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Hydraulic jump
From Wikipedia, the free encyclopedia
A hydraulic jump is a phenomenon in the science of hydraulics.
A hydraulic jump represents a wave that desires to move in the upstream direction, but which is unable to do so, as the flow velocity of the water exceeds the wave's velocity with respect to the water. A hydraulic jump can be recognised through an increasing waterlevel in the downstream direction. Tidal bores are formed in this manner, and can be thought of as moving hydraulic jumps.
A hydraulic jump marks the border between supercritical and subcritical flow.
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The wave celerity (velocity of individual waves, as opposed to the velocity of a group of waves) of gravity waves in shallow water is given by:
which is similar to 
for small d;
In which:
- c = wave celerity (m/s)
- g = gravitational acceleration (9.81 m/s²)
- d = water depth (m)
wave number
A daily example of a hydraulic jump can be seen when brushing your teeth: in the sink. Around the place where the tap water hits the sink, you will see a smooth looking flow pattern. A little further away, you will see a sudden 'jump' in the water level. This is a hydraulic jump.
The energy considered in a hydraulic jump is in two forms - kinetic and potential. Kinetic energy required to stop the water, while potential energy is when the water is water is due to gravity that makes water flow downhill. If the water is on top of a hill it has more gravitational potential energy. As it flows to the bottom of the hill, however, some of the potential energy will be converted into kinetic energy.
For a given amount of energy, the flow can either be subcritical or supercritical depending on Froude number of the flow. Supercritical is faster and shallower, while subcritical is slower and deeper.
For a typical hydraulic jump the water coming toward the jump is supercritical. It comes upon a blockage/impediment. Initially the blockage is can be a wall/dam or some stones in the river. But after the water builds up behind it the blockage becomes the water itself. Across the jump momentum and mass are conserved so that in passing over the blockage the flow will transfer energy from kinetic to potential and in doing so changes from supercritical to subcritical. The total energy into the jump in the form of kinetic and potential is not conserved across the jump and this typically turned into undular waves, turbulence in the jump. Due to the dissipation of energy engineers use hydraulic jumps to reduce the destructive power of the water without wearing out dams and other man-made structures.