Unlined Canal design

UNLINED CANAL DESIGN
Submitted By:
Preetpal Singh
B.Tech Civil Engineering (4th Sem)
GU14-0005

Canal Design
Drainage
Channel
Design
Irrigation
Channel
Design
Canal Design Types

Design Parameters
 The design considerations naturally vary according to the type of
soil.
 Velocity of flow in the canal should be critical.
 Design of canals which are known as ‘Kennedy’s theory’ and
‘Lacey’s theory’ are based on the characteristics of sediment
load (i.e. silt) in canal water

Important Terms Related to Canal Design
 Alluvial soil
 Non-alluvial soil
 Silt factor
 Co-efficient of rugosity
 Mean velocity
 Critical velocity
 Critical velocity ratio (C.V.R.), m
 Regime channel
 Hydraulic mean depth (R)
 Full supply Level
 Economical section

Alluvial Soil
The soil which is formed by the continuous deposition of silt is
known as alluvial soil. The river carries heavy charge of silt in rainy
season. When the river overflows its banks during the flood, the
silt particles get deposited on the adjoining areas. This deposition of
silt continues year after year. This type of soil is found in deltaic region
of a river. This soil is permeable and soft and very fertile. The river
passing through this type of soil has a tendency to change its course.

Non-alluvial Soil
The soil which is formed by the disintegration of rock formations
is known as non-alluvial soil. It is found in the mountainous
region of a river. The soil is hard and impermeable in nature.
This is not fertile. The river passing through this type of soil has
no tendency to change its course.

Silt Factor
During the investigations works in various canals in alluvial
soil, Gerald Lacey established the effect of silt on the
determination of discharge and the canal section. So, Lacey
introduced a factor which is known as ‘silt factor’.
It depends on the mean particle size of silt. It is denoted by ‘f’. The
silt factor is determined by the expression,
f = 1.76 dmm
where dmm = mean particle size of silt in mm
Particle Particle size (in mm) Silt factor
Very fine Silt 0.05 0.40
Fine Silt 0.12 0.60
Medium Silt 0.23 0.85
Coarse Silt 0.32 1.00

Coefficient of Rugosity (n)
The roughness of the canal bed affects the velocity of flow. The
roughness is caused due to the ripples formed on the bed of the
canal. So, a coefficient was introduced by R.G Kennedy for
calculating the mean velocity of flow. This coefficient is known as
coefficient of rugosity and it is denoted by ‘n’. The value of ‘n’
depends on the type of bed materials of the canal.
Materials Value of n
Earth 0.0225
Masonry 0.02
Concrete 0.13 to 0.018

D
0.6 D
Mean Velocity
It is found by observations that the velocity at a depth 0.6D
represents the mean velocity (V), where ‘D’ is the depth of water
in the canal or river.
(a) Mean Velocity By Chezy’s Expression:
V= C√RS
(b) Mean Velocity By Manning’s Expression:
V=(1/n)x(R^⅔)x(S^ ⅟₂)
Mean Depth

Critical Velocity
When the velocity of flow is such that there is no silting or
scouring action in the canal bed, then that velocity is known as
critical velocity. It is denoted by ‘Vo’. The value of Vo was given
by Kennedy according to the following expression,
Vo = 0.546 D0.64
; where, D = Depth of water
D

Critical Velocity Ratio (C.V.R.)
The ratio of mean velocity ‘V’ to the critical velocity ‘V₀’ is
known as critical velocity ratio (C.V.R.). It is denoted by ‘m’ i.e.
C.V.R. (m)=V/V₀
When m = 1, there will be no silting or scouring
When m > 1, scouring will occur
When m < 1, silting will occur
So , by finding the value of m, the condition of the canal can be
predicted whether it will have silting or scouring

Regime Channel
When the character of the bed and bank materials of the channel
are same as that of the transported materials and when the silt
charge and silt grade are constant, then the channel is said to be
in its regime and the channel is called regime channel. This ideal
condition is not practically possible.

Hydraulic Mean Depth
The ratio of the cross-sectional area of flow to the wetted
perimeter of the channel is known as hydraulic mean depth. It is
generally denoted by R.
R = A/P
Where, A= Cross-sectional area
P = Wetted perimeter

Full Supply Level
The maximum capacity of the canal for which it is designed, is
known as full supply discharge. The water level of the canal
corresponding to the full supply discharge is known as full supply
level (F.S.L).
FSL

Cutting Area Balancing depth
Economical Section
If a canal section is such that the earth obtained from cutting
(i.e. excavation) can be fully utilized in forming the banks, then that
section is known as economical section. Again, the discharge
will be maximum with minimum cross-section area. Here, no extra
earth is required from borrow pit and no earth is in excess to form
the spoil bank. This condition can only arise in case of partial cutting
and partial banking. Sometimes, this condition is designated as
balancing of cutting and banking. Here, the depth of cutting is
called balancing depth.
Filling Area

Unlined Canal Design on Alluvial soil by
Kennedy’s Theory
After long investigations, R.G Kennedy arrived at a theory which
states that, the silt carried by flowing water in a channel is kept in
suspension by the vertical component of eddy current which is
formed over the entire bed width of the channel and the suspended
silt rises up gently towards the surface.
The following assumptions are made in support of his theory:
 The eddy current is developed due to the roughness of the bed.
 The quality of the suspended silt is proportional to bed width.
 It is applicable to those channels which are flowing through
the bed consisting of sandy silt or same grade of silt.
 It is applicable to those channels which are flowing through
the bed consisting of sandy silt or same grade of silt.

He established the idea of critical velocity ‘Vo’ which will make a
channel free from silting or scouring. From, long observations, he
established a relation between the critical velocity and the full
supply depth as follows,
Vo = CDn
The values of C and n where found out as 0.546 and 0.64
respectively, thus Vo = 0.546 D0.64
Again, he realized that the critical velocity was affected by the
grade of silt. So, he introduced another factor (m) which is known
as critical velocity ratio (C.V.R).
Vo = 0.546mD0.64

Drawbacks of Kennedy’s Theory
The theory is limited to average regime channel only.
The design of channel is based on the trial and error method.
The value of m was fixed arbitrarily.
Silt charge and silt grade are not considered.
 There is no equation for determining the bed slope and it
depends on Kutter’s equation only.
The ratio of ‘B’ to ‘D’ has no significance in his theory.

Critical Velocity, Vo = 0.546 D0.64
Mean Velocity
B/D ratio is assumed accordingly
Discharge, Q = A V
Where, A = Cross-section area in m2,
V = mean velocity in m/sec
The full supply depth is fixed by trial to satisfy the value of ‘m’.
Generally, the trial depth is assumed between 1 m to 2 m. If the
condition is not satisfied within this limit, then it may be assumed
accordingly.
Design Procedure

Unlined Canal Design on Alluvial soil by
Lacey’s Theory
Lacey’s theory is based on the concept of regime condition of the
channel. The regime condition will be satisfied if,
The channel flows uniformly in unlimited incoherent alluvium of
the same character which is transported by the channel.
The silt grade and silt charge remains constant.
The discharge remains constant.
But in practice, all these conditions can never be satisfied. And,
therefore artificial channels can never be in ‘True regime’.

Initial Regime and Final Regime
When only the bed slope of a channel varies due to dropping of silt ,
and its cross-section or wetted perimeter remains unaffected, even
them the channel can exhibit ‘no silting no scouring’ properties,
called INITIAL REGIME.
IF there is no resistance from the sides, and all the variables such as
perimeter, depth, slope etc. are equally free to vary and get
adjusted according to discharge and silt grade, then the channel is
said to have achieved permanent stability, called FINAL REGIME.

Design Procedure
 Calculate the velocity from equation
Where, Q is discharge in cumecs,
V is velocity in m/s
f is silt factor
 Workout the hydraulic mean depth (R) from the equation
 Compute area(A) of channel section by using

 Compute the wetted perimeter, P
 Knowing these values, the channel section is known; and finally
the bed slope (S) is determined by the equation
 B/D ratio of channel is assumed accordingly.

Drawbacks of Lacey’s Theory
 The concept of true regime is theoretical and con not be
achieved practically.
 The various equations are derived by considering the silt factor
f which is not at all constant.
 The concentration of silt is not taken into account.
 Silt grade and silt charge is not taken into account.
 The equations are empirical and based on the available data from
a particular type of channel. So, it may not be true for a different
type of channel.
 The characteristics of regime channel may not be same for all cases.

Kennedy’s Theory Lacey’s Theory
It states that the silt carried by the
flowing water is kept in suspension
by the vertical component of eddies
which are generated from the bed of
the channel.
It states that the silt carried by the
flowing water is kept in
suspension by the vertical
component of eddies which are
generated from the entire
wetted perimeterof the channel.
It gives relation between ‘V’ and ‘D’. It gives relation between ‘V’ and ‘R’.
In this theory, a factor known as
critical velocity ratio ‘m’ is introduced
to make the equation applicable to
different channels with different silt
grades
In this theory, a factor known as silt
factor ‘f’ is introduced to make the
equation applicable to different
channels with different silt grades.
In this theory, Kutter’s equation is
used for finding the mean velocity.
This theory gives an equation for
finding the mean velocity.
This theory gives no equation for bed
slope.
This theory gives an equation for
bed slope.
In this theory, the design is based on
trial and error method.
This theory does not involve trial
anderror method.
Comparison of Kutter’s & Lacey’s Theory

Unlined Canal design

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Unlined Canal design