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.

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. Thank you for watching, and let me know what you think!