--- title: "Purpose and types of spillways" subject: "Hydro Power" chapter: 6 key: "semester_6/hydro_power:spillway-purpose-types" source: https://www.bctengineer.com/q/semester_6/hydro_power/spillway-purpose-types site: BCT Engineering --- # Purpose and types of spillways Hydro Power ยท Chapter 6: Spillways - Asked in: 2080 Asoj, 1+5 marks; 2081 Asoj, 1+3 marks; 2073 Bhadra, 1+5 marks; 2074 Bhadra, 4 marks (asked 4 times) - Video: [#spillway SPILLWAYS: TYPES OF SPILLWAYS & ITS CLASSIFICATION #spillway #civilengineering](https://www.youtube.com/watch?v=1w-MmF0jujY) โ€” ๐๐ฅ๐จ๐จ๐ฆ ๐ฐ๐ข๐ญ๐ก ๐€๐ง๐š๐ฆ๐ข๐ค๐š - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/spillway-purpose-types - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-6 ## Question What is the purpose of providing spillway? List out different types of spillways with neat sketches. ## Short answer A spillway safely passes surplus floodwater from the reservoir to downstream, preventing dam overtopping and maintaining reservoir level below maximum water level. Common types include free overfall, ogee (overflow), chute (trough), side channel, shaft (morning glory), siphon, tunnel/conduit, labyrinth, and stepped spillways, each suited to specific site conditions and dam types. ## Answer A **spillway** is a structure built with a dam to safely pass surplus flood water from the reservoir to the downstream river when the reservoir is full. #### Purpose of a spillway - To protect the dam from **overtopping** (especially earth and rockfill dams) during floods. - To keep the reservoir level below the maximum water level (MWL). - To release water safely without eroding the dam toe or downstream bed. #### Types of spillways 1. **Free overfall (straight drop) spillway:** water falls freely from a thin crest; used for low weirs and thin arch dams on firm rock. ```text ~~~~~~~|\ | \ nappe | \ _______|___\~~~~~ ``` 2. **Ogee (overflow) spillway:** crest shaped to fit the lower nappe of a sharp-crested weir; most common for concrete gravity dams. ```text ~~~~~~~_ / \__ | \ | \___ _____|___________\___~~ ``` 3. **Chute (trough) spillway:** water flows over a short crest into a steep open channel (chute) to the river; used with earth dams. ```text reservoir crest ~~~~~~~~~~|----\ \ chute \______ stilling basin ``` 4. **Side channel spillway:** crest is parallel to the channel; water spills sideways into a trough and then into a chute or tunnel; used in narrow valleys. ```text ~~~~~~~ reservoir ~~~~~~ ======== crest ========= | side channel ---> | -> chute ``` 5. **Shaft (morning glory) spillway:** water enters a circular funnel crest, drops through a vertical shaft and leaves through a horizontal tunnel. ```text ~~~\ /~~~ | | shaft | |______ |_________ -> tunnel ``` 6. **Siphon spillway:** closed duct shaped like an inverted U; flow primes automatically when water rises above crest, giving a large discharge for small head rise. ```text ____ / __ \ ~|~| | \ |_| | \__ outlet ``` 7. **Tunnel (conduit) spillway:** flow is carried through a closed tunnel or conduit around or under the dam. 8. **Labyrinth and stepped spillways** are also used (longer crest in plan; steps to dissipate energy). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The main purpose of a spillway is to A. pass surplus flood water safely and prevent overtopping of the dam B. generate electricity from floods C. store sediment D. raise the reservoir above MWL Answer: A. It protects the dam, especially earth and rockfill dams. (Key point: Purpose) 2. The ogee spillway is most common for A. earth dams B. concrete gravity dams C. arch dams only on firm rock D. tunnel diversion only Answer: B. Its crest fits the lower nappe of a sharp-crested weir. (Key point: Ogee) 3. A chute spillway is mainly used with A. concrete gravity dams as an overflow section B. siphon systems C. earth dams, as a steep open channel to the river D. low arch dams for free fall Answer: C. Water cannot flow over earth dams, so a separate chute is used. (Key point: Chute) 4. A siphon spillway is characterised by A. a vertical funnel shaft B. a long zigzag crest C. a crest parallel to a side trough D. an inverted U duct that primes automatically, giving large discharge for small rise in level Answer: D. Siphon: closed inverted-U duct priming when water exceeds crest. (Key point: Siphon) ## Related in Hydro Power - [Spillway classification and ogee spillway](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-classification-ogee) โ€” Hydro Power, chapter 6, asked 2ร— - [Cavitation over spillways](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acavitation-over-spillway) โ€” Hydro Power, chapter 6 - [Spillway function, vertical and radial gates](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-function-vertical-radial-gate) โ€” Hydro Power, chapter 6 - [Spillway types suitable for Nepalese rivers](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-types-nepal) โ€” Hydro Power, chapter 6 ## Related in other subjects - [Purpose and types of spillways](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-purpose-types-explain-one) โ€” Hydropower Engineering, chapter 4, asked 4ร— - [Spillway need; chute, shaft; ski-jump conditions](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-chute-shaft-skijump-conditions) โ€” Hydropower Engineering, chapter 4 - [Methods of providing superelevation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asuper-methods) โ€” Transportation Engineering I, chapter 3, asked 5ร— - [Purposes of spillway; types of gates](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-purposes-and-gate-types) โ€” Hydropower Engineering, chapter 4