---
title: "Spillway definition; siphon and shaft spillway"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:spillway-define-siphon-shaft"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spillway-define-siphon-shaft
site: BCT Engineering
---

# Spillway definition; siphon and shaft spillway

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2080 Bhadra, 1+3+3 marks
- Video: [Siphon Spillway](https://www.youtube.com/watch?v=E3c_Qw0GPwA) — Ammar Kamel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spillway-define-siphon-shaft
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

Define spillway. Explain siphon and shaft spillway with neat sketches.

## Short answer

A spillway is a structure that passes surplus flood water safely downstream. A siphon spillway is an inverted U-shaped conduit that primes automatically and regulates level without gates. A shaft spillway has a circular crest feeding a vertical shaft and tunnel, suitable for narrow gorges.

## Answer

#### Definition
A **spillway** is a hydraulic structure, usually part of a dam, that passes the surplus flood water from the reservoir to the downstream river without damaging the dam or the reservoir.

#### Siphon spillway
It is a closed conduit shaped like an inverted U, with its crest at the normal reservoir level, its inlet below the water surface and its outlet on the downstream side. Discharge starts when the reservoir rises above the crest: the water spilling over the crest seals the outlet and carries the air out of the bend, and once the air is removed the siphon primes. The conduit then flows full under a head equal to the difference between the reservoir level and the outlet (tail water) level.

```text
        air vent
          |   ___crest
     ~~~~~|__/   \__
 res.     /         \
 ~~~~~~~~/  inlet    \ throat
 ========            \_______
                     outlet (sealed in tail pool)
```
- Priming is automatic, and the siphon breaks when the reservoir drops (air enters through the vent), so it regulates itself and keeps a nearly constant reservoir level.
- Large capacity in a small space with a small rise in head; no gates are needed.
- Disadvantages: limited capacity for very large floods, clogging by floating debris, and vibration or cavitation when it is not fully primed.

#### Shaft (morning-glory) spillway
Water spills over a circular overflow crest (a funnel-shaped intake) into a vertical or inclined shaft, and then through a horizontal or sloping tunnel to the river below the dam.

```text
        crest ring
      ~~~~\______/~~~~    reservoir
          |    |
          | shaft |       elbow
          |    |
          \____|_________  ==> tunnel to river
```
- At low head it acts as a ring weir ($Q\propto H^{3/2}$); at higher head the crest is drowned and it behaves like an orifice or full-pipe flow, so the discharge then rises only slowly.
- Suitable for narrow gorges and steep rock abutments, and where a diversion tunnel can be reused.
- Needs anti-vortex piers and air vents to avoid cavitation and pulsation.

## Check yourself

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

1. What is a spillway?
   A. A structure passing surplus flood water from reservoir to the d/s river without damaging the dam
   B. A structure for diverting water to the turbines
   C. A structure for fish passage only
   D. A structure storing sediment
   Answer: A. A spillway safely releases excess flood water downstream. (Key point: Definition)

2. How does a siphon spillway regulate the reservoir level?
   A. It primes when water exceeds the crest and breaks when air enters via the vent as level drops
   B. It needs gates operated manually
   C. It works only with the outlet open to air
   D. It never stops flowing once started
   Answer: A. Priming is automatic and breaks when air enters, giving self-regulation at nearly constant level. (Key point: Siphon priming)

3. How does a shaft spillway behave as head increases?
   A. Acts as a ring weir (Q proportional to H^1.5), then as orifice or full pipe flow with slowly rising discharge
   B. Discharge increases as H^3
   C. Discharge stays proportional to H^1.5 always
   D. Discharge falls with head
   Answer: A. At higher head the crest drowns and flow is controlled by the shaft/tunnel, so discharge rises slowly. (Key point: Shaft spillway flow)

4. Where is a shaft spillway suitable?
   A. Narrow gorges with steep rock abutments, where a diversion tunnel can be reused
   B. Wide flat valleys with soft soil
   C. Low heads in canals
   D. Dams with heavy floating debris and no trash booms
   Answer: A. It needs little space at the dam and can use the diversion tunnel. (Key point: Shaft suitability)

## Related in Hydropower Engineering

- [Spillway need; chute, shaft; ski-jump conditions](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-chute-shaft-skijump-conditions) — 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
- [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

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