---
title: "Fixing design discharge of undersluices"
subject: "Irrigation and Drainage Engineering"
chapter: 5
key: "semester_6/irrigation_and_drainage_engineering:undersluice-design-discharge"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/undersluice-design-discharge
site: BCT Engineering
---

# Fixing design discharge of undersluices

Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks

- Asked in: 2076 Bhadra, 2 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/undersluice-design-discharge
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5

## Question

How do you fix the design discharge of undersluices?

## Short answer

The design discharge is the largest of: twice the canal discharge, 10–15% of maximum flood discharge, or the dry season low flood. It ensures sufficient scouring velocity. The waterway is then sized by the weir formula with the head difference between pond and tailwater.

## Answer

The design discharge of the undersluices is fixed on the basis of the scouring needs:

- It should be at least **twice the canal discharge** (to give enough scouring velocity in the pocket), or
- **10 to 15% of the maximum flood discharge of the river**, or
- the **dry season (low flood) discharge** that the river carries,

whichever is the largest. Sometimes the design discharge is the flow that passes when the pond level is kept and the weir crest is not overtopped. The head is the difference between the pond level and the d/s level at that discharge. Then the waterway of the undersluices is found from the discharge formula for free-flow $Q = C\,L\,H^{3/2}$.

## Check yourself

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

1. Which is a criterion for undersluice design discharge?
   A. Twice the canal discharge, or 10-15% of max flood, or dry-season flow, whichever is largest
   B. Half the canal discharge
   C. Equal to max flood
   D. Always 1% of weir discharge
   Answer: A. The largest of the three scouring criteria governs. (Key point: Criteria)

2. How is the sluice waterway found?
   A. From Lacey's formula
   B. From the free-flow formula Q = C L H^(3/2)
   C. From Manning's formula
   D. From Khosla's constants
   Answer: B. Waterway L comes from Q = C L H^(3/2) with head = pond level minus d/s level. (Key point: Waterway)

3. Why at least twice the canal discharge?
   A. To store water
   B. To fill the canal
   C. To give enough scouring velocity in the pocket
   D. To lower the pond
   Answer: C. A larger discharge gives scouring velocity in the pocket. (Key point: Purpose of criterion)

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