---
title: "Divide wall, under sluice, fish ladder functions"
subject: "Irrigation and Drainage Engineering"
chapter: 5
key: "semester_6/irrigation_and_drainage_engineering:divide-wall-undersluice-fish-ladder"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/divide-wall-undersluice-fish-ladder
site: BCT Engineering
---

# Divide wall, under sluice, fish ladder functions

Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks

- Asked in: 2079 Chaitra, 6 marks; 2080 Chaitra, 3 marks (asked 2 times)
- Video: [Part 20 ! Under sluice ! Divide wall ! Parts of diversion head work ! Scouring sluice !Divide groyne](https://www.youtube.com/watch?v=UOVHo7nn-6c) — NARAYANA COACHING CLASSES
- 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/divide-wall-undersluice-fish-ladder
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5

## Question

Describe briefly the key functions of fish ladder, divide wall and under sluices in a diversion headworks.

## Short answer

Fish ladder lets fish bypass the weir via a baffled slope. Divide wall separates the quiet pocket from weir flow, preventing cross-current damage and scour interference. Undersluices are low-level gates that scour pocket silt, maintain a clear channel to the head regulator, and pass low floods without dropping shutters.

## Answer

- **Fish ladder.** A fish ladder is a sloping channel (generally 1:10 to 1:15) with baffles or steps, built beside the divide wall at the weir. It lets fish swim upstream past the weir to their breeding places. The baffles slow the flow and form resting pools. It is necessary wherever fish migration must be protected, as in Nepal's rivers.
- **Divide wall.** A masonry or concrete wall built at right angles to the weir axis, between the weir and the undersluices. Its functions: (i) separates the quiet pocket in front of the head regulator from the weir flow, so that the silt in the pocket can be scoured by the sluices; (ii) keeps the cross-currents from the weir away from the sluices and the regulator; (iii) prevents the high-velocity flow over the weir from damaging the sluices; (iv) acts as the side wall of the fish ladder, if provided; (v) separates the weir floor from the undersluice floor, which is at a lower level.
- **Undersluices (scouring sluices).** Gated openings in the weir, next to the head regulator, with the crest at a low level (river bed). Functions: (i) scour out the silt deposited in the pocket by opening the gates; (ii) maintain a clear, deep channel in front of the canal head regulator; (iii) pass the low floods without dropping the weir shutters, so the pond level is kept; (iv) induce a curved flow so that the bed silt goes to the sluices and the canal gets the clearer upper water; (v) pass part of the high flood.

## Check yourself

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

1. What is the purpose of a fish ladder and its usual slope?
   A. To scour silt, slope 1:2
   B. To let fish pass upstream, slope 1:10 to 1:15
   C. To measure flow, slope 1:50
   D. To dissipate energy, slope 1:1
   Answer: B. A baffled sloping channel (1:10 to 1:15) lets fish migrate upstream to breeding places. (Key point: Fish ladder)

2. Which is a function of the divide wall?
   A. Passes low floods
   B. Draws clear upper water
   C. Keeps cross-currents from the weir away from the sluices and separates the pocket
   D. Raises the pond level
   Answer: C. It separates the quiet pocket from the weir and protects the sluices. (Key point: Divide wall)

3. What is a key function of the undersluices?
   A. Gate the canal discharge
   B. Hold fish
   C. Create a hydraulic jump on the weir
   D. Scour silt deposited in the pocket
   Answer: D. Opening the undersluice gates scours deposits and keeps a clear channel before the regulator. (Key point: Undersluice)

## Related in Irrigation and Drainage Engineering

- [Functions of divide wall, undersluice, silt devices](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adivide-wall-undersluice-silt-devices-functions) — Irrigation and Drainage Engineering, chapter 5
- [Fish ladder position and necessity](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Afish-ladder-necessity) — Irrigation and Drainage Engineering, chapter 5
- [Components and layout of diversion headworks](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aheadworks-components-layout) — Irrigation and Drainage Engineering, chapter 5, asked 2×
- [Headworks layout and cross sections](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aheadworks-layout-sections) — Irrigation and Drainage Engineering, chapter 5

## Related in other subjects

- [Design process for masonry wall under lateral loads](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Adesign-wall-lateral-loading) — Concrete Technology and Masonry Structure, chapter 9