up speed and becomes supercritical. When this supercritical flow transitions back
to subcritical flow in the slower moving water downstream, it creates a hydraulic jump as
you can see here in my model flume. It’s easy to see why these types of structures
could pose a threat to those using the waterway for recreation. Any location with fast moving water and high
turbulence can be dangerous to swimmers or kayakers, but the location of this hydraulic
jump can turn a manageable risk into an almost surefire way to drown. The depth of the flow downstream of a dam
is called the tailwater, and it controls the location of the hydraulic jump. In my model, I can adjust the elevation of
the tailwater by adding or removing these stoplogs. When tailwater is low, the hydraulic jump
forms away from the dam. This is a fully developed jump that follows
the traditional shape and flow patterns. If I send down this piece of wood as a kayaker
surrogate, it experiences some turbulence as it passes over the weir and through the
jump but, it doesn’t have much trouble escaping downstream. But, as the tailwater rises the jump moves
closer and closer to the dam. Eventually if the tailwater is high enough,
the hydraulic jump will reach the dam. This condition is called a submerged or drowned
jump. It may look fairly innocuous, but this is
when things get dangerous. Let’s send down our kayaker surrogate to
see why. A submerged hydraulic jump creates an area
of recirculation immediately downstream of the dam sometimes called a “keeper” for
obvious reasons. The jet of the hydraulic jump surfaces downstream
causing a boil point. Sometimes this is easy to see and sometimes
it’s not. Either way, objects or people can will only
be able to escape a submerged hydraulic jump if they are able to get beyond this boil point. And, any rescuers who approach a submerged
jump from downstream run the risk of being drawing into the hydraulic themselves. The recirculating currents that trap recreators
is dangerous enough on its own but there are other factors contributing to the danger at
low head dams. These currents also trap large debris between
the strong hydraulic forces and the hard concrete surface of the dam which can batter someone
trapped in the keeper. The water is often cold, increasing the potential
for hypothermia and further disorientation. The turbulence of the hydraulic jump entrains
a lot of air, reducing the buoyancy of a swimmer. And, low head dams often span the entire width
of the river, meaning there is no still water nearby that can be used as a safe haven. This is exactly why the low head dam is called
the perfect drowning machine. All these factors added together create a
situation that’s almost impossible to survive. There are a lot of ways to mitigate this issue. The simplest option is just to keep people
away from these structures. Some states require that exclusion zones be
established to make sure that kayakers safely portage dams instead of trying to run them. Good signage and buoys as warnings can sometimes
be enough to keep people safe. Another option is to modify the structure
to reduce the potential for recirculating currents. Researchers have proposed various retrofits
to existing dams to improve flow conditions when tailwater is high. Of course, the most obvious (but also most
expensive) way to address the issue is to remove these dams altogether. In many cases they are no longer serving an
important role, and removing dams can help restore ecosystems and improve connectivity
for aquatic species in addition to removing a hazard. If you’re swimming or paddling on a river
with a low head dam, don’t underestimate the danger of these powerful hydraulic forces. Different flow conditions on the river can
dramatically change the behaviour of the hydraulic jump, as we saw, so be careful. Thank you for watching and let me know what
you think! If you’re new to the channel, you might not
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100 thoughts on “The Most Dangerous Dams

  1. There is an abandoned low head damn in Troy NY and people die there almost every year

  2. You logged onto the internet as a kid? UHG,…..I was about 25 or 26 when I got online the first time.

  3. That piece of wood made my stomach turn just as much as it was. This is an incredible demonstration.

  4. Hmmm so these are useful for people who want to drown. Interesting! Btw, what's your favorite modus moriendi?

  5. I've always had a fear of deep dark water… but I'm slowly realizing that deep dark water might just be the safest water out there!
    Safe looking shallow rivers can sweep you off your feet, inviting looking natural warm springs have brain eating parasites, wimpy looking small dams can drown you with ease, and even a small amount of water in a pipe other container can easily have the force to kill or seriously injure and destroy.

    Water is scary.
    I'm gonna keep a life vest near my toilet from now on. You can never be too safe! ;D

  6. Ever since I was little, we all knew about quicksand because it was so common in games, even though I've never seen it before in my life.

    I think I'm going to put this in a game sometime. I wonder how many will be caught off guard…

  7. I mean, why would anyone get close enough to the dam to get caught in the hydro jump?
    Rhetorical, kinda, but really, why?

  8. hydraulic jump….. it's in the master plumbers test if you plan on taking the test anytime soon learn about it

  9. my hometown of saskatoon sk has one of the most dangerous rivers in the world because the university of saskatchawan draws water from a wier for the heating plant but what i dont get is you can be sucked under really far away from the weir itsself like outta town even why is this??

  10. First time learnt the terminology Boil Point in Hydraulics. Thanks very much.

  11. Hoover Dam is the most dangerous dam. I went there once and was almost killed by the legion.

  12. I've gotten stuck in one of those before but luckily for me I was tall enough to stand up. Water went right up to my shoulders just above.. It was very difficult to stand. Let alone trying to swim out of that.. I had a kick away from the back wall like a Olympic swimmer in I had enough thrust to get out

  13. what structure can be used to slow the flow and increase depth both before and downstream of structure?

  14. Impressive. I'd take a look at primochill vue PC coolant, your graphics are top notch already, but itll take it to the next level.

  15. In Ohio many of the dams are huge environmental issues. They were built to supply steel mills with water. The dams now hold heavy metals that are released when removed. Water quality is improving but dam removal adds to the complexity.

  16. Watched this video because we have a low head dam in my hometown, then I saw that dam at 5:02 and had to do a double take.

  17. My dad (former kayaker) alway warned me to stay away from these at all costs. Their uniform hydrology serves to trap you underwater and makes them incredibly dangerous.

  18. This is the second most important safety video I've seen. The first would be the delta p video.

  19. I almost got swamped in a small boat by this. It was 60 seconds of sheer terror that our 10hp outboard BARELY got us out of with paddling. I thought we were going in. There was 3 of us, surely someone wouldn't have gone home. This one even looked scary, so it was probably way worse than that in reality. Got lucky that day.

  20. I live in S.C., and our damns like this broke and we had a 1000 year flood. I live 2 blocks from on of the areas that flooded. I was way above that area even livening 2 blocks away.

  21. Friends and I used to cross a local river by jumping between the raised sections of the weir, usually barefoot. Stuff was covered in moss too, I'm surprised no one died.

  22. The biggest issue of public safety is the lack of grey matter in the average persons skull. I don't feel sorry for kayakers or sky divers or people who swim with sharks, etc.. They put themselves in harms way on purpose so no sympathy here. I run 2 hydroelectric facilities and see this crap all the time, many times with the persons entire family in the fishing boat while trying to get some muddy fish. They don't have a clue as to surface tension in the boil areas or the phenomena that this man so expertly showed with this video. Everyone stay safe.

  23. The explanation about the different flows hitting each other could use some more explanation otherwise great!

  24. We have a levee that runs through my town it has those spill way dams i grew up on the river and as such am a strong swimmer i loved going down the canal through those tucking my legs and arms in and letting it spin me in circles

  25. PSA: I'm a kiwi/NZ'er, done a fair amount of swimming in white-water rivers, and was a strong swimmer too. Getting caught in a keeper only happened a few times, but my response was to dive to the bottom, lie flat on it as close as possible (so water can't get under me to lift me up), and swim downstream. It isn't difficult as water against the bottom is going downstream too. When the turbulence stops, swim a few more strokes then surface. Fighting the gyre will exhaust you damned fast because it's very hard to get your head above water to breathe (as you said, the amount of air churned into the water reduces your buoyancy greatly). It's better to use the water flow to your advantage. Success has two very important prerequisites: not panicking, and understanding the situation you're in. Your video explains that excellently. Note: you can't do as I suggest if you're wearing a flotation device. You've got a very tricky decision to make if you are and you're caught in a gyre.

  26. I live near a low head dam. When it floods, trees the size of cars get in there and never leave.

  27. Oct 2018 in Malaysia.. six rescue diver trap and drown while trying to search for drowned teenage at low head dam

  28. Hey! Your channel inspired me to pursue further education. Been out of school for many years. Getting a degree seemed kind of out of my league to start, so I'm taking a technologist program that allows transfer into 3rd year of a degree upon completion. Doing great so far, and almost done! So excited to continue onward. 😀

  29. I guess there’s no way to escape out of that boil point.
    What to do would be nice…

  30. These things scare the piss out of me. one of my biggest fears, and have caused nightmares, is getting trapped in the local river past a specific bridge.

    Currently! My hometown has been a part of the Great Miami River canoeing trails where you can paddle free and whatnot to your leisure. HOWEVER! from all plans I see the city put out they NEVER planned to remove to low head dam behind the, currently decomissioned, power plant and future plans seem to indicate it staying in tact as a park is constructed in the footprint of the power plant; right up against the low level dam. CRAZY!

  31. you spend the whole video talking about hydraulic jumps but you never explain what is or show where it is in the flow. you just assume we know. standard engineer. so concerned with details you cant see your own simple mistakes

  32. Ok, so in case the situation occurs where i get into this boiling spot is there something at least I can attempt to do? I'd like to know all my possibilities here. I am no cayaker or river swimmer but fuck knows if situation arises and my brain starts flash backing like crazy to search for possible solutions or at least shots at surviving… What can I do to attempt to escape the boiling spot or the hydraulic jump?

  33. As a Kayaker i really want to thank you for this video. This issue is so big and usually doesn’t get the media coverage it deserves.

  34. Such a dam exists not far from where I grew up. Each generation has lost someone to it. In a small rural area the stories abound. It's a surreal place. It's topped by an iron truss bridge so you can walk over/above it. The dam is part of one of the largest public parks in the area.

  35. A pair of two young brothers in my town died from swimming near one of these. One of them jumped in and when the other noticed he didn’t come up he jumped in to try and save him. They both ended up drowning. This actually scared me because I had always wanted to swim there but never did. I’m glad now. Please be careful y’all.

  36. The key is to swim down deep if you’re caught in the vortex. You want to get below the arrows at 4:52, notice how that water will end up on the outside. Swim down to the bottom, the water in a low-head dam is shallow by definition. Then swim out for as long as you can before resurfacing, you have to get past the jet. If you got past the jet the water will now carry you away from the dam, if you didn’t you’re headed back towards the dam, try again and remember to get down deep and swim out far.

    If you can clear your lungs and take a deep breath to get out CO2 and take in O2 before trying to swim out, even better. Remember, get down deep and then swim as far away as you can.

  37. Could they use this to make a different hydroelectric dam the generator is right we’re the water is circulating

  38. And if you do found yourself into one of those your best bet to escape it is to remove your life jacket and try to swim down stream underwatrer

  39. There is one of these near my hometown everyone swims at the falls , not dong that next time I’m back in town anymore.

  40. 5:40 To be honest. if you're in that you have bigger problems than hydraulic jumps…

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