---
title: "Non-modular, semi-modular outlet; module requirements"
subject: "Irrigation and Drainage Engineering"
chapter: 7
key: "semester_6/irrigation_and_drainage_engineering:nonmodular-semimodular-working-module-requirements"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/nonmodular-semimodular-working-module-requirements
site: BCT Engineering
---

# Non-modular, semi-modular outlet; module requirements

Irrigation and Drainage Engineering · Chapter 7: Regulating Structures

- Asked in: 2072 Asoj, 2+2 marks
- Video: [Types of canal outlet with Live video Example || Irrigation canal module types](https://www.youtube.com/watch?v=sw5seo8N21M) — Gyan Tokri
- 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/nonmodular-semimodular-working-module-requirements
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-7

## Question

Explain the working principle of non-modular and semi-modular outlet. What are the requirements of a good module?

## Short answer

Non-modular: discharge depends on both distributary and water course levels. Semi-modular: discharge depends only on distributary head (free orifice), unaffected by water course level. Good module: simple, cheap, robust, low head, proportionate silt, flexibility near 1, tamper-proof.

## Answer

#### Non-modular outlet (working)
Its discharge depends on the water level in both the distributary and the water course: $q = C_d\,a\sqrt{2g(H_d - H_w)}$. Raising the distributary level increases $q$; raising the water course level (silting) reduces $q$. Example: open sluice, drowned pipe.

#### Semi-modular (flexible) outlet
Its discharge depends on the distributary level only, provided a minimum working head exists. The flow leaves as a free jet, so the water course level has no effect. Example: Kennedy's gauge outlet, Crump's adjustable orifice semi-module (AOSM), free pipe outlet.

```text
  distributary ~~FSL~~|--orifice--> free jet ~~ water course
  discharge = f(H_d) only; independent of H_w
```

#### Requirements of a good module (outlet)
1. **Simple and robust:** no moving parts that can jam or be tampered with.
2. **Cheap** and easy to construct and maintain.
3. **Minimum working head** requirement, so that command is maintained.
4. **Correct share of silt:** it should draw its proportionate share of the silt carried by the distributary (not set too low or too high).
5. **Proportional (flexibility near one) or at least sub-proportional**, for equitable distribution.
6. **Tamper-proof:** the discharge cannot be changed by the farmer.
7. **Adjustable** to suit changes in the crop, if required (semi-modular).
8. **Little sensitivity** to water level fluctuations.

## Check yourself

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

1. What determines the discharge of a semi-modular outlet?
   A. Water course level only
   B. Distributary level only, provided a minimum working head exists
   C. Both levels equally
   D. Neither level, it is fixed
   Answer: B. It discharges as a free jet, so the water course level does not affect q if minimum head is available. (Key point: Semi-modular working)

2. A rise in the water course level has what effect on a non-modular outlet compared with a semi-modular outlet?
   A. Reduces both discharges equally
   B. Increases both discharges
   C. No effect on non-modular, reduces semi-modular
   D. Reduces the non-modular discharge but has no effect on the semi-modular outlet
   Answer: D. Non-modular q depends on (Hd - Hw); the semi-modular free jet is independent of Hw. (Key point: Non-modular vs semi-modular behaviour)

3. Which is an example of a semi-modular outlet?
   A. Crump's adjustable orifice semi-module
   B. Open sluice
   C. Drowned pipe outlet
   D. Submerged pipe outlet
   Answer: A. Kennedy's gauge outlet, Crump's AOSM and free pipe outlet are semi-modular; open sluice and drowned pipe are non-modular. (Key point: Examples)

4. What silt requirement should a good module meet?
   A. Draw no silt at all
   B. Draw as much silt as possible
   C. Draw its proportionate share of the silt carried by the distributary
   D. Draw only coarse silt
   Answer: C. Setting the outlet too low or too high would over- or under-draw silt, so it should take its fair share. (Key point: Silt requirement of a module)

## Related in Irrigation and Drainage Engineering

- [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
- [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
- [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
- [Modular, semi-modular, non-modular outlets](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aoutlets-modular-semimodular-nonmodular) — Irrigation and Drainage Engineering, chapter 7, asked 3×
- [Adjustable orifice semi-module design](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aadjustable-orifice-semimodule-022) — Irrigation and Drainage Engineering, chapter 7
- [Canal outlet and module performance criteria](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-outlet-module-performance-criteria) — Irrigation and Drainage Engineering, chapter 7

## Related in other subjects

- [Good insulating material requirements and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-application) — Electric Machine Design, chapter 1, asked 2×
- [Requirements of a good pipe material](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arequirements-good-pipe-material) — Water Supply Engineering, chapter 8, asked 2×
- [Good pipe requirements; expansion joint](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apipe-requirements-expansion-joint) — Water Supply Engineering, chapter 8, asked 2×
- [Requirements of a good radio receiver; superheterodyne](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Agood-receiver-superheterodyne) — Communication Systems, chapter 2