---
title: "Fish ladder position and necessity"
subject: "Irrigation and Drainage Engineering"
chapter: 5
key: "semester_6/irrigation_and_drainage_engineering:fish-ladder-necessity"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/fish-ladder-necessity
site: BCT Engineering
---

# Fish ladder position and necessity

Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks

- Asked in: 2068 Chaitra (old course), 5 marks
- Video: [COMPONENTS OF DIVERSION HEADWORK | ANIMATED VIDEO LECTURE | GATE | ASSISTANT ENGINEER | SUB ENGINEER](https://www.youtube.com/watch?v=oBisIdBDcLo) — Amaze Academy for Civil Engineering
- 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/fish-ladder-necessity
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5

## Question

Showing the position of a fish ladder, write its necessity in headworks.

## Short answer

A fish ladder is a stepped channel placed along the divide wall or bank, allowing fish to migrate past the barrage. It is necessary because weirs/barrages block spawning migration; the ladder with baffles provides low velocity (<2–3 m/s) and resting pools, preserving fish populations and complying with environmental regulations.

## Answer

A **fish ladder** is a sloping channel with a series of baffles or steps and pools, built along the divide wall (on the weir side or the river bank side) as shown in the plan below.

```text
 Left  <------ river flow ------>
 bank  ________________________________
  |    | weir bays |DW| sluices |..| head
  |    |           |  | pocket     | reg
  |    |  fish ladder  <-- along DW |
  |____|__________________________|
```

#### Necessity

- A weir or barrage across a river blocks the migration of fish, which swim upstream to spawn (e.g. mahseer and snow trout in Nepali rivers). The ladder allows them to pass through the structure.
- Fish need a velocity they can handle, usually below 2-3 m/s; the baffles and pools reduce the velocity and give them resting places.
- It keeps the aquatic ecology and fish population of the river, which provides livelihood and is required in environmental clearance (EIA) of the headworks.
- The ladder is usually 1:10 to 1:15 in slope, with entrance at the lowest point below the weir and exit at the pond level.

## Check yourself

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

1. Why is a fish ladder necessary?
   A. To scour silt
   B. The weir blocks fish that swim upstream to spawn
   C. To reduce afflux
   D. To measure flow
   Answer: B. A weir or barrage blocks fish migration upstream; the ladder lets them pass. (Key point: Why needed)

2. What do the baffles and pools do?
   A. Increase velocity
   B. Trap silt
   C. Reduce velocity and give resting places
   D. Raise the pond
   Answer: C. Baffles slow the flow below what fish can handle and provide rest. (Key point: Baffles)

3. What are the slope, entrance and exit?
   A. 1:2; entrance at pond level
   B. 1:10 to 1:15; entrance at the highest point
   C. 1:50; exit at tailwater
   D. 1:10 to 1:15; entrance below the weir, exit at pond level
   Answer: D. The ladder is gently sloped, entering low below the weir and exiting at pond level. (Key point: Slope and levels)

## Related in Irrigation and Drainage Engineering

- [Divide wall, under sluice, fish ladder functions](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adivide-wall-undersluice-fish-ladder) — Irrigation and Drainage Engineering, chapter 5, asked 2×
- [Headworks sketch, regulator section, protection works](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aheadworks-sketch-regulator-section-dissipators) — Irrigation and Drainage Engineering, chapter 7

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- [RoR headworks sketch; intake importance and location](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-headworks-arrangement-intake-location) — Hydropower Engineering, chapter 5