---
title: "Inverted siphon"
subject: "Sanitary Engineering"
chapter: 4
key: "semester_6/sanitary_engineering:inverted-siphon"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/inverted-siphon
site: BCT Engineering
---

# Inverted siphon

Sanitary Engineering · Chapter 4: Sewer Appurtenances

- Asked in: 2070 Bhadra, 4 marks
- Video: [Inverted siphon](https://www.youtube.com/watch?v=2_ofCZJnYaU) — Vidhyarani Kshirsagar
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/inverted-siphon
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4

## Question

What is an inverted siphon? In what circumstances is such a structure provided? What are the purposes served by it? Describe briefly with neat sketch.

## Short answer

An inverted siphon is a pressurized sewer section dipping below obstructions like rivers or railways to maintain gravity flow. It uses inlet/outlet chambers with multiple pipes (dry/wet weather) to ensure self-cleansing velocity (0.9-1.0 m/s). Constructed of cast iron, steel, or RCC, it avoids pumping and aqueduct costs while minimizing head loss.

## Answer

An **inverted siphon** (depressed sewer) is a sewer section which dips below the hydraulic gradient line to pass under an obstruction such as a stream, a railway, a road or a depression, and then rises to the original level. It runs full under pressure; it is not a true siphon.

#### Circumstances for providing it
- To cross a **river, canal, stream or drainage channel** below the bed.
- To pass under **railway lines, highways, tunnels or underground utilities**.
- To cross a **deep valley or depression** where the ground drops below the sewer's gradient and a bridge would be costly.

#### Purposes served
1. Lets the sewer cross an obstruction without pumping, using only the head available.
2. Avoids the cost of an aqueduct or bridge.
3. Keeps the sewage flowing by gravity with only a small loss of head.

#### Construction
```text
 Inlet                          Outlet
 chamber                        chamber
  __                              __
 |  |-\                        /-|  |-->
 |  |  \  Ground level         /  |  |
 ----    \______________/ ------
         depressed sewer (below stream)
```
- The sewer is made of **cast iron or steel** (or RCC encased in concrete) pipes, laid below the obstruction.
- An **inlet chamber** and an **outlet chamber** are built at both ends. The inlet chamber has an overflow, and sluice gates or stop planks so that one of the pipes can be shut.
- Usually **two or more pipes** (dry weather pipe and wet weather pipe) are provided, so that the velocity in the dry weather flow is not less than 0.9-1.0 m/s, which prevents silting.
- The pipes are also provided with cleaning access (manholes and washouts).
- The head loss is equal to $\dfrac{v^2}{2g}$ plus friction, and the levels are fixed so that the loss is available.

## Check yourself

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

1. What is an inverted siphon?
   A. A tank that flushes sewers
   B. A pipe that vents gas
   C. A pump station
   D. A sewer section dipping below the hydraulic gradient line to pass under an obstruction and run full under pressure
   Answer: D. It runs full under pressure; not a true siphon. (Key point: Definition)

2. When is it provided?
   A. To cross rivers, railways, roads or depressions below them
   B. To store sewage
   C. To join sewers at same level
   D. For ventilation
   Answer: A. It crosses obstructions without pumping. (Key point: Use)

3. Why are two or more pipes (dry and wet weather) provided?
   A. To double the cost
   B. To keep velocity at least 0.9-1.0 m/s in dry flow and avoid silting
   C. To allow less flow
   D. To reduce head
   Answer: B. A small pipe maintains self-cleansing velocity in dry weather. (Key point: Dry weather pipe)

4. Which pipe materials are used?
   A. Stoneware only
   B. Plain PVC unprotected
   C. Cast iron or steel, or RCC encased in concrete
   D. Clay tile
   Answer: C. Strong pressure-resisting pipes are needed. (Key point: Material)

## Related in other subjects

- [Short note: aqueducts and inverted siphon](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aaqueduct-siphon-note) — Transportation Engineering I, chapter 4, asked 2×
- [NDT circumstances; ultrasonic pulse velocity test](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Andt-circumstances-upv) — Concrete Technology and Masonry Structure, chapter 5