---
title: "Waterway and crest level at alluvial zone"
subject: "Irrigation and Drainage Engineering"
chapter: 5
key: "semester_6/irrigation_and_drainage_engineering:waterway-crest-level-alluvial-structures"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/waterway-crest-level-alluvial-structures
site: BCT Engineering
---

# Waterway and crest level at alluvial zone

Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks

- Asked in: 2079 Jestha, 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/waterway-crest-level-alluvial-structures
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5

## Question

How do you calculate the length of the waterway and crest level of the diversion structures (under sluices, weir and canal head regulator) at the alluvial zone.

## Short answer

Total waterway = Lacey's wetted perimeter P = 4.75√Q (max flood), effective width L ≈ P. Undersluices get 10–20% width. Weir crest is the lower of the flood-required level or pond level minus shutter height. Undersluice crest at river-bed level; head regulator crest 1.2–2 m higher. Pond level = FSL + 0.3–0.5 m.

## Answer

#### 1. Length of waterway (total)

The regime width of the river is found from Lacey's wetted perimeter

$$
P = 4.75\sqrt{Q}
$$

where $Q$ is the maximum flood discharge (m³/s). The **total waterway** of the headworks is taken as about $L = P$ (up to $1.2P$ where needed), but not more than the natural river width. The clear (effective) waterway is $L$ minus the total thickness of the piers, and the afflux produced (usually 0.5-1.2 m) must be checked to be within the permissible limit.

Distribution: the undersluices occupy about 10-20% of $L$ (sluice bays of 5-8 m); the rest is the weir. The waterway of the canal head regulator is found from its discharge formula for the canal full supply discharge, with the head equal to the pond level minus the regulator crest level.

#### 2. Crest levels

- **Pond level** $=$ canal full supply level (FSL) $+$ losses through the head regulator (0.3-0.5 m), so that the canal can draw its full supply.
- **Weir crest level** is found in two ways and the lower value is adopted:
  - from the flood: crest $=$ HFL after afflux $-$ head $H$ needed to pass the design flood, where $H$ comes from $Q = C L H^{3/2}$ ($C \approx 1.7$);
  - from ponding: crest $=$ pond level $-$ height of shutters (0.9-2.0 m) if shutters or gates are used (or crest $=$ pond level for a weir without shutters).
- **Undersluice crest level** $=$ average (low-water) river-bed level at the site, about 1.2-2.0 m lower than the head regulator crest.
- **Head regulator crest level** $=$ undersluice crest $+ (1.2$-$2.0)$ m; it must be above the sluice crest to keep the bed silt away from the canal.

HFL (after construction) $=$ HFL (before construction) $+$ afflux, and the head over the weir and sluices is checked with the discharge formulae.

## Check yourself

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

1. How is the regime width of the river found?
   A. P = 4.75 sqrt(Q) (Lacey's wetted perimeter)
   B. P = 1.35 Q
   C. P = Q/4.75
   D. P = sqrt(Q)/4.75
   Answer: A. Lacey's P = 4.75 sqrt(Q), with Q the max flood discharge. (Key point: Regime width)

2. What total waterway is adopted?
   A. 0.1 P
   B. About 1.0-1.2 P, minus pier widths for effective waterway
   C. 5 P
   D. Equal to canal width
   Answer: B. L = 1.0-1.2 P; effective waterway subtracts the piers. (Key point: Total waterway)

3. What is the undersluice crest level?
   A. At HFL
   B. At pond level
   C. Average river-bed level, 1.2-2.0 m below head regulator crest
   D. 1.2-2.0 m above the regulator crest
   Answer: C. The sluice crest sits at river-bed level, below the regulator crest. (Key point: Undersluice crest)

4. What mainly decides the weir crest level?
   A. Fish ladder slope
   B. River width
   C. Pier thickness
   D. Canal FSL plus regulator losses (pond level)
   Answer: D. Weir crest follows from canal FSL + head regulator losses + required depth. (Key point: Weir crest)

## Related in Irrigation and Drainage Engineering

- [Weir waterway and crest levels, 16000 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aweir-waterway-crest-q16000) — Irrigation and Drainage Engineering, chapter 5
- [Canal head regulator design, Q 200](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-head-regulator-design-q200) — Irrigation and Drainage Engineering, chapter 7
- [Canal head regulator design, Q 150](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-head-regulator-design-q150) — Irrigation and Drainage Engineering, chapter 7
- [Diversion weir: Bligh and Khosla, 4000 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adiversion-weir-bligh-khosla-q4000) — Irrigation and Drainage Engineering, chapter 5
- [Crest level of fall, Q 30 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Afall-crest-level-30cumec-broad-crested) — Irrigation and Drainage Engineering, chapter 7
- [Weir design by Bligh's theory, 3200 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adiversion-weir-vertical-drop-bligh-q3200) — Irrigation and Drainage Engineering, chapter 5