---
title: "Short note: aqueducts and inverted siphon"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:aqueduct-siphon-note"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/aqueduct-siphon-note
site: BCT Engineering
---

# Short note: aqueducts and inverted siphon

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2063 Kartik (old course), 4 marks; 2062 Jestha (old course), 4 marks (asked 2 times)
- Video: [The inverted siphon](https://www.youtube.com/watch?v=SUZMd9NyA78) — Hubert Chanson
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/aqueduct-siphon-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

Write a short note on aqueducts and inverted siphon.

## Short answer

An aqueduct carries a canal over a drain or road when the canal bed is above the high flood level; the drain passes below. An inverted siphon is a closed pipe running full under pressure below an obstacle, used when there is insufficient head. Siphon velocity should be 2 to 3 m/s with silt trap and air vent.

## Answer

#### Aqueduct
An **aqueduct** is a structure that carries a canal or open channel over a natural drainage, river, or road. The canal bed is above the High Flood Level (HFL) of the drainage. The canal flows in a trough supported on piers or abutments, and the drainage water passes below, like a bridge for water. Where a road meets an irrigation canal, an aqueduct (canal over road) lets the road pass freely beneath.

Types: (i) a canal trough of RCC/steel on piers (like a bridge); (ii) a *siphon aqueduct*, where the HFL of the drain is above the canal bed, so the drain flows under pressure through barrels beneath the canal.

```text
   canal trough ===============   (canal over drain)
   ============================
   |pier|     drain flows     |pier|
   |____|~~~~~~~~~~~~~~~~~~~~~|____|
```

#### Inverted siphon
An **inverted siphon** is a closed conduit (pipe or barrel) that runs below an obstacle: the water goes down, runs full under pressure, and rises again on the other side. It is used where a drain, stream or canal must cross a road, canal or valley and there is not enough head to pass above it. In road drainage, a drain that passes beneath a canal or road at a level below its hydraulic gradient line forms an inverted siphon.

```text
  inlet |\               /| outlet
        | \_____________/ |    <- pipe full, under
        |  (below road/canal)    pressure
```

- Velocity should be 2 to 3 m/s to prevent silting.
- Needs a silt trap and an air vent.
- Head loss must be checked.

An aqueduct is chosen when the canal bed is well above the HFL of the drain. An inverted siphon is used when the obstacle (canal or road) lies across the water's path at or above its water level, so the water must pass beneath it under pressure.

## Check yourself

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

1. An aqueduct is:
   A. A causeway
   B. A pipe carrying drain under a canal
   C. A structure carrying a canal over a drainage, river or road
   D. A side drain
   Answer: C. The canal bed is above the HFL of the drainage and flows in a trough. (Key point: Aqueduct)

2. An inverted siphon is:
   A. A trough on piers
   B. A closed conduit running full under pressure below the obstacle
   C. An open drain
   D. A bridge
   Answer: B. Water goes down, runs full and rises again on the other side. (Key point: Inverted siphon)

3. When is an inverted siphon chosen rather than an aqueduct?
   A. When canal level is higher than HFL of the drain
   B. When the canal or road is lower than the HFL
   C. When no water flows
   D. Always
   Answer: B. An aqueduct suits canal above HFL; a siphon suits canal/road below the HFL. (Key point: Choice)

## Related in other subjects

- [Inverted siphon](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Ainverted-siphon) — Sanitary Engineering, chapter 4