--- title: "Cavitation erosion and energy dissipators" subject: "Hydro Power" chapter: 6 key: "semester_6/hydro_power:cavitation-erosion-energy-dissipators" source: https://www.bctengineer.com/q/semester_6/hydro_power/cavitation-erosion-energy-dissipators site: BCT Engineering --- # Cavitation erosion and energy dissipators Hydro Power · Chapter 6: Spillways - Asked in: 2070 Magh, 2+4 marks - Video: [What is a Hydraulic Jump?](https://www.youtube.com/watch?v=7tjf8HWiR3Y) — Practical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/cavitation-erosion-energy-dissipators - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-6 ## Question Define cavitation erosion in spillway. Explain with neat sketch of different types of energy dissipator used on hydropower plant. ## Short answer Cavitation erosion is pitting and concrete removal caused by repeated collapse of vapour bubbles on spillway surfaces. Energy dissipators include hydraulic-jump stilling basins, ski-jump (flip) buckets, roller buckets, and plunge pools; they destroy excess kinetic energy to prevent downstream scour. ## Answer #### 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). ```text 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). ```text \ \ _..--''--.._ \____/ jet '. bucket \ plunge pool _________________________\__~~~ ``` - **Solid and slotted roller bucket:** submerged bucket where tailwater is high; water forms surface and ground rollers that dissipate energy. ```text ~~~~~~~~~~~~~~~ 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What causes cavitation erosion of spillway concrete? A. Chemical attack by dissolved salts B. Repeated collapse of vapour bubbles near the surface producing high impact pressures C. Freezing of water in cracks D. Weight of the water on the crest Answer: B. Bubbles collapse near the surface and the repeated high impact pressures gradually pit the concrete. (Key point: Cavitation erosion) 2. Why are energy dissipators provided below a spillway? A. To increase the velocity of water B. To store water for irrigation C. To destroy the high kinetic energy of the water so it does not scour the riverbed and endanger the dam D. To create cavitation intentionally Answer: C. Spillway water carries high kinetic energy that would scour the bed; dissipators remove it before water enters the river. (Key point: Purpose of energy dissipators) 3. How does a hydraulic jump stilling basin dissipate energy? A. Supercritical flow changes to subcritical flow on an apron, dissipating energy in turbulence B. Water is thrown into the air far downstream C. Water falls freely into a deep pool only D. Flow changes from subcritical to supercritical Answer: A. The jump converts fast supercritical flow to slow subcritical flow, losing energy in turbulence. (Key point: Hydraulic jump basin) 4. When is a ski-jump (flip) bucket suitable? A. Where tailwater is high and rock is weak B. Where tailwater is low and the rock is sound, so the jet lands far away in a plunge pool C. Only when a hydraulic jump is easily formed D. Only for very low dams without a river Answer: B. The jet is thrown far from the dam into a plunge pool, which needs low tailwater and sound rock. (Key point: Ski-jump bucket use) ## Related in Hydro Power - [Cavitation over spillways](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acavitation-over-spillway) — Hydro Power, chapter 6 - [Spillway types suitable for Nepalese rivers](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-types-nepal) — Hydro Power, chapter 6 - [Firm power and plant load factors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afirm-power-load-factors) — Hydro Power, chapter 3 - [Intake site selection for RoR plant](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aintake-site-selection-ror) — Hydro Power, chapter 5 - [Primary and secondary energy at 45% exceedance](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aprimary-secondary-energy-45pct) — Hydro Power, chapter 3 - [Primary and secondary energy with 10% environmental flow](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aenergy-with-environmental-flow-950) — Hydro Power, chapter 3 ## Related in other subjects - [Equipment used in hydropower construction](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Ahydropower-equipment) — Construction Management, chapter 4 - [Schematic of a PROR hydropower plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apror-schematic-components) — Power Plant Design, chapter 3, asked 2× - [Hydropower plant components and functions](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydropower-components-functions) — Power Plant Design, chapter 3, asked 2× - [Steady state operation of hydropower plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Ahydropower-steady-state-operation) — Power Plant Equipment, chapter 1