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A river to examine
Appropriate equipment to traverse a riparian environment
Understanding hydraulic jumps
Discussing real world examples of hydraulic jumps
Tips for navigating hydraulic jumps
In this lesson, Trevor is discussing the principal of hydraulic jumps. Building upon our knowledge of Flow States, we will be taking our understanding of critical flow zones further by identifying the 4 types of hydraulic jumps found in critical flow zones.
Understanding how hydraulic jumps work is closely related to flow states. If you are still a little unsure about how river flow works, please check out the Flow States Lesson before diving deeper into this lesson.
A hydraulic jump refers to the depth of the flow at the tailwater and the lip of the drop structure (below and above) an area of Critical Flow. The simplest way to put it is that a hydraulic jump occurs where the super critical flow impacts the sub critical flow and you see a jump in water depth. The water depth will suddenly increase at the tailwater as the super critical flow loses momentum. In hydrology a drop structure refers to any natural or man-made point at which the water falls creating super critical flow.
The connotations for boaters are that a hole isn’t just a hole because there are 4 types of hydraulic jumps which may create what we traditionally know as a “hole”. Each of these jumps has a different connotation for what will likely happen to rafts and swimmers.
High velocity super critical flow will have a lot of momentum and that momentum can push a lot of water out of the way. The main characteristic of the type A jump is “swept out tailwater”. In scientific terms the super critical flow has so much momentum that the hydraulic jump is pushed back well into the tailwater behind a drop structure. As rafters we typically encounter these types of features in low volume scenarios or whenever the water has a lot of momentum. Some things to look for with type A jumps:
Swept out tailwater
Minimal recirculation
Powerful super critical flow going over the drop
The critical flow forms well away from the drop structure
Water level of the tail water is well below the lip of the drop structure
Generally safe to swim because it will flush swimmers downstream
When the tailwater rises to a significant depth that the super critical flow can no longer sweep the hydraulic jump away from the drop structure a type B hydraulic jump begins to form. The type B jump forms directly at the base of a drop structure and is commonly what rafters call a surf wave. This is the first type of feature rafters tend to call a “hole”, but for our purposes this term is too broad in scope and does not accurately reflect what a type B jump will do. Type B jumps will tend to have all of their current moving downstream with the exception of a slight recirculation zone which can provide a violent surf. Here are some things to lookout for in a Type B Jump:
Critical flow at the base of a drop
The majority of the water moving downstream
No defined boil zone, but chaotic water flushing through the sub-critical flow
Small amounts of surface recirculation
Generally safe to swim because it will flush swimmers downstream
As the tail water continues to rise at the base of a drop the hydraulic jump can become submerged with the critical flow just below the surface of the water. This is the most dangerous type of hydraulic jump because it will have a strong recirculation zone. It can also be tricky to navigate, because rafts can lose a lot of momentum due to the opposing momentum of the sub-critical flow. As air becomes entrained in the critical flow, those turbulent bubbles get driven to the bottom of the river below the drop structure. From there the entrained air rises to the surface bringing water with it and creating a strong boil zone below the drop structure. Some clues you can use to help identify type C jumps are:
Critical flow at the base of a drop slightly under water
The majority of the water up the boil line back into the critical flow
Large amounts of recirculation
Produces less violent surfs, but is particularly unsafe to swim due to the possibility of flush drowning
Strong sub-critical flow will tend to stop the momentum of the raft and flush it back into a surf
When the tail water rises to be near even with the top of the drop structure the critical flow becomes washed out, but it does begin to oscillate in a predictable pattern. This pattern typically manifests in the from of a wave train on the surface of the water. The majority of the water will be moving downstream and poses little hazard to swimmers as the supercritical flow simply pushes over the sub-critical flow. Here are some tips on identifying type D jumps:
Critical flow is hard to locate below the drop structure
Smooth predictable wave trains
The drop structure is well underwater
Generally safe for swimmers as most of the water flushed downstream
Typically encountered in high water scenarios or big water rivers
We want to say a huge thanks n this video to Dr. Rollin Hotchkiss from BYU who has been doing some fantastic work on identifying lowhead dams throughout the country. if you would like to help support his work you can download the free app and report lowhead dams to the database.
