DESIGN OF ERODIBLE AND NON-ERODIBLE CHANNELS

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DESIGN OF ERODIBLE AND NON-ERODIBLE CHANNELS
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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.
m Critical velocity ratio 1.1 to 1.2 for
coarse sand 0.8 to 0.9 for fine sand
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KENNEDYS METHOD OF CHANNEL DESIGN PROCEDURE

Q A x V
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• Assume a depth of flow d, m
• Compute the critical velocity from kennadys
  formula
• Compute are of c/s of flow Q/Vc
• Assuming a side slope of channel, say 0.51
  compute the bed width
• Compute the wetted perimeter for the assumed
  depth abd computed bed width
• Calculate C from Kutters formula and then the
  velocity of flow by Chezys equation
• If the Velocity computed now is same as found by
  kennadys method the design depth is correct
• Otherwise repeat the above steps by assuming
  different depth of flow

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CWPC PRACTICE FOR n
Type of soil Canal discharge (cumecs) Value of n
1. Soil other than rock Up to 0.014 0.14 to 1.4 1.4 to 14 Above 14 0.03 0.025 0.0225 0.020
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

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Channel of condition Value of n
1. Very good 0.0225
2. Good 0.025
3. Indifferent 0.0275
4. Poor 0.03
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He also defined critical velocity as non-silting
non-scouring velocity and gave a relation
between critical velocities to the depth of
flowing water.
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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
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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.
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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

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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.
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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 silt 0.70
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Value of m
Type of silt Value of m
Light sandy silt in the rivers of northern India 1.00
Somewhat coarser light sandy silt 1.1
Sandy loamy silt 1.2
Rather coarser silt or debris of hard soil 1.3
Silt of river Indus in sindhu 0.7
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• Drawbacks in Kennedys 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 kutters equation for the
  determination of mean velocity and therefore the
  limitations kutters equation got incorporated in
  Kennedys theory of channel design

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PRESENTED BY PREETHA DEVI.R BTE-06-026