---
title: "Semi-theoretical, Kennedy, Lacey approaches"
subject: "Irrigation and Drainage Engineering"
chapter: 4
key: "semester_6/irrigation_and_drainage_engineering:semi-theoretical-kennedy-lacey-compare"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/semi-theoretical-kennedy-lacey-compare
site: BCT Engineering
---

# Semi-theoretical, Kennedy, Lacey approaches

Irrigation and Drainage Engineering · Chapter 4: Design of Canals

- Asked in: 2064 Kartik (old course), 8 marks; 2063 Baisakh (old course), 8 marks (asked 2 times)
- Video: [Difference Between Lacey and Kennedy Theory in Nepali | Kennedy Vs Lacey |   लेसी Vs केनेडी](https://www.youtube.com/watch?v=eKBOp76v6oE) — Civil Engineering Power
- 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-kennedy-lacey-compare
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-4

## Question

Differentiate (make critical comparisons) among semi-theoretical, Kennedy's and Lacey's approaches of canal design.

## Short answer

The semi‑theoretical method uses Manning's equation with empirical non‑silting velocity limits and an assumed B/D, requiring slope. Kennedy's method uses critical velocity V0=0.55mD^0.64 and Kutter's formula by trial, needing slope. Lacey's regime theory directly gives V, R, P and S from Q and f without trial or roughness coefficient.

## Answer

#### Semi-theoretical approach

Uses a basic flow formula (Manning's or Chezy's) with **empirical limits** for non-silting, non-scouring velocity and a chosen $B/D$ ratio (from standards). The section is found from $Q = AV$ and $V = \frac{1}{n}R^{2/3}S^{1/2}$, with $V$ and $B/D$ fixed by experience, and the slope is checked against the allowable value.

#### Kennedy's approach

Based on the critical velocity concept, $V_0 = 0.55mD^{0.64}$ (vertical eddies from the bed). Kutter's formula gives $V$. Trial and error on $D$ gives the section for a known $S$.

#### Lacey's approach

Regime theory for alluvial channels with a silt factor $f$; $V$, $R$, $P$ and $S$ come directly from $Q$ and $f$.

#### Critical comparison

| Basis | Semi-theoretical | Kennedy | Lacey |
|---|---|---|---|
| Principle | Flow equation + empirical limits | Critical velocity (bed eddies) | Regime (eddies on whole perimeter) |
| Silt effect | Through limiting velocity | Through $m$ | Through $f = 1.76\sqrt{d}$ |
| Roughness $n$ | Required | Required (Kutter) | Not required |
| Slope | Given / assumed | Given | Computed |
| $B/D$ | Assumed from standards | Obtained by trial | Obtained from $P$ and $A$ |
| Work | Simple | Trial and error | Direct, quick |
| Use | Non-alluvial and lined canals | Old alluvial canals | New alluvial canals |
| Limits | Little theoretical backing | Tied to Upper Bari Doab data, $m$ uncertain | Regime relations derived from field data; valid for fine silt |

Semi-theoretical designs are the practical choice for canals where silt is not a problem (hill irrigation systems, non-alluvial soils). Kennedy and Lacey apply to alluvial soils, with Lacey's method preferred for new works.

## 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. A flow formula (Manning/Chezy) with empirical limits on velocity and B/D from standards
   B. A silt factor from grain size
   C. Critical velocity V0 = 0.55mD^0.64
   D. Regime perimeter P = 4.75 sqrt(Q)
   Answer: A. It combines Manning's equation with experience-based limits. (Key point: Semi-theoretical basis)

2. Which approaches need the roughness coefficient n?
   A. Only Lacey
   B. Semi-theoretical and Kennedy
   C. Only Lacey and Kennedy
   D. None of them
   Answer: B. Lacey's regime formulae do not need n; the other two use Manning/Kutter. (Key point: Roughness coefficient)

3. How is the bed slope treated in Lacey's method?
   A. It is given or assumed
   B. It is found by trial and error from V0
   C. It is computed as an output from Q and f
   D. It is taken from standards tables
   Answer: C. Kennedy and semi-theoretical methods start with a known slope, Lacey computes it. (Key point: Slope treatment)

4. Which approach is the practical choice for hill or non-alluvial canals where silt is not a problem?
   A. Lacey's regime theory
   B. Kennedy's silt theory
   C. Neither; no design is possible
   D. Semi-theoretical
   Answer: D. Regime theories need alluvial channels; semi-theoretical design suits lined and non-alluvial canals. (Key point: Where each is used)

## Related in Irrigation and Drainage Engineering

- [Semi-theoretical approach in canal design](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asemi-theoretical-approach-canal) — Irrigation and Drainage Engineering, chapter 4
- [Lacey and Kennedy bed width, Q 40](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alacey-kennedy-q40-comparison) — Irrigation and Drainage Engineering, chapter 4
- [Kennedy and Lacey silt theories compared](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akennedy-lacey-theories-compare) — Irrigation and Drainage Engineering, chapter 4, asked 4×
- [Kennedy design: Q 10, n 0.0245](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akennedy-q10-n00245) — Irrigation and Drainage Engineering, chapter 4, asked 3×
- [Kennedy design: Q 40, n 0.018](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akennedy-q40-n018-s00020) — Irrigation and Drainage Engineering, chapter 4, asked 3×
- [Lacey design: Q 20, f 1.5](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alacey-q20-f15) — Irrigation and Drainage Engineering, chapter 4, asked 2×

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