---
title: "Spillway and ogee spillway"
subject: "Hydro Power"
chapter: 6
key: "semester_6/hydro_power:spillway-ogee-brief"
source: https://www.bctengineer.com/q/semester_6/hydro_power/spillway-ogee-brief
site: BCT Engineering
---

# Spillway and ogee spillway

Hydro Power · Chapter 6: Spillways

- Asked in: 2074 Bhadra, 4 marks
- 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-ogee-brief
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-6

## Question

What do you mean by a spillway? Briefly describe on ogee spillway.

## Short answer

A spillway safely discharges excess floodwater to prevent dam overtopping. An ogee spillway is an overflow type with an S-shaped crest profile matching the lower nappe of a sharp-crested weir at design head. Its downstream face follows the WES standard curve (x^1.85 = 2H_d^0.85 y) ending in a bucket. Discharge is Q = C L H^3/2 (C≈2.2). At design head, pressures are near atmospheric, giving high efficiency; used for concrete gravity dams and barrages, gated or ungated.

## Answer

A **spillway** is a structure built as part of a dam to pass excess flood water safely from the reservoir to the river downstream, so that the reservoir level never rises above the maximum water level and the dam is not overtopped. It is the safety valve of a dam.

#### Ogee spillway
The **ogee spillway** is an overflow spillway whose crest and downstream face follow the shape of the **lower surface of the nappe** of water falling over a sharp-crested weir at the design head. Its profile is an S-shaped (ogee) curve.

```text
 ~~~~~~~~~~~~~~~~~~__ 
                 _/  \_   crest
                |      \
   dam body     |       \  d/s profile
                |        \
 _______________|_________\____~~~~ TWL
                     bucket
```

- **Upstream portion:** compound curves joining the upstream face to the crest.
- **Downstream profile:** WES shape $x^{1.85} = 2\,H_d^{0.85}\,y$ ($H_d$ = design head), followed by a straight slope and a **reverse curve/bucket** that delivers water to an energy dissipator.
- **Discharge:** $Q = C\,L\,H^{3/2}$, with $C \approx 2.2$ (SI units).
- At design head the pressure on the face is nearly atmospheric, giving high efficiency with no cavitation. At higher heads negative pressure (risk of cavitation) and at lower heads positive pressure (lower discharge coefficient) occur.
- Widely used for concrete gravity dams and barrages; may be gated or ungated.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A spillway is
   A. a structure passing excess flood water safely so the reservoir stays below MWL
   B. a structure to store flood water permanently
   C. a channel only for irrigation
   D. an energy-generating unit
   Answer: A. It is the safety valve of a dam. (Key point: Definition)

2. The ogee profile follows the
   A. upper surface of a free jet
   B. lower surface of the nappe over a sharp-crested weir at design head
   C. dam face slope
   D. river bed
   Answer: B. S-shaped curve fits the lower nappe. (Key point: Ogee profile)

3. The ogee discharge equation is
   A. Q = C L H^2
   B. Q = C L H^(1/2)
   C. Q = C L H^(3/2), with C about 2.2 (SI)
   D. Q = C H^(3/2) / L
   Answer: C. Weir-type relation with H^(3/2). (Key point: Discharge)

4. At the design head the pressure on the ogee face is
   A. strongly negative
   B. equal to reservoir pressure
   C. double atmospheric
   D. nearly atmospheric
   Answer: D. Hence high efficiency without cavitation. (Key point: Pressure)

## Related in Hydro Power

- [Spillway classification and ogee spillway](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aspillway-classification-ogee) — Hydro Power, chapter 6, asked 2×

## 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
- [Advantages and suitability of spillway types](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-types-advantages-suitability) — Hydropower Engineering, chapter 4