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Dams are used to generate electric power by running the water through turbines. This uses the potential energy of the water, first converting it to kinetic energy of linear water movement, then rotation of the turbine.

Dams can get filled up so much that there is a risk of over-topping the dam. This poses a severe danger to the structural integrity of the dam. Because of this, a dam has a way to prevent uncontrolled over-topping by means of a spillway. This is a gate slightly below the maximal accepted water level that begins to over-top at a lower water level, and can guide and control the water. It can not, however, limit the total amount of water released.

In it's simplest form, it is not more than a reinforced path leading water to the foot of the dam to release it there when it spills over a specific section of the dam.

What is often used in practice is a simple straight channel, which converts most of the potential energy to kinetic energy of the linear movement of the water. At the end of the path, the energy is handled in some way.

The method I like to understand is as follows: At ground level, the water stream is disrupted and shot upwards away from the dam over a large water body, such that it is dispersed to a spray of droplets as a wide stream or cloud.

Where does the energy go?

  • The turbulent flow during spray creation heats the water
  • The spray creates strong wind
  • There is significant sound
  • Much water evaporates, which takes a lot of energy cooling the spray
  • Some of the spray just rains down to the water, and may take heat with it.

Is there something about surface tension and separating droplets of water?

What are the main factors in dispersing the energy?

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  • $\begingroup$ In the end, the main mechanism is viscous dissipation which converts the potential energy and the kinetic energy into increased internal energy (a little higher temperature) of the water (which then gets shared with the surrounding air and ground). $\endgroup$ – Chet Miller Oct 31 '19 at 12:59
  • $\begingroup$ @ChetMiller That comment contains interesting points for an answer, I'd say... $\endgroup$ – Volker Siegel Oct 31 '19 at 13:03
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You are correct. Of the mechanisms listed, the primary dissipation mechanism is violently turbulent mixing, which then heats the water, radiates sound, entrains air and forces it into motion, and creates surface area against the forces of surface tension.

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