---
title: "Short note: Design of lined canals"
subject: "Irrigation and Drainage Engineering"
chapter: 4
key: "semester_6/irrigation_and_drainage_engineering:lined-canal-design-note"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/lined-canal-design-note
site: BCT Engineering
---

# Short note: Design of lined canals

Irrigation and Drainage Engineering · Chapter 4: Design of Canals

- Asked in: 2062 Kartik (old course), 4 marks
- Video: [4.8 Design of Lined Canal](https://www.youtube.com/watch?v=qXW1OrFAz78) — Sanjay Bonkile
- 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/lined-canal-design-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-4

## Question

Write a short note on design of lined canals.

## Short answer

Design uses Manning's formula with limiting velocity. For most economical section, R=D/2, B=2D(√(1+z²)-z), then solve for D and B. If velocity exceeds limit, fix V=lim, find R, A, P to get wider shallower section. Provide freeboard, lining thickness, joints, and drains.

## Answer

Design of a lined canal follows the principle of a rigid-boundary channel: the section need not be silt-regime, so Manning's formula is used with the limiting velocity of the lining.

#### Data needed

Discharge $Q$, available bed slope $S$, side slope $z$ (1:1 to 1.5:1 for concrete), roughness $n$ (0.014-0.016 for concrete) and limiting velocity (about 2.0-2.5 m/s for concrete; lower for brick and soil cement).

#### Procedure

1. Choose the **economical (best hydraulic) section**: $R = D/2$, $B = 2D(\sqrt{1+z^2} - z)$ and $A = D^2(2\sqrt{1+z^2} - z)$.
2. Insert into Manning's formula, $Q = \frac{1}{n}AR^{2/3}S^{1/2}$, and solve for $D$, then $B$.
3. Compute $V$. If $V \le V_{lim}$, the section is accepted.
4. If $V > V_{lim}$: fix $V = V_{lim}$, find $R = (Vn/\sqrt S)^{3/2}$, $A = Q/V$, $P = A/R$ and solve for $B$ and $D$ from $A = (B + zD)D$ and $P = B + 2D\sqrt{1+z^2}$ (a wider and shallower section results, using the permitted slope).
5. Provide **freeboard** (0.5-0.75 m for medium canals) and **lining thickness** (5-10 cm for concrete), expansion joints and drains behind the lining to relieve back pressure.
6. Check the **economic justification** of the lining (saving in seepage water, maintenance and land against the cost of lining).

## Check yourself

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

1. Why can a lined canal be designed with Manning's formula and a limiting velocity instead of silt theory?
   A. Because lined canals carry no discharge variation
   B. Because it is a rigid-boundary channel and need not be in silt regime
   C. Because Manning's n is zero for concrete
   D. Because lining stops all seepage
   Answer: B. The lining fixes the boundary, so no silt regime is needed; the limit is the permissible velocity of the lining. (Key point: Design principle)

2. For the best hydraulic (economical) section, what is the hydraulic radius R?
   A. R = D
   B. R = D/2
   C. R = 2D
   D. R = D/4
   Answer: B. The economical section has R = D/2, with B = 2D(sqrt(1+z^2) - z). (Key point: Economical section)

3. If the computed velocity exceeds the limiting velocity, what is done?
   A. Increase the bed slope
   B. Reduce the roughness n
   C. Fix V = Vlim, get R = (Vn/sqrt S)^(3/2), A = Q/V, then solve for B and D
   D. Accept the section anyway
   Answer: C. V is fixed at Vlim, giving a wider, shallower section that uses the permitted slope. (Key point: Handling V > Vlim)

4. Which provision relieves back pressure behind the lining?
   A. Higher freeboard
   B. Thicker lining of 20 cm
   C. Steeper side slopes
   D. Drains behind the lining along with expansion joints
   Answer: D. Drains behind the lining relieve back pressure; expansion joints take temperature movement. (Key point: Details of lining design)

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