---
title: "Spillway purposes and ogee advantages"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:spillway-purposes-ogee-advantages"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spillway-purposes-ogee-advantages
site: BCT Engineering
---

# Spillway purposes and ogee advantages

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2074 Ashwin, 2+2 marks
- Video: [Hydraulic Design of an Ogee Spillway](https://www.youtube.com/watch?v=mWBzCfeTLAg) — WIT Solapur - Professional Learning Community
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spillway-purposes-ogee-advantages
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

What are the purposes of a spillway? What are the advantages of ogee shape spillway? Explain.

## Short answer

A spillway safely passes floods, controls reservoir level, and dissipates energy. An ogee crest provides the highest discharge efficiency (C≈2.2), maintains near-atmospheric pressure at design head, ensures smooth flow, and integrates well with concrete dams.

## Answer

#### Purposes of a spillway
- **Safe passage of floods**: it passes surplus flood water from the reservoir to the d/s river and so prevents overtopping of the dam (a major cause of failure, especially for earth dams).
- **Reservoir level control**: keeps the reservoir below the maximum water level and protects the dam, intake and powerhouse.
- **Energy dissipation**: together with a stilling basin or bucket, it reduces the energy of the falling water, preventing scour at the dam toe.
- **Flood routing and sluicing**: gives a controlled release, helps pass debris, ice and silt, and allows lowering of the reservoir in an emergency.

#### Advantages of an ogee-shaped spillway
The ogee crest has an S-shaped profile; the upper curve follows the lower surface of a free-falling nappe over a sharp-crested weir, and the lower curve is tangent to the downstream face.

```text
        ~~~~~~~~ H
   ___  ~~~~~   .
  /   \ ~  nappe ' .
 |  crest \          '.
 |      ogee curve      '.   d/s face 1:0.8
 |                          '.
```
- **Highest discharge efficiency**: $Q=C\,L\,H^{3/2}$ with $C\approx2.2$ ($C_d\approx0.74$), larger than for broad-crested or round-crested weirs, so a shorter crest can pass the same flood.
- **Pressure at crest close to atmospheric** at design head, so there is no cavitation or vibration; cost of the spillway is low.
- **Smooth, stable flow** with little energy loss and no separation; the nappe is supported by the crest.
- It fits well as the **overflow section of a concrete dam**, can be fitted with gates, and links smoothly to a stilling basin, bucket or ski-jump.
- Ability to pass debris and ice over the crest without clogging.

## Check yourself

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

1. Which purpose of a spillway prevents a major cause of dam failure?
   A. Safe passage of floods to prevent overtopping
   B. Raising the reservoir level
   C. Storing silt
   D. Producing power
   Answer: A. Passing surplus flood prevents overtopping, a major failure cause especially for earth dams. (Key point: Overtopping)

2. What defines the ogee crest shape?
   A. The upper curve follows the lower surface of a free-falling nappe over a sharp-crested weir, with the lower curve tangent to the d/s face
   B. A flat horizontal top
   C. A semicircle
   D. A triangular wedge
   Answer: A. The S-shaped profile matches the nappe, so the water is supported by the crest. (Key point: Ogee shape)

3. Why can an ogee spillway use a shorter crest than a broad-crested weir for the same flood?
   A. Its discharge coefficient (C about 2.2) is higher
   B. Its crest is lower
   C. It has no head
   D. It has more piers
   Answer: A. Q = C L H^1.5 with higher C means less length for the same discharge, and pressure stays near atmospheric. (Key point: Ogee advantages)

## Related in Hydropower Engineering

- [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
- [Purposes of spillway; types of gates](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-purposes-and-gate-types) — 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

## Related in other subjects

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