CHAPTER 5- Water Conveynance and Control-1.pptx

CHAPTER :FIVE
1
WATER CONVEYANCE &CONTROL

5.1 IRRIGATION DISTRIBUTION SYSTEMS
The purpose of any irrigation network is water intake,
conveyance, distribution and field application to the
crops.
The conveyance and distribution of water will be
affected by canals.
Irrigation canals are irrigation structures (open
channels) through which irrigation water is conveyed
and distributed with in the irrigation system.
Drainage canals are open channels which evacuate
excess water (either excess irrigation water or excess
water from rainfall) away from the irrigation area.
2

CON…
The basic differences between irrigation and
drainage canals are:
•Irrigation canals carry clean water
•drainage canals carry generally dirty (saline or
alkaline) water;
•The flow in irrigation canals is from the high
level to the lower level canals.
•flow in drainage canals is from the lower level
canals to the higher level canals.
3

PRELIMINARY LAYOUT OF IRRIGATION AND DRAINAGE
CANALS
 The layout on irrigation and drainage systems should take into
consideration:
The physical (technical) feasibility and
The economic feasibility
As a very general principle for the layout of irrigation and
drainage canals: drainage canals are located in the lower part of the
area and irrigation canals on the higher parts of the area.
The reason for this principle is that from irrigation canals water
has to flow out by gravity to the fields located at the lower areas
while water has to flow into the drainage canals located at the lower
parts of the area.
4

CONT…
5
Irrigation
canal
Drains

Classification of irrigation canals
Based on the alignment, irrigation canals can be classified into
three:
Contour canal
Watershed canal (ridge canal)
Side slope canal
Main (Primary canal)
Branch canal
Secondary canals
Tertiary canals
Field canals
6
Based on the sizes and importance, irrigation canals can be classified as:

DESIGN OF LINED AND UNLINED CANALS
An irrigation and drainage network should be designed and
operated in such a way that:
the required discharge flows are passed at design water
levels;
no erosion of canal bottoms and banks will occur, and
any sediment that enters the system will not settle in the
network but will be carried along and discharged through
the outlets, to the fields or to the natural drainage system.
Design discharge
Design discharge also called canal capacity is the maximum
discharge that the cross section of a canal reach is designed
for.
This discharge is the maximum expected flow in the canal
during peak periods of peak flow.
8

Determination of deign discharge
The design discharge of a canal can be determined as follows:
I. Determine the total area irrigated from the canal under consideration;
II. Based on the crop water requirement, determine the water demand
for each unit irrigated in l/s/ha;
III. For each outlet in the canal reach, determine the outflow by
multiplying the area by the water requirement;
IV. Determine the outflow into the next reach of the canal;
V. Add all the outlet discharges and the outflow into the next reach of
the canal to determine the design discharge of the canal reach under
consideration.
9

CONT…
The outlet discharges are;
Where:-
A= Area to be irrigated from the outlet
q= peak water demand rate (l/s/ha)
The design discharge of the reach is thus,
Q= Q1+Q2+Q3+…+ Qo+ losses
Where:
Q1, Q2, Q3… are outlet discharges downstream of the reach
Qo outflow into the next reach.
10
q
A
Qi *


CANAL CROSS SECTION
Irrigation and drainage canals can be constructed of earth
(unlined canals) or lined canals.
Irrigation canals are usually designed as trapezoidal sections;
however lined irrigation canals can also be designed as
rectangular sections.
11
b
d
1
m
Canal
depth

CONT…
Freeboard: Vertical distance between the highest water level
anticipated in the design and the top of the retaining banks. It is a
safety factor to prevent the overtopping of structures.
Side Slope (m): The ratio of the horizontal to vertical distance of
the sides of the channel. m= e/d = e’/D
13
Sand, Soft Clay 3: 1 (Horizontal: Vertical)
Sandy Clay, Silt Loam,
Sandy Loam
2:1
Fine Clay, Clay Loam 1.5:1
Heavy Clay 1:1
Stiff Clay with Concrete
Lining
0.5 to 1:1
Lined Canals 1.5:1

CONT…
The design of irrigation canal cross section basically
means fixing appropriate bottom width b, depth of flow d,
total depth and canal side slope m.
The other parameter used in the design canals is the
longitudinal slopes of the canal.
The following longitudinal slopes can be adopted for
preliminary design of canals:
Large canals, Q > 15 m3/s, …………….0.10 to 0.30 %
Intermediate canals,0.3< Q<15 m3/s …0.20 to 0.40 %
Small canals, Q < 0.3 m3/s,…………….0.30 to 0.50 %
14

DESIGN OF LINED CANALS
In lined canals the cross section of the canals
is covered (lined) with some kind of harder
material than earth (soil) to provide resistance
against erosion and avoid seepage losses.
Design of lined canals is usually done based
on the permissible velocity approach.
It is defined as the mean velocity at or bottom
which the channel bottom and sides are not
15

Recommended Permissible Velocities∗
.
16
Material V(m/s)
Fine sand 0.6
Coarse sand 1.2
sandy silt 0.6
Silt clay 1.1
clay 1.8

CONT….
For the design of lined canals, uniform flow equations for open
channel flow can be used. This can be the Chezy equation or
Manning’s formula:
 Continuity equation
 Chezy equation;
Manning’s Formula
17
V
A
Q *

RS
C
A
Q *

2
1
3
2
1
* S
R
n
A
Q 
Where:-
Q = design discharge, m3/s
A = the x-sectional area of
flow
C = Chezy constant
R = hydraulic radius, m
S = longitudinal slope of the
canal
n = Manning’s coefficient

CONT…
The Manning’s n and the Chezy C are functions of the kind
of lining and the condition (roughness) of the surface.
The following are recommended values of the Manning’s n
for different linings.
Lining material value of n
Concrete lining………………..…………0.013 to 0.018
Masonry lining with random stone………0.017 to 0.020
Brick lining……………………………….0.014 to 0.017
Asphalt lining……………………………..0.013 to 0.016
18

DESIGN PROCEDURE FOR LINED CANAL
1. For a known permissible velocity V, Manning’s n, and
longitudinal slope S of the canal, determine the hydraulic
radius from Manning’s or Chezy equation;
2. Form two equations with two unknowns, bottom width b and
depth of flow y.
Trapezoidal canal: Rectangular canal
3. In the equations * and **, there are two unknowns b and y, which
can be solved simultaneously to determine the flow depth y and
bottom width b.
19
*
..
..........
2
my
by
A 

*
*
.
..........
1
*
2 2
m
y
b
P 


by
A 
P
A
R 

CONT…
The best hydraulic section is the one with minimum wetted
perimeter for a given discharge.
The best hydraulic section for a trapezoidal canal is one when R=
y/2. The bed width/depth ratio, b/y, is usually unity.
Table: Values for m and b/y for lined trapezoidal canals; for
rectangular: b/y=2
20
Q(m3/s) 0.3 0.3 – 2.0 2.0 – 30.0 >30.0
m 1 or 1.25 1 or 1.25 1.25 or 1.5 1.5 – 2.0
b/y 1 0.03 * Q + 1.0

DESIGN OF UNLINED CANALS
Unlined canals can be classified into two classes based on the
stability of the boundaries of the canal for design purposes:
Canals with stable (non-erodible) bed and
Canals with erodible bed (Alluvial canal)
Design of non-erodible (stable) canals
Non-erodible canals are canals with fairly stable boundary.
When a channel conveying clear water is to be lined, or the earth
used for its construction is non-erodible in the normal range of
canal velocities, Manning's equation is used.
21
2
1
3
2
1
* S
R
n
A
Q 
Manning's n can also be got from Tables or
estimated using the Strickler equation: n = 0.038
* d1/6,d is the particle size diameter (m)

CONT…
There are two approaches for the design of such canals:
The recommended (b/y ratio) approach and
The Tractive force (permissible velocity approach)
Design of canals on b/y ratio approach
The highest flow velocity in a section will occur when the
hydraulic radius R is maximized.
For any given hydraulic gradient S and side slope m, an infinite
range of, b/y, could be selected.
Minimizing the wetted perimeter by equating the first derivative
on y to zero results in a ‘best’ hydraulic cross section if:
22
 
 
z
z
y
b 


5
.
0
2
1
2

CONT…
23
In earth canals, the best hydraulic section is
seldom applied because:
The cross section would not be stable;
The excavation would be too deep;
A change of flow heavily affects the depth of
water and velocity distribution in deep and narrow
canals.

CONT…
For a preliminary design, recommended side slopes
that are stable under normal conditions are as
follows:
Table: Side slopes in canals; m = cotangent of side slope
24
Material m
Rock 0
Stiff clay 0.5
Cohesive medium soils 1.0-1.5
Sand 2
Fine sand, porous clay, soft peat 3

CONT…
The bed width/depth ratio of a small earthen irrigation canal is
often close to unity; the ratio is gradually increased for larger
canals.
The recommended ratios as related to the design discharge of
the canal Q are as follows:
In most small and medium size earth canals, the freeboard varies
from 50% to 60% of the depth y, with a minimum of 0.15 or 0.20
m.
25
(b/y)recom. =1.76*Q0.35 , for Q > 0.2 m3/s
(b/y)recom. = 1.0, for Q ≤ 0.2 m3/s

CONT…
Recommended values for m and b/y for unlined irrigation
canals constructed in earth (cohesive medium soils)
26
Q (m3/s) 0.2 0.2 – 0.5 0.5 – 10.0 >10.0
m 0.8 – 1 1 – 1.5 1.5 - 2 > 2
b/y 1 1.76 * Q0.35

DESIGN OF CANALS ON TRACTIVE FORCE
(PERMISSIBLE VELOCITY) APPROACH
The balance of forces on a sediment flow is an important aspect
that often controls the cross section of earthen irrigation canals.
Sediment transport in irrigation canals is a function of velocity
of flow.
Tractive force theory
Tractive force or shear force is the force applied by the flowing
water on the canal bed and sides in the direction of flow.
This force per unit area is called Unit tractive force or shear
stress.
27
S
R
Wetted
ALS
force
Tractive
.
.
L
*
P
area
Wetted





DESIGN OF ERODIBLE CHANNELS
Erodible canals are canals with movable bed.
In canals designed on erodible (alluvial deposits), not only have erosion
problem but also in most cases the water carries sediments with it.
The design of such canals can be made based on the maximum and
minimum permissible velocities
Procedure For Design
i) Determine the maximum permissible velocity .
ii) With the permissible velocity equal to Q/A, determine A.
iii) With permissible velocity = 1/n S1/2 R2/3 for known value Slope, s
and n.
iv) R = A/P , so determine P as A/R
v) Then A = b y + m y2 and
P = b+ 2 y (m2 + 1)1/2 ,
solve and obtain values of b and y
28

CONT…
29
Material
Clear water Loaded water
Velocity, m/s Tc (N/m2) Velocity, m/s Tc (N/m2)
Fine sand, 0.46 1.30 0.76 3.61
Sandy loam, 0.53 1.78 0.76 3.61
Silt loam, 0.61 2.31 0.91 5.29
Alluvial silts, 0.61 2.31 1.07 7.22
Volcanic ash 0.76 3.61 1.07 7.22
Stiff clay, 1.14 12.51 1.52 22.13
Alluvial silts, 1.14 12.51 1.52 22.13

REGIME CHANNELS
Canals designed for non-silting and non-scouring
velocity are called regime canals.
A channel is in state of regime means that whatever
sediment entering the canal at the head is kept in
suspension so that it will not settle and local
sediments are not produced by erosion of the canals
beds.
There are two researchers called R.G. Kennedy and
Lacey from India who have done a remarkable
research for finding a solution for design of stable
(non-silting and non-scouring) alluvial canals.
31

KENNEDY’S THEORY
Kennedy selected some straight reach of a canal which had not
caused serious silting and scouring for the previous more than 30
years.
He concluded that whether a sediment particle will be kept in
suspension or will settle down is a function of generation of
eddies that rise to the surface.
According to Kennedy, a critical velocity is the velocity which
will just keep the canal free from silting and scouring.
Where:
Vo = the critical velocity
y= the depth of flow, m
0.55 and 0.64=constants which depend on silt charge.
32
64
.
0
*
55
.
0 y
Vo 

CONT…
33
64
.
0
.
.
55
.
0 y
m
Vo 
Sediment type m
Light sandy silt 0.9 to 1.1
Sandy, loamy silt 1.2
Hard soil 1.3
As this formula is purely empirical depending on
observations, in order to apply it to other canals reaches,
some factor which takes into account the soil type.
He introduced a factor called critical velocity ratio (m),
which depends on the size of the silts.

Procedure for design of regime canals on Kennedy’s theory
The following procedure can be used for canal design:
1. Assume a trial depth of flow y and determine the critical velocity
Vo;
2. Determine the area of flow, A from A=Q/Vo;
3. workout the canal cross sectional parameters;
4. calculate the actual mean velocity V in the canal from the Kutter’s
formula, Manning’s formula or Chezy equation;
5. Compare V and Vo. If the same, the assumed depth of flow y is
right if not the same assume another y and repeat steps 1 through
4.
34
RS
R
n
S
S
n
V *
*
00155
.
0
23
1
00155
.
0
23
1





























Kutter’s Formula
Where: V is flow velocity,
m/s
R is Hydraulic radius, m
S is Slope of the canal
n is roughness coefficient

LACEY’S REGIME THEORY
Lacey carried out investigations for the design of regime canals
on alluvial deposits.
He came up with three kinds of regimes called initial, true and
final regimes.
He mentioned that the regime theory can be applied to only
channels in true regime or final regime.
A canal in true regime is a canal in which discharge, velocity,
depth of flow, amount of silt and size of silt are constant.
35

CONT…
Procedure for design of regime canals on Lacey’s Theory
1. Evaluate the flow velocity from
Where V is in m/s
Q is design discharge in m3/s
f is silt factor
where d is mean particle size, mm
2. Determine the hydraulic radius, R from
36
6
1
2
140







Qf
V
d
f *
75
.
1










f
V
R
2
*
2
5

CONT…
3. Calculate the area of flow,
4. Calculate wetted perimeter, P
5. Workout y and b from the known, A, P and R.
6. Compute the canal bed slope, S
or S = 0.0003 f 5/3/Q 1/6
37
V
Q
A 
Q
P *
75
.
4










6
1
3
5
.
3340 Q
f
S

6.2 FLOW MEASUREMENT
Critical-flow flumes and broad-crested weir are devices used to
measure flow in open channels.
These devices are adaptable to a variety of measurement
applications in both natural and man-made channels, and both new
and existing canal systems.
Weir
 It is a barrier (structure) constructed across a river to raise the
water level in the river behind it so as to enable regulated
diversion of water.
 There are two types of weirs in common use:
 Sharp-crested weirs and the broad-crested weirs.
• The sharp-crested weirs are commonly used in irrigation practice.
38

6.3 RELATED HYDRAULIC STRUCTURE
Water levels in canals need to be lowered in some cases for
topographical reasons.
Lowering of water levels will be attended by a loss of energy of the
flow.
Structures used for this purpose are called canal drops or falls.
Falls (Canal drops)
A canal drop is a RC or masonry structure provided in canal to lower
down the water level and the bed level of a canal.
A canal drop consists of a water level lowering structure and an energy
dissipating structure.
It is required when the natural ground slope along the alignment is
steeper than the bed slope of the canal.
The location of this structure is depend on topography and economy.
39

Chute
A chute is a pipe or open channel lowering the water level from a
higher canal to a lower one.
It is used when the required crop height is large (5m and so) and
water has to be conveyed over a longer distance.
Cross Drainage Structures
Cross- drainage structure is structures which is constructed at the
crossing of a canal and natural drainage channels like river and
stream, so as dispose of drainage water with out interrupting the
continuous canal supplies.
As cross drainage structures are expensive, the alignment of the
canal should be that which minimizes the cross drainage works as
much as possible.
40

CONT…
Based on the relative position of the canal and drainage, cross
drainage works can be classified as:-
1. Canal over the drainage: by passing the canal over the drainage.
This is when bed level of the canal is well above the High Flood
Level(HFL) of drainage.
Ex. Aqueduct, Siphon aqueduct
2. Canal below the drainage: by passing the canal below the drainage.
provided when the bed level of drain is well above the Full Supply
Level (FSL) of the canal.
Ex. Super passage or inverted (canal) siphons
3. Canal and drainage at the same level: by passing the drain through
the canal. Provided when HFL and FSL of drain and canal are at the
same level. In this case the canal water &the drainage water are
allowed to mix to each other.
41

QUIZ(10%)
Attempt Any One Question
1. Design an irrigation channel(D,B,T,&S)to carry
a discharge of 30cumecs by Kennedy's theory.
Take B/D=8.0,n=0.0225 and m=1
2. Design regime channel (using lacy`s theory)
carrying discharge of 27.67cumecs and the
average particle size in 0.323mm.
42

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