--- title: "Cavitation over spillways" subject: "Hydro Power" chapter: 6 key: "semester_6/hydro_power:cavitation-over-spillway" source: https://www.bctengineer.com/q/semester_6/hydro_power/cavitation-over-spillway site: BCT Engineering --- # Cavitation over spillways Hydro Power · Chapter 6: Spillways - Asked in: 2071 Magh, 2+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/cavitation-over-spillway - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-6 ## Question Define cavitation. Write about cavitation phenomenon over the spillway. ## Short answer Cavitation is the formation and violent collapse of vapour bubbles due to local pressure dropping to vapour pressure, causing erosion, noise and vibration. Over spillways, high flow velocities, operation above design head, surface offsets and gate slots create low-pressure regions that initiate cavitation. ## Answer **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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Cavitation is best defined as: A. Dissolving of air in water at high pressure B. Formation of vapour bubbles where local pressure falls to vapour pressure, followed by their violent collapse in higher pressure C. Boiling of water due to a rise in temperature D. Separation of sediment from flowing water Answer: B. Bubbles form at vapour pressure and collapse in higher-pressure regions, causing pitting and erosion. (Key point: Definition of cavitation) 2. Why does cavitation tend to occur on a high-dam spillway chute? A. Velocity is very high, so p/gamma = H - V^2/2g can fall to vapour pressure B. The water is too cold C. The flow is very slow and stagnant D. Pressure is always highly positive Answer: A. At 20-40 m/s the velocity head is large so the pressure head can drop towards vapour pressure. (Key point: Cause on spillway) 3. What happens on an ogee crest when the actual head exceeds about 1.33 Hd? A. The nappe is pressed onto the crest and pressure rises B. The flow becomes subcritical C. The nappe springs away from the surface, creating sub-atmospheric pressure and risk of cavitation D. The crest operates with maximum efficiency Answer: C. Above the design head the nappe lifts off the crest, producing negative pressure on the crest surface. (Key point: Operation above design head) 4. Which measure best prevents cavitation damage on a chute? A. Increasing roughness of the surface B. Using aerators to supply air (5-8% near the boundary) C. Making gate slots with sharp corners D. Operating always above design head Answer: B. Air in the boundary layer cushions bubble collapse, and 5-8% concentration almost eliminates damage. 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