---
title: "Kennedy's theory features and limitations"
subject: "Irrigation and Drainage Engineering"
chapter: 4
key: "semester_6/irrigation_and_drainage_engineering:kennedy-theory-features-limitations"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/kennedy-theory-features-limitations
site: BCT Engineering
---

# Kennedy's theory features and limitations

Irrigation and Drainage Engineering · Chapter 4: Design of Canals

- Asked in: 2078 Chaitra, 5 marks
- Video: [Important Theory Question from Irrigation Engineering | 6th Sem | Prashant YT |CTEVT| Diploma Civil|](https://www.youtube.com/watch?v=mvRRwMtZ364) — Prashant YT
- 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/kennedy-theory-features-limitations
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-4

## Question

Discuss the salient features of Kennedy's theory for the design of earthen channels based on the critical velocity concept and mention its limitations.

## Short answer

Kennedy's theory uses critical velocity V0 = 0.55 m D^0.64 to keep silt suspended by bed eddies, with trial-and-error design using Kutter's formula. Limitations include ignoring side eddies, empirical m selection, non-unique dimensions, reliance on Kutter's n, regional data bias, uncontrolled B/D ratio, and no silt load consideration.

## Answer

**Kennedy's theory** (R.G. Kennedy, 1895, from the Upper Bari Doab canal, Punjab) is for the design of alluvial earthen channels that neither silt nor scour.

#### Salient features

- Silt is kept in suspension by the **vertical components of eddies** formed at the channel **bed**. The channel remains stable when the mean velocity equals the critical velocity $V_0$.
- Critical velocity: $V_0 = 0.55\,m\,D^{0.64}$, $D$ = depth (m), $m$ = critical velocity ratio $V/V_0$ ($m$ = 1.1 for coarse silt, 0.8 for very fine silt, 1.0 for normal).
- The mean velocity in the channel is found from Kutter's formula (or Manning's) with a rugosity coefficient $n$.
- For a given $Q$ and $S$ the section is found by **trial and error** so that $V = V_0$, with $A = Q/V$.
- Valid for stable channels in alluvium which carry fine silt, with trapezoidal section and side slopes taken from soil types.
- A tabular/graphical (Garret's diagram) solution can be used to reduce the work.

#### Limitations

1. Considers eddies from the bed only; sides ignored.
2. $m$ is chosen by judgement; there is no relation to the silt grade or the silt charge.
3. A single equation ($V_0$) is used for a design that actually involves several unknowns ($B, D, S$), so the depth and width are not uniquely determined unless $S$ or $B/D$ is assumed.
4. Requires Kutter's $n$, which is not well defined for silt-laden flow.
5. Based on the data of one region (Upper Bari Doab canals), not on fully regime channels.
6. Trial-and-error design; the bed width/depth relation $B/D$ is not controlled, and the ratio can become unreasonable.
7. Does not account for the silt load carried, so cannot be used where the canal carries heavy silt.

## Check yourself

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

1. What is Kennedy's basic concept?
   A. Vertical eddies from the bed keep silt in suspension; no silting/scouring when V = V0
   B. Eddies from the whole perimeter
   C. Silt kept by side slope shear
   D. Regime with initial and final stages
   Answer: A. Stable condition is mean velocity equal to critical velocity. (Key point: Kennedy's principle)

2. What is m in V0 = 0.55 m D^0.64?
   A. Manning's roughness
   B. Critical velocity ratio V/V0, chosen by judgement (about 0.8 to 1.1)
   C. Silt factor from grain size
   D. Side slope
   Answer: B. m varies with silt grade, but the choice is judgemental, a limitation. (Key point: Role of m)

3. How is the actual mean velocity found?
   A. Lacey's V = (Qf^2/140)^(1/6)
   B. V = Q/P
   C. Kutter's (or Manning's) formula with a rugosity coefficient
   D. Garret's diagram only
   Answer: C. Needing n is a limitation as n is poorly defined for silt-laden flow. (Key point: Velocity formula)

4. Which is a limitation of Kennedy's theory?
   A. It considers eddies from sides too
   B. It requires no roughness coefficient
   C. It is based on data from many countries
   D. Only the bed eddies are considered; trial and error needed and B/D not controlled
   Answer: D. It ignores sides, uses one equation for several unknowns, and relies on Upper Bari Doab data. (Key point: A limitation)

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