Cavitation erosion and energy dissipators

Cavitation erosion in spillways

Cavitation erosion is the damage (pitting and removal of concrete) on a spillway surface caused by the repeated collapse of vapour bubbles. In high-velocity flow over the spillway, pressure falls to the vapour pressure at low-pressure points (crest operating above design head, surface offsets, gate slots, curves); vapour bubbles form and then collapse near the surface, producing very high impact pressures that gradually erode the concrete. It is prevented by smooth surfaces, correct crest design and air entrainment (aerators).

Energy dissipators

Water leaving a spillway has high kinetic energy, which would scour the riverbed and endanger the dam. Energy dissipators destroy this energy before the water enters the river.

1. Hydraulic jump type stilling basins A hydraulic jump is formed on a concrete apron where supercritical flow changes to subcritical, dissipating energy in turbulence. USBR standard basins: Type I (simple apron), Type II (chute blocks + dentated end sill, high dams), Type III (chute blocks + baffle blocks + end sill, $V < 18$ m/s), Type IV (oscillating jump).

 spillway
   \
    \___   jump     
        \_/~~~\~~~~~~~~ TWL
  ________|_|_|___|__
  chute  baffle end sill
  blocks blocks

2. Bucket type dissipators (used when tailwater is too high for a jump or too low)

• Ski-jump (trajectory/flip) bucket: water is thrown high into the air and falls far from the dam into a plunge pool; used where tailwater is low and rock is sound (common in Nepal).

   \
    \      _..--''--.._
     \____/  jet        '.
      bucket             \ plunge pool
 _________________________\__~~~

• Solid and slotted roller bucket: submerged bucket where tailwater is high; water forms surface and ground rollers that dissipate energy.

 ~~~~~~~~~~~~~~~ TWL
   \    (roller)
    \___  ___
        \/   | lip
 ____________|____

3. Plunge pool / free fall: jet from a free overfall or flip bucket falls into a deep pool, which absorbs the energy.

4. Impact and baffle type: baffle blocks, impact walls, stepped spillways and dentated sills break up the flow.

Selection depends on tailwater rating curve versus jump-height curve, rock quality and economy.