DESIGN OF ERODIBLE AND NON-ERODIBLE

CHANNELS

According to Kennedy the critical velocity ratio Vc in a channel may be defined as the mean velocity of flow which will just keep the channel free from silting or scouring.

His investigations pertain to Upper bari Doab canal in UP.

64.0..55.0 dmVc m = Critical velocity ratio

= 1.1 to 1.2 for coarse sand

= 0.8 to 0.9 for fine sand

KENNEDY’S METHOD OF CHANNEL DESIGN PROCEDURE

Q = A x V

V

R

nS

SnC

00155.0231

00155.023

1

RSCV

64.0.. dmCV vc

Assume a depth of flow = d, m

Compute the critical velocity from kennady’s formula

Compute are of c/s of flow = Q/Vc

Assuming a side slope of channel, say 0.5:1 compute the bed width

Compute the wetted perimeter for the assumed depth abd computed bed width

Calculate C from Kutter’s formula and then the velocity of flow by Chezy’s equation

If the Velocity computed now is same as found by kennady’s method the design depth is correct

Otherwise repeat the above steps by assuming different depth of flow

CWPC PRACTICE FOR “n”

Type of soil Canal discharge (cumecs) Value of n

1. Soil other than rock

Up to 0.0140.14 to 1.41.4 to 14Above 14

0.030.0250.02250.020

2. Rocky cuts 1. When rock portion at least 15 cm above the excavated bed level is left out in working out cross sectional area.

0.035 to 0.05

2. When no portion above bed level is left out

0.05 to 0.080

Channel of condition

Value of n

1. Very good 0.02252. Good 0.0253. Indifferent 0.02754. Poor 0.03

He also defined critical velocity as non-silting –non-scouring velocity and gave a relation between critical velocities to the depth of flowing water.

The relation is, V0 = 0.55 D0.64

(OR V0 = 0.84 D0.64 in F.P.S Units

In general V0 = CDn

V0 = Critical velocity, in (m/s)

D = Depth of water over bed portions of a channel in m

n = any index number

The equation has been derived on the basis of observations on one canal only, it is applicable to only those channels, which are flowing, in sandy silt of the same quality or grade as that of Upper Bari Doab system.

Kennedy later realized the importance of silt grade on critical velocity and introduced a factor ‘m’ known as critical velocity ratio (C.V.R) in his equation. The equation is then written as

V0 = 0.55 m D0.64

Where, m = C.V.R = V/ V0

Sand coarser than the standard was assigned value of m from 1.1 to 1.2 and those finer than the standard from 0.9 to 0.8. Generally, in a system of canal, higher C.V.R. is assumed in head reaches and lower value of C.V.R is assumed towards its tail end.

The value of constant C in equation for various grades of material may be assumed as follows:

Types of material Value of C

Light sandy silt 0.53

Coarser light soil 0.59

Sandy loam 0.65

Coarse silt0.70

Type of silt

Value of m

Light sandy silt in the rivers

of northern India1.00

Somewhat coarser light

sandy silt1.1

Sandy loamy silt 1.2

Rather coarser silt or debris

of hard soil1.3

Silt of river Indus in sindhu 0.7

Value of m

Drawbacks in Kennedy’s theory•Kennedy did not notice importance of B/D ratio.•He aimed to find out only the average regime conditions for the design of a channel.•No account was taken of silt concentration and bed load, and the complex silt-carrying phenomenon was incorporated in a single factor m.•Silt grade and silt charge were not defined.•Kennedy did not give any slope equation.•Kennedy use kutter’s equation for the determination of mean velocity and therefore the limitations kutter’s equation got incorporated in Kennedy’s theory of channel design

PRESENTED BY

PREETHA DEVI.RBTE-06-026

THANK YOU