Cavitation over spillways

Cavitation is the formation of vapour bubbles in a flowing liquid where the local pressure falls to the vapour pressure of the liquid, followed by the violent collapse of these bubbles when they move into a region of higher pressure. The collapse causes very high local pressures (hundreds of MPa), noise, vibration and pitting (erosion) of the boundary.

Cavitation over spillways

1. Cause – negative pressure: On a spillway the flow velocity is very high (20–40 m/s on high dams), so the pressure head $p/\gamma = H - V^2/2g$ can fall below atmospheric. When the pressure drops to the vapour pressure (about −10 m of water gauge), vapour cavities form.
2. Operation above design head: an ogee crest designed for head $H_d$ follows the nappe; when the actual head exceeds about 1.33 $H_d$, the nappe tends to spring away from the surface, creating sub-atmospheric pressure on the crest and leading to cavitation.
3. Surface irregularities: offsets, joints, protruding aggregate, rough patches, gate slots, sudden changes in slope or curvature cause local flow separation and low pressure zones in the high-velocity flow.
4. Gate slots and piers: sharp corners at gate grooves and pier ends generate vortices with low pressure cores.
5. Location: most damage occurs on the lower chute, the toe/bucket, baffle blocks of stilling basins and downstream of gate slots, where velocities are highest.

Effects

• Pitting and erosion of concrete; large holes can form in a few hours of high flood flow (e.g. damage at Glen Canyon dam spillway tunnels, 1983).
• Vibration and noise; reduction in discharge capacity.

Prevention

• Design crest for adequate head (design head close to maximum head) and smooth profiles.
• Strict surface tolerances; remove offsets and protrusions; use high-strength concrete or steel lining.
• Aeration: provide aerators (ramps, grooves, offsets) on chutes to supply air to the flow; 5–8% air concentration near the boundary almost eliminates cavitation damage.
• Streamlined gate slots and pier noses.
• Cavitation index check: keep $\sigma = \dfrac{p - p_v}{\rho V^2/2}$ above the critical value for the surface.