---
title: "Modular, semi-modular, non-modular outlets"
subject: "Irrigation and Drainage Engineering"
chapter: 7
key: "semester_6/irrigation_and_drainage_engineering:outlets-modular-semimodular-nonmodular"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/outlets-modular-semimodular-nonmodular
site: BCT Engineering
---

# Modular, semi-modular, non-modular outlets

Irrigation and Drainage Engineering · Chapter 7: Regulating Structures

- Asked in: 2063 Asoj (old course), 4 marks; 2070 Chaitra (old course), 4 marks; 2064 Kartik (old course), 4 marks (asked 3 times)
- Video: [Types of Canal Outlets | Modular, Semi-Modular & Non-Modular | Irrigation Engineering](https://www.youtube.com/watch?v=Fwo7xk6zdGQ) — Polytechnic Class (READBY) 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/outlets-modular-semimodular-nonmodular
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-7

## Question

Explain the working of modular, semi-modular and non-modular outlets (with neat sketches).

## Short answer

Non-modular outlet discharge depends on both canal and watercourse levels (e.g., submerged pipe). Semi-modular outlet discharge depends only on distributary level if free flow is ensured (e.g., AOSM). Modular outlet provides constant discharge independent of both levels, using energy dissipating mechanisms (e.g., Gibb’s module).

## Answer

#### Non-modular outlet
Its discharge depends on the water levels in **both** the distributary and the water course: $q = C\,A\sqrt{2g\,(H_d - H_w)}$ when submerged. Any change of either level changes the discharge. Examples: open sluice, drowned pipe outlet.

```text
  distributary FSL ~~~~~|--pipe--|~~~~ water course
                       (submerged)
```

#### Semi-modular outlet
The discharge depends **only on the water level of the distributary** if the minimum working head is available, the water course level not influencing the flow (the jet discharges freely or the flow is made independent by a flume). Examples: pipe outlet with free discharge, Kennedy's gauge outlet, Crump's open flume, adjustable orifice semi-module.

```text
  Crump's adjustable orifice semi-module (AOSM)
 distributary                          water course
 ~~~~~ FSL ~~~~|_roof block_|~~ flume ~~
 ______ bed ___|  orifice   |__expanding__
                free jet, hydraulic jump in flume
```
The roof block can be raised or lowered to set the opening; the diverging flume gives a free flow.

#### Modular outlet
Discharge is practically **constant** and independent of the water levels in both distributary and water course, within a working range. Rigid modules have no moving parts: extra head is destroyed by a standing wave against baffles in a spiral eddy chamber (Gibb's module) or by a sloping roof with air vents (Khanna's module). Examples: Gibb's module, Khanna's module.

```text
  Gibb's rigid module
  distributary ~~~~ | eddy chamber with |~~~~ water course
                    | baffles (no moving parts) |
```
Modular outlets are accurate but costly and need careful maintenance, so semi-modular outlets are preferred in practice.

## Check yourself

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

1. Why does a non-modular outlet's discharge vary?
   A. It depends on the heads in both channels, q = CA sqrt(2g(Hd - Hw)) when submerged
   B. It depends only on the distributary level
   C. It depends on neither level
   D. It is regulated by a float
   Answer: A. Any change in either level changes the head and hence discharge. (Key point: Non-modular working)

2. What makes a semi-modular outlet independent of the water course?
   A. A float that adjusts the opening
   B. Free jet or flume flow, provided minimum working head is available
   C. Being fully submerged
   D. A very wide opening
   Answer: B. Free discharge breaks the influence of the water course level. (Key point: Semi-modular working)

3. How does a modular outlet keep a constant discharge?
   A. The orifice is fixed and large
   B. Water course level controls the flow
   C. A device such as a float or plunger adjusts the opening automatically
   D. Silt blocks the flow
   Answer: C. Gibb's module varies the orifice to compensate for level changes. (Key point: Modular working)

4. Why are semi-modular outlets preferred in practice?
   A. Modular ones are inaccurate
   B. Semi-modular ones give constant discharge
   C. Non-modular ones cost more
   D. Modular ones are costly and need careful maintenance
   Answer: D. Semi-modular outlets are cheap and give equitable distribution. (Key point: Practical choice)

## Related in Irrigation and Drainage Engineering

- [Non-modular, semi-modular outlet; module requirements](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Anonmodular-semimodular-working-module-requirements) — Irrigation and Drainage Engineering, chapter 7
- [Non-modular outlet working, merits, demerits](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Anon-modular-outlet-working-pros-cons) — Irrigation and Drainage Engineering, chapter 7
- [Outlet, requirements, modular vs semi-modular](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aoutlet-requirements-modular-distinction) — Irrigation and Drainage Engineering, chapter 7
- [Semi-modular pipe outlet design, flexibility 0.9](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asemimodular-pipe-outlet-flexibility-09) — Irrigation and Drainage Engineering, chapter 7