---
title: "Semi-theoretical approach in canal design"
subject: "Irrigation and Drainage Engineering"
chapter: 4
key: "semester_6/irrigation_and_drainage_engineering:semi-theoretical-approach-canal"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/semi-theoretical-approach-canal
site: BCT Engineering
---

# Semi-theoretical approach in canal design

Irrigation and Drainage Engineering · Chapter 4: Design of Canals

- Asked in: 2074 Bhadra, 3 marks
- Video: [Irrigation- Kennedy's theory // Design Steps with numerical (In Hindi)](https://www.youtube.com/watch?v=OUV8beq8K1g) — Yogendra Ram Meena
- 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/semi-theoretical-approach-canal
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-4

## Question

Describe briefly the semi-theoretical approach in canal design.

## Short answer

The semi-theoretical approach uses Manning's formula with empirical velocity limits (0.3-0.6 m/s minimum, maximum non-scouring velocity) and standard B/D ratios. Slope is adapted to ground slope, and section is determined by trial. It suits non-alluvial canals like lined or hilly channels where regime theories don't apply.

## Answer

The **semi-theoretical approach** designs a canal using a standard flow formula together with empirical limits obtained from experience.

- Flow equation: Manning's, $V = \frac{1}{n}R^{2/3}S^{1/2}$, with $Q = AV$ (or Chezy's formula).
- Non-silting and non-scouring conditions are met by keeping $V$ between permissible limits: not less than about 0.3-0.6 m/s (to avoid silting and weed growth) and not more than the maximum non-scouring velocity of the boundary material.
- The ratio $B/D$ and side slopes are taken from standard tables of design practice for the given discharge range.
- The slope is chosen to suit the land slope, so the design is checked by trial and adjusted until the section carries $Q$ at the desired velocity.

It is used for canals which are not fully alluvial (hilly regions, lined canals, canals in rigid boundaries) where regime theories are not applicable. It is simple but depends on experience (empirical limits), unlike Kennedy's and Lacey's theories, which are based on silt transport concepts.

## Check yourself

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

1. What does the semi-theoretical approach use?
   A. Manning's or Chezy's formula with empirical limits from experience
   B. Silt factor and regime equations
   C. Critical velocity from bed eddies
   D. Tractive force only
   Answer: A. A standard flow formula is combined with empirical velocity limits. (Key point: Basis)

2. What limits are placed on V?
   A. Exactly equal to V0
   B. Above a minimum to avoid silting/weeds and below the non-scouring velocity of the boundary
   C. Always above 3 m/s
   D. No limits
   Answer: B. V is kept between permissible limits. (Key point: Velocity limits)

3. Where is it used?
   A. Only fully alluvial canals
   B. Only river training
   C. Non-alluvial, lined or hilly canals where regime theories do not apply
   D. Only drainage channels
   Answer: C. Regime theories need alluvial channels. (Key point: Where used)

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