--- title: "Spillway classification and ogee spillway" subject: "Hydro Power" chapter: 6 key: "semester_6/hydro_power:spillway-classification-ogee" source: https://www.bctengineer.com/q/semester_6/hydro_power/spillway-classification-ogee site: BCT Engineering --- # Spillway classification and ogee spillway Hydro Power · Chapter 6: Spillways - Asked in: 2082 Chaitra, 1+2+4 marks; 2078 Chaitra, 1+1+4 marks (asked 2 times) - Video: [DHS Module5 [Part05] - Ogee Spillway | Effective Length of Spillway Crest | Ogee Profile](https://www.youtube.com/watch?v=F_xPXgoXT3Y) — AM CIVIL ACADEMY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/spillway-classification-ogee - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-6 ## Question What is a spillway? Classify spillways based on their prominent features. With a neat sketch, describe an Ogee spillway. ## Short answer A spillway passes floodwater to prevent overtopping. Classified by control (gated/ungated), function (main/auxiliary/emergency), shape (free overfall, ogee, chute, side channel, shaft, siphon, tunnel, labyrinth, stepped), and location. An ogee spillway has an S-shaped crest matching the lower nappe at design head, with WES downstream curve (x^1.85 = 2H_d^0.85 y) and a bucket. Discharge Q = C L H^3/2 (C≈2.2). At design head pressures are near atmospheric, ensuring high efficiency. ## Answer A **spillway** is a safety structure provided with a dam to pass surplus flood water from the reservoir to the downstream channel safely, so that the dam is not overtopped. #### Classification of spillways | Basis | Types | |---|---| | Control | Uncontrolled (ungated); Controlled (gated) | | Function / use | Main (service), Auxiliary, Emergency | | Shape and flow path | Free overfall, Ogee (overflow), Chute (trough), Side channel, Shaft (morning glory), Siphon, Tunnel/conduit, Labyrinth, Stepped | | Location | On the dam body (overflow); separate from dam (chute, side channel, shaft) | #### Ogee spillway An **ogee spillway** is an overflow spillway whose crest and downstream face are shaped to follow the profile of the **lower nappe of a fully aerated sheet of water** flowing over a sharp-crested weir. The profile looks like an elongated "S" (ogee curve). ```text upstream crest ~~~~~~~~~~~~~~~~~~~__ water surface _/ \__ upstream face | \_ downstream (vertical or | \ profile sloping) | \ (x^n = K H_d^(n-1) y) | \ | \___ reverse curve ________________|_______________\___~~~~~~ tailwater bucket / stilling basin ``` **Features:** - **Upstream crest curve:** compound circular curves (radii about $0.5H_d$ and $0.2H_d$) from the upstream face to the crest. - **Downstream profile:** WES standard shape, $x^{1.85} = 2.0\,H_d^{0.85}\,y$ for a vertical upstream face, where $H_d$ is design head. - **Straight tangent** portion following the dam slope, ending in a **reverse curve (bucket)** which turns the flow into a stilling basin or ski-jump bucket. **Discharge:** $Q = C\,L\,H_e^{3/2}$ with $C \approx 2.2$ (SI) at design head. **Working and merits:** - Since the face fits the nappe, pressures on the surface are near atmospheric at design head, giving high discharge efficiency without damage. - At heads above design head, negative pressures develop (cavitation risk); below design head, positive pressures reduce efficiency. - It is simple, reliable, and suitable for concrete gravity and buttress dams; can be gated or ungated. - Not suitable alone for earth dams, since water would flow over the dam body. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The ogee crest profile follows A. the lower nappe of a fully aerated sheet over a sharp-crested weir B. the upper nappe of a submerged weir C. a straight inclined line D. a circular arc only Answer: A. That gives near-atmospheric pressure at design head. (Key point: Ogee shape) 2. At heads above the design head on an ogee crest A. pressures stay atmospheric B. negative pressures develop, with cavitation risk C. discharge efficiency rises without limit D. the crest becomes a siphon Answer: B. Above design head negative pressures occur; below it positive pressures lower efficiency. (Key point: Below design head) 3. Spillways classified by control are A. main and auxiliary B. ogee and chute C. uncontrolled (ungated) and controlled (gated) D. overflow and side channel Answer: C. Control basis: gated or ungated. (Key point: Classification) 4. The WES downstream profile for a vertical upstream face is A. x^2 = 2 Hd y B. x^1.5 = Hd y C. y^1.85 = 2 Hd^0.85 x D. x^1.85 = 2.0 Hd^0.85 y Answer: D. Standard WES shape with Hd the design head. (Key point: WES profile) ## Related in Hydro Power - [Spillway and ogee spillway](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-ogee-brief) — Hydro Power, chapter 6 - [Purpose and types of spillways](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-purpose-types) — Hydro Power, chapter 6, asked 4× - [Cavitation over spillways](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acavitation-over-spillway) — Hydro Power, chapter 6 ## Related in other subjects - [Spillway purposes and ogee advantages](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-purposes-ogee-advantages) — Hydropower Engineering, chapter 4 - [Discharge through chute spillway ogee crest](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-chute-discharge-ogee-250m) — Hydropower Engineering, chapter 4 - [Purpose and types of spillways](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-purpose-types-explain-one) — Hydropower Engineering, chapter 4, asked 4× - [Features of microprocessor based instrumentation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-features) — Instrumentation II, chapter 1, asked 4×