---
title: "Design considerations: free and submerged outlets"
subject: "Irrigation and Drainage Engineering"
chapter: 7
key: "semester_6/irrigation_and_drainage_engineering:free-submerged-outlets-design"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/free-submerged-outlets-design
site: BCT Engineering
---

# Design considerations: free and submerged outlets

Irrigation and Drainage Engineering · Chapter 7: Regulating Structures

- Asked in: 2081 Chaitra, 4 marks
- 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/free-submerged-outlets-design
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-7

## Question

Discuss design considerations for free and submerged outlets.

## Short answer

Free outlets discharge as a free jet; size from q = Cd a sqrt(2gH), set sill for desired flexibility, and ensure minimum head to avoid drowning. Submerged outlets depend on head difference between distributary and water course; size from q = Cd a sqrt(2g(Hd - Hw)), ensure submergence, and prevent tampering.

## Answer

#### Free (semi-modular) outlets
Water leaves as a free jet, so the discharge depends on the distributary level only: $q = C_d\,a\sqrt{2g\,H}$, with $H$ the head over the centre of the opening.

Design considerations:
1. Fix the required discharge $q = \dfrac{A\,\Delta}{T}$ from the command area, the depth of water (duty) and the period.
2. Choose the **setting** $H/D$ (sill level) to give the required flexibility, $F = \dfrac{m}{n}\dfrac{D}{H}$: a deeper (lower) setting gives a lower flexibility but draws more silt; a higher setting gives a higher flexibility. For proportionality, $H/D = m/n$.
3. Provide a minimum working head so that the jet is not drowned by the water course level (modular limit).
4. Size the opening from the discharge equation; a pipe also needs a check on its length and friction.
5. Set the sill at a suitable height above the distributary bed so that it draws its share of silt.
6. Provide a free fall or stilling space d/s, and protect the water course with stone pitching.

#### Submerged (non-modular) outlets
The water level in the water course is higher than the outlet crest, so the discharge is $q = C_d\,a\sqrt{2g\,(H_d - H_w)}$ and depends on both levels.

Design considerations:
1. Required discharge as above.
2. Determine the available head difference $(H_d - H_w)$ from the levels in the distributary and the water course at FSL.
3. Size the opening: $a = \dfrac{q}{C_d\sqrt{2g(H_d - H_w)}}$.
4. Check that the outlet remains submerged over the range of flows and does not become hyper-proportional.
5. Prevent tampering: the opening should be fixed, preferably a pipe set in masonry.
6. Provide a settling basin in the water course if silt is a problem.

Free outlets are preferred, since they are not affected by the water course.

## Check yourself

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

1. On what does the discharge of a free (semi-modular) outlet depend?
   A. Water course level
   B. Head over the centre of the opening from the distributary level only
   C. Difference of distributary and water course levels
   D. Only the pipe length
   Answer: B. The free jet gives q = Cd a sqrt(2gH) with H the head over the opening centre in the distributary. (Key point: Free outlet discharge)

2. How is the required discharge of the outlet fixed?
   A. From the distributary width
   B. From the pipe diameter
   C. From the silt factor
   D. From q = A Δ/T using command area, water depth (duty) and period
   Answer: D. The discharge follows from area x depth divided by the irrigation period. (Key point: Required discharge)

3. How is the opening area of a submerged outlet found?
   A. a = q/(Cd sqrt(2g(Hd - Hw))) using the available head difference
   B. a = q/(Cd sqrt(2g Hd))
   C. a = q x Cd
   D. a = Hd - Hw
   Answer: A. A submerged outlet flows under the head difference between the two levels, so area uses (Hd - Hw). (Key point: Submerged outlet sizing)

4. Why are free outlets preferred over submerged ones?
   A. They need no opening
   B. They are not affected by the water course level
   C. They are always hyper-proportional
   D. They need no head
   Answer: B. Free outlets give a discharge independent of the water course, so tampering or silting there does not change it. (Key point: Preference)

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