---
title: "Longitudinal section of a head regulator"
subject: "Irrigation and Drainage Engineering"
chapter: 7
key: "semester_6/irrigation_and_drainage_engineering:head-regulator-longitudinal-section"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/head-regulator-longitudinal-section
site: BCT Engineering
---

# Longitudinal section of a head regulator

Irrigation and Drainage Engineering · Chapter 7: Regulating Structures

- Asked in: 2066 Bhadra (old course), 4 marks
- Video: [what is head regulator in canal | what is free board in canal | sluice gate](https://www.youtube.com/watch?v=SVjfgX0DCWA) — 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/head-regulator-longitudinal-section
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-7

## Question

Draw a neat longitudinal section through a head regulator of an irrigation head works showing all components.

## Short answer

A head regulator longitudinal section shows a raised crest with gates, upstream cut-off and floor, downstream stilling basin with energy dissipation, downstream cut-off, and protection works of inverted filter and stone apron. These elements lengthen seepage, reduce uplift, and prevent scour.

## Answer

A canal head regulator is built at the off-take of a canal from the headworks pond (u/s of the weir). A longitudinal section along the axis of the canal shows the following components.

```text
 Pond level ~~~~~~~~|  gate  |~~~~~~~ FSL canal ~~~~~~
                    |  hoist |
                 ___|  bridge|___
 crest level ___/ piers + gates \_____
 |u/s    |  u/s    |  crest     | d/s floor |  d/s
 |cut-off|  floor  |  (raised)  | (stilling)|  protection
 |  d1   |         |            |           | inv. filter
 |       |         |            |           | + apron
 |_______|_________|____________|___________|___ canal bed
                 <----- impervious floor ----->
```

**Components**
1. **Upstream cut-off (pile) and upstream floor:** lengthen the seepage path and reduce the uplift and exit gradient.
2. **Crest wall:** raised about 1.2 m above the under-sluice crest or river bed to exclude bed load.
3. **Gates and piers** (vertical lift gates) with a bridge and hoist.
4. **Downstream floor** (stilling basin): dissipates energy and withstands uplift.
5. **Downstream cut-off:** keeps the exit gradient safe and protects the floor against scour.
6. **Protection works:** inverted filter, loose stone apron, and wing walls splayed into the canal.

## Check yourself

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

1. Why is the crest raised above the river bed?
   A. To exclude bed load from entering the canal
   B. To store more water
   C. To resist uplift
   D. To reduce velocity
   Answer: A. A raised crest (about 1.2 m) keeps silt out. (Key point: Crest wall)

2. What do the u/s cut-off and floor do?
   A. Dissipate energy
   B. Reduce uplift and exit gradient
   C. Pass the flood
   D. Raise the pond
   Answer: B. They lengthen the seepage path. (Key point: Floor and cut-offs)

3. What is the role of the downstream floor/stilling basin?
   A. Hold gate hoist
   B. Exclude silt
   C. Dissipate energy and withstand uplift
   D. Provide a bridge only
   Answer: C. Plus an inverted filter and apron protect d/s. (Key point: D/s stilling basin)

## Related in Irrigation and Drainage Engineering

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- [Functions of head and cross regulator](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ahead-cross-regulator-functions-sketch) — Irrigation and Drainage Engineering, chapter 7, asked 6×
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