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What is a Hydraulic Jump?

Controlling the flow of water is one of the
fundamental objectives of modern infrastructure, from flooding rivers to irrigation canals,
stormwater drainage facilities to aqueducts, and even the spillways of dams. So, engineers need to be able to predict how
water will behave in order to design structures that manage or control it. And fluids don’t always behave the way you’d
expect. Hey, I’m Grady, and this is Practical Engineering. On today’s episode, we’re talking about
one of the most interesting phenomena in open-channel flow: the hydraulic jump. This video is sponsored by Nord VPN. Feel safe online: anywhere, anytime. More on that later. Fluid dynamics might sound as complicated
as rocket science, but unlike rockets, you probably already have some intuitions about
how water flows. The study of how water with a free surface
behaves, that is not confined within a pipe, is known as open channel hydraulics. This field is especially useful in civil engineering
where structures can’t usually be tested at scale. We can’t build a dam, cause a flood to see
how well the spillway works, and then rebuild it if the performance isn’t up to standards. Instead, engineers need to be able to predict
how how well hydraulic structures will perform before they’re ever constructed. This is the definition of engineering: to
take theoretical knowledge of science and physics (in this case fluid dynamics), and
apply that information to make decisions about the real world. One of the most important parameters in fluid
dynamics is velocity, or how quickly the water flows. Sometimes velocity is a good thing, like when
you’re trying to move a lot of water quickly, for example in a flood. Sometimes velocity is a bad thing, like if
you’re trying to avoid erosion. Either way, it’s almost always a key criterion
when designing hydraulic structures. But the velocity of flow isn’t the only
velocity that’s important in fluid dynamics. We also care about the velocity of waves or
how quickly pressure disturbances in a fluid can travel. If the flow velocity is exactly equal to the
wave speed, we call the flow critical. But it’s more likely that these two velocities
are different. Slow, tranquil flow conditions are called
subcritical. In this case, the wave speed is faster than
the flow velocity. You can see that the waves can travel against
the flow direction. Because of this, the depth is controlled by
downstream conditions. You can see that anything I do upstream isn’t
changing the depth of this flow. Fast moving flow is called supercritical. In this case, the flow velocity is faster
than the wave speed. You can see that waves aren’t able to propagate
upstream. Supercritical flow is controlled on the upstream
side, so nothing I do downstream affects the depth of the supercritical flow above. A flow profile can naturally transition from
subcritical to supercritical (that is from slow to fast), for example if a channel changes
to a steeper slope or a cliff. Many types of flow measurement devices rely
on forcing a flow to transition from sub- to supercritical because there will be a unique
relationship between flow rate and depth for a given geometry. Maybe we’ll talk more about flow measurement
in a future video. But, when flow transitions the other direction
– when a fast-moving supercritical flow transitions to a more tranquil subcritical condition – something
much more interesting happens: a hydraulic jump. The classic demonstration of a hydraulic jump
can be seen at the bottom of your sink. Open the faucet and watch how the flow behaves. You can see the fast moving water right as
the flow hits the sink and the abrupt transition of the hydraulic jump to a slower moving flow. But the sink demo isn’t the best example
because it happens due to surface tension, not gravity. Plus it’s kind of a boring. So I built this flume in my garage to give
you a better look at the hydraulics. If I open the upstream gate by just a little
bit, I can create supercritical flow in the flume. Now, if I obstruct the area downstream, I
can force the flow to transition into subcritical. Right where the flow transitions, you can
clearly see the hydraulic jump. This phenomenon happens naturally in certain
locations. Steep mountain streams often have supercritical
flow crashing into rocks and changing slopes, creating whitewater and turbulence and the
occasional hydraulic jump. Also, a tidal bore occurs when an incoming
tide forms a wave that travels upstream against a river. These events only occur in a few places across
the world, but it’s fascinating if you get to see it in person. In many cases, the bore travels as a moving
hydraulic jump, similar to what you see here in my flume. But, jumps aren’t just natural phenomena. They’re important in hydraulic structures
as well, especially for energy dissipation. A major part of the job of a civil engineer
working in the field of hydraulics is designing against erosion from the flow of water. When we try to control flow of water, it often
leads to the potential of having fast moving, erosive conditions. For example, when we put water in a culvert
rather than allowing to flow over a roadway, it can pick up speed in the pipe. When we line a ditch or creek with concrete,
the smoothness speeds up the flow compared to natural conditions. And when we make releases from a reservoir
behind a dam into a spillway, the water can come roaring down at extremely high velocities. This supercritical flow can cause erosion
and eventually lead to failure of the structure. So, most hydraulic structures will be equipped
with some form of energy dissipator on the downstream end to reduce the velocity of flow
and protect against erosion. There are all kinds of hydraulic energy dissipators,
but for large structures like spillways, the most common types rely on the formation of
a hydraulic jump. Because a hydraulic jump causes so much turbulence,
it is able to effectively dissipate hydraulic energy as heat. So many energy dissipators, also called stilling
basins, are designed to force a hydraulic jump to occur. There are many types of stilling basins, but
most use different combinations of blocks, end sills, and overall geometry to control
how the hydraulic jump forms. The turbulence stays within the stilling basin
with the objective of having smooth, tranquil, subcritical flow leaving downstream, minimizing
the potential for erosion which would otherwise threaten the integrity of the structure. Hydraulic jumps don’t just serve utilitarian
purposes. Recreational whitewater courses can be found
across the world, and many of these courses make use of hydraulic jumps as artificial
rapids. In fact, many kayak parks started out as obsolete
dams in need of removal, a perfect opportunity for replacement with something more beneficial
to the community and the environment. Freestyle kayaking, also known as playboating,
involves performing tricks in a single spot. Playboaters use natural and artificial hydraulic
jumps to stay in one spot. I’ve never tried this myself but it looks
like a lot of fun. Next time you see water flowing in a open
channel, try to identify if it’s sub- or supercritical, and keep your eye out for hydraulic
jumps. Thanks to Nord VPN for sponsoring this video. I’m really happy to share this sponsor,
because, in a small way at least, I owe my marriage to VPNs. If you’re not familiar, a Virtual Private Network, or VPN, is a way to tunnel your web traffic so it can’t be seen or recorded from the outside. Well before my wife and I were married, she
spent 2 years living and working in Beijing, China. China’s heavy-handed regulation of the internet,
sometimes known as the Great Firewall, made it difficult for us to stay in touch during
this time. We used a VPN to get around the censorship
of certain websites and services we used to communicate. I remember then it was so complicated and
complex to get running. Now I use NordVPN, and they’re offering
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an extra month for free. Thank you for watching, and let me know what
you think!


  1. Rob * Author

    Love the series! It's piqued my interest in sewer and storm drain systems. Drop structures involve way more hydraulic analysis than i'd thought. I skimmed this PhD thesis — — and that led me to the concept of , which i found very interesting.

  2. The Illuminaughty Author

    What if there were two or more spillway outflow points aimed at each other? Could the energy of each water course be dissipated by crashing it into its mirror opposite?

  3. Anton Richter Author

    I live around 300m downstream from a dam, I can see it every time I go out to the backyard.
    The water level rises and falls around 1m everyday

  4. clayton kawasaki Author

    I absolutely love you videos and I enjoy how you get into calculations and design on how dams are engineered and altered for a specific task
    This might seem a bit off topic but could u do a nuclear reactor engineering video ?
    I would love to learn on why certain reactors like light water or heavy water reactors are chosen for a task and mabe even how to calculate the mass of uranium needed for the energy demand
    Thank you

  5. Promasree Majumdar Author

    Fluid dynamics is my fav subject and this video just incredibly made me more interested towards this subject. Thanks a lot.

  6. Daniel Tracy Author

    I just wanna lay in your lap and soak up your knowledge and calming voice all night. Very informative video. I never imagined the fluid dynamic of a dam being as complex as you have exposed it to be.

  7. James Staab Author

    I know the video is meant to be informative, but I just find it a bit therapeutic with your narration and the different pics of flowing water.

  8. Adhunik Axom Author

    Thanks a lot. A question related to this topic came in a govt exam I appeared few months back.
    The question was something like this
    1. Hydraulic jumps help in
    A) dissipating energy
    B) releasing silt etc coming with flow
    C) increasing energy
    D) ….. (Forgot)

    And I guessed it to be (B) .

  9. Tommy Frerking Author

    5:55 that's where I live, Saint Anthony Falls on the Mississippi River in Minneapolis! I pass over that bridge on my way home every day and I love watching the water flow!

  10. William 1221 Author

    there are some dams near me that where made in the 1800s and 1900s how have they with stood erosion as they don't seem to have hydraulic jumps

  11. Samsng Device Author

    I used this video as reference
    with my customers who have issues with flooding basements.
    By creating a hydraulic JUMP at the base of the roof's gutter system's down spouts, we achieved sucess by eliminating scour, with out digging a complex underground drain system.

  12. linoox lee Author

    The Hongqi River project is to introduce Tibet's water (60 billion cubic meters per year) into Xinjiang and the Yellow River. Hongqi's Chinese meaning is "red flag". The total investment of Hongqi river Project is about 4 trillion yuan (600 billion US dollars), and the construction period is ten years. After the completion of the Hongqi River project, the 300,000-square-kilometer desert in Xinjiang has become a fertile land with a sunny and flat terrain suitable for large-scale mechanized farming. The cost of forming a farm per square meter is about 20 yuan($3). After the water in Tibet is injected into the Yellow River, it can solve the problem of insufficient water resources in northern China.
    The following video is a report by the Chinese Academy of Sciences academician Tsinghua University Professor Wang Hao on the Hongqi River project: (

    According to the plan of the experts of the Chinese Academy of Engineering, the Hongqi River has a large amount of engineering. The Hongqi River project consists of four major components:
    1. Barrage: A total of 19 barrage dams were installed along the Hongqi River. Among them, 12 are dam high above 100 meters, and the remaining 7 are less than 100 meters.
    2. Diversion tunnel: This is one of the most important parts of the project. The same tunnel consists of 3 to 5 holes ranging from 15 meters to 12 meters. The total length of the single line is about 2,200 kilometers.
    3. Open channel: According to the current planning and design of Hongqi River, after the Liujiaxia Reservoir of the Yellow River, water is diverted to the Yumen, Hotan and Kashi sections through open channels and a small number of tunnels, mainly in the form of open channels. The total length is 3,700 kilometers. Since there was a branch of the Hongyan River diversion before this section, there were also the branch line Mobei and the Chunfeng River diversion. The average flow after the diversion and the water diversion scale of the South-to-North Water Diversion Middle Line (95-135 billion cubic meters/year) The scale is similar, and the mileage is 2.5 times that of the South-North Water Transfer.
    4. Branch line: 3 branch lines, totaling 411 kilometers of tunnel and 2589 kilometers of open channel.
    The Hongqi River project will increase China's cultivated land by 25% and food production by 50%. It not only has a profound impact on China, but also seriously affects other countries and regions of the world.
    China will convert large quantities of imported agricultural products into large quantities of export agricultural products, and the output costs of agricultural products are very low. It will destroy the farming and animal husbandry in the Netherlands, Australia and New Zealand. Agriculture in the United States, Brazil and Argentina has also been greatly affected, causing many farmers to lose their jobs.
    The Hongqi River project will greatly enhance China's economic power. Not only does it increase agricultural output, it also frees China to release large amounts of land to meet the needs of Chinese industry. As China's economic strength has increased, China's political and economic influence on the world has also increased, and the world's political and economic structure will undergo great changes.

  13. Jiro Martinez Author

    Hi cool guy im a mechanical engineer here well could you make a video regarding direct pumping dos and donts and tip us for safety in the system will you will you????????

  14. Woody615 Author

    Geographer by education here. Love your channel. However, I disagree with your comment that tidal bores only happen in a few places. Tidal bores are only a function of tides going up stream. The geometry of the local hydraulic system dictates the size and scale of the bore. Amazon and China have some famous large bores.

    However, I've seen them as small as 6 inches traveling up concrete channelized river beds, the San Gabriel River in Long Beach, CA specifically. The end of the channel is a couple of feet above the low tide level but it is also a couple of feet below high tide level. Thus as the tide comes in and progresses up the channel, a tidal bore is produced. I've seen the tide come in and slowly creep up the channel producing a 6 inch tidal bore. Quite fascinating. And this happens almost 3 3/4 miles up stream from the open ocean, so it is not based on open ocean breakers.

  15. Melodic Humming Bird Author

    I can only confirm, as every single day, in my house or any place I go to free myself of some H2O with NaCL, I powerfully control the water stream with all kinds of taps or muscles.

  16. Melodic Humming Bird Author

    From now on, when I take a pee, I will make sure to create a massive hydraulic jump by jumping as high as I can while peeing, in order to minimize the Erosion in my toilet system.

  17. Jacob Author

    Playing in a hydraulic jump is literally asking to be drowned, you will be definitely be taken on a ride……of your life, it will be very exhilarating… you die. Yes you will stay in the same spot……underwater…..with no air.

  18. russell379x Author

    i have concrete flip buckets at the end of my flood spillways, been testing coal tar epoxy on them for cavitation erosion testing, so far so good ✌🏻super critical flow is what the oroville spillway failed from, when it would go over the drop you can see it just turn white from cavitation…. it’s like little fire crackers going off … what the seismic sensors were picking up. (the jump down)They need to slow the speed down at that point

  19. Igrowtowers Permaculture Author

    luckily water is always level…that means flat……just like the water in your glass, in your bathtub, in the lake and in the ocean……so….where does the curve start?…right….never?
    so does this prove the earth is flat?

  20. Ronan Rogers Author

    …and when civil engineers get the flow wrong, that simply presents an opportunity for other engineers to innovate with things like the salmon cannon.

  21. Walter Mrak Author

    :::::::::::::::>>>>> Allegedly photons are still hitting telescope lenses on earth from the start of the universe 14+ billion years ago… how are the exempt from entropy?

  22. SlimThrull Author

    Dumb question. You mention velocity is bad for erosion. So, say we have a river that's flowing at X meters per second. Over the course of a year, we would expect it to erode Y units. If we doubled X (made the water flow twice as fast) would Y also be doubled (the erosion would happen twice as fast)? Or, I guess in simpler terms, is the velocity to erosion graph linear or something else?

  23. Cracked Emerald Author

    2:46 i read somewhere that shallow water flow is analogous to supersonic flow (or something like it) so could you compare subcritical flow to subsonic flow and supercritical flow to supersonic flow?


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