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5.1 Irrigation Distribution Systems
5.2 Methods of Water Measurement
5.3 Related Hydraulic Structures
Instructional objectives
On completion of this lesson, the student shall learn the following:
1. The need for structures of a canal irrigation system for conveying water from
one point to another.
2. Structures for conveying water across, over or under natural streams
3. Transitions in canals at change of cross section
3.8.0 Introduction
A canal conveying water from the head works has to run for large distances and
has to maintain the water levels appropriately, as designed along its length. It has
to run through terrains which generally would have a different slope small than the
canal. The surrounding areas would invariably have its own drainage system
ranging from small streams to large rivers. The canal has to carry the water across
these water bodies as well as across artificial obstacles like railway line or roads.
The main structures of a canal system for conveyance of canal flow and control of
water levels are as follows.
1. Pipe conduits, culverts and inverted siphons to carry flow under railways and
highways.
2. Aqueducts, siphon aqueducts, super-passage, canal siphon or level crossings
across natural drainage courses or other depressions.
3. Transitions at changes in cross sections.
This lesson deals with the concepts of planning, layout and design of canal
structures for flow conveyance across artificial and natural obstacles.
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3.8.1 Structures for crossing canals across roads and railway lines
These are structural elements to convey canal water under roads or railway
lines. For small roads, carrying relatively less traffic, the pipe conduit is sufficient.
A general view of the pipe conduit is shown in Figure 1 and its typical plan and
cross section in Figure 2. For canals crossing under major highways and railway
tracks, reinforced concrete culverts are more commonly adopted. These roads or
railway crossings are usually having a straight profile along its length. The water
level in the canal for this type of crossing is lower than the level of the obstruction it crosses, as
may be noticed from Figure 2 and the flow through the pipe may be free or under mild pressure.
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Pipe road crossings are relatively economical, easily designed and built, and
have proven a reliable means of conveying water under a roadway. Pipe
installations are normally installed by cut and cover method below minor roads but
for important roads, where traffic cannot be interrupted, it may be accomplished by
jacking the pipe through the roadway foundation.
The inverted siphons are structures for canal water conveyance below roads,
railway lines and other structures (Figure 3). The longitudinal profile is not exactly
in a straight line and the central portion is seen to sag beneath the object to be
crossed. The inverted siphon, therefore, is provided where the water level in the
canal is about the same as the level of the obstruction (Figure 4).
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The inverted siphon is a closed conduit designed to run full and under pressure.
If made of pressure pipes, they should be able to withstand the load of cover and
wheel from outside and the hydrostatic head from inside. Transitions for changes in
cross sections are nearly always used at inlet and outlet of a siphon to reduce head
losses and prevent erosion in unlined canals caused by the velocity changes between
the canal and the pipe.
3.8.2 Structures for crossing canals across natural streams
(Cross drainage works)
These structural elements are required for conveying the canals across natural
drainage. When a canal layout is planned, it is usually seen to cross a number of
channels draining the area, varying from small and shallow depressions to large
rivers. It is not generally possible to construct cross-drainage structures for each of
the small streams. Some of the small drainage courses are, therefore, diverted into
one big channel and allowed to cross the canal. However, for larger streams and
river, where the cost of diversion becomes costlier than providing a separate crossdrainage work, individual structures to cross the canal across the stream is
provided.
There could be a variety of combinations of the relative position of the canal with
respect the natural channel that is to be crossed. These conditions are shown in
Figures 5 to 9. The notations used in the figures are as follows:
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(a) CBL: Canal Bed Level;
(b) SBL: Stream Bed Level;
(c) FSL: Canal Full Supply Level;
and (d) HFL: Stream High Flood Level
Figure 5 shows the relative position of canal (shown in cross-section) with respect to
a natural stream (shown in longitudinal section), when canal bed level is higher
than stream high flood level.
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Figure 6 shows the relative position of a canal whose bed level is below but full
supply level is above the stream high flood level.
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Figure 9 shows the relative position of canal with respect to the natural stream
where the canal full supply level is below the stream bed level.
In general , the solution for all the illustrated conditions possible for conveying an
irrigation canal across a natural channel is by providing a water conveying
structure which may:
(a) Carry the canal over the natural stream;
(b) Carry the canal beneath the natural stream; or
(c) Carry the canal at the same level of the natural stream. These three broad types
of structures are discussed further in this lesson.
3.8.3 Structures to carry canal water over a natural stream
Conveying a canal over a natural watercourse may be accomplished in two ways:
(a) Normal canal section is reduced to a rectangular section and carried across the
natural stream in the form of a bridge resting on piers and foundations (Figure 10).
This type of structure is called a trough type aqueduct.
(b) Normal canal section is continued across the natural stream but the stream
section is flumed to pass through ‘barrels’ or rectangular passages (Figure 11). This
type is called a barrel type aqueduct.
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For the aqueducts, it may be observed from Figures 12 and 13 that the HFL of
the natural stream is lower than the bottom of the trough (or the roof of the barrel).
In this case, the flow is not under pressure, that is, it has a free surface exposed to
atmospheric pressure.
In case the HFL of the natural stream goes above the trough bottom level (TBL)
or the barrel roof level (BRL), then the flow in the natural watercourse would be
pressured and the sections are modified to form which is known as siphon
aqueducts (Figures 14 and 15).
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It may be observed that the trough type aqueduct or siphon aqueduct would
be suitable for the canal crossing a larger stream or river, whereas the barrel type is
suitable if the natural stream is rather small. The relative economics of the two
types has to be established on case to case basis.
Further, the following points maybe noted for the two types of aqueducts or
siphon aqueducts:
Trough type: The canal is flumed to not less than 75 percent of the bed width
keeping in view the permissible head loss in the canal .Transitions 3:1 on the
upstream and 5:1 on the downstream side are provided to join the flumed section to
the normal canal section . For the trough-type siphon aqueduct the designer must
consider the upward thrust also that might act during high floods in the natural
stream when the stream water flows under pressure below the trough base and for
worst condition, the canal may be assumed to be dry at that time. The dead weight
of the trough may be made more than that of the upward thrust or it may be
suitably anchored to the piers in order to may be counteract the uplift condition
mentioned
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Barrel type: The barrel may be made up of RCC, which could be single or
multi-cell, circular or rectangular in cross section. Many of the earlier structures
were made of masonry walls and arch roofing. Precast RCC pipes may be
economical for small discharges. For barrel-type siphon aqueducts, the barrel is
horizontal in the central portion but slopes upwards on the upstream and
downstream side at about an inclination of 3H: 1V and 4H: 1V respectively. A selfcleaning velocity of 6m/s and 3m/s is considered while designing RCC and masonry
barrels respectively.
3.8.4 Structures to carry canal water below a natural stream
A canal can be conveyed below a natural stream with the help of structures
like a super-passage or a siphon. These are exactly opposite in function to that of
the aqueducts and siphon aqueducts, which are used to carry the canal water above
the natural stream. The natural stream is flumed and made to pass in a trough
above the canal. If the canal water flows with a free surface, that is, without
touching the bottom of the trough, it is called a super-passage (Figure 16). Else,
when the canal passes below the trough as a pressure flow, then it is termed as a
syphon or a canal syphon.
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Instead of a trough, the canal flow may be conveyed below the natural stream
using small pre-cast RCC pipes (for small discharges) and rectangular or circular
barrels, either in single or multiple cells, may be used (for large discharges), as
shown in Figure
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3.8.5 Structures to carry canal water at the same level as a natural stream
A structure in which the water of the stream is allowed to flow into the canal
from one side and allowed to leave from the other, known as a level crossing , falls
into this category (Figure 18)
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This type of structure is provided when a canal approaches a large sized
drainage with high flood discharges at almost the same level. The flow control is
usually provided on either side of the canal and on the outlet side of the drain. As
such, this type of arrangement is very similar to canal head-works with a barrage.
Advantage may be taken of the flow of the natural drainage to augment the flow of
the outgoing canal. The barrage type regulator is kept closed during low flows to
head up the water and allows the lean season drainage flow to enter the outgoing
canal. During flood seasons, the barrage gates may be opened to allow much of the
silt-laden drainage discharge to flow down. Another structure, called an inlet, is
sometimes provided which allows the entry of the stream water into the canal
through an opening in the canal bank, suitably protected by pitching the bed and
sides for a certain distance upstream and downstream of the inlet. If the natural
stream water is not utilized in the canal then an outlet, which is an opening on the
opposite bank of the canal is provided. The canal bed and sides suitably pitched for
protection.
3.8.6 Transitions at changes in canal cross-sections
A canal cross section may change gradually, in which case suitable flaring of the
walls may be made to match the two sections (Figure 19)
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For more abrupt changes, like a normal canal section being changed to a
vertical walled aqueduct, suitable transitions have been designed which would
avoid formation of any hydraulic with consequent loss of energy. A typical view of
transition of a normal canal bank to a vertical walled flume section is shown in
(Figure 20).
As may be observed, the banks of the normal canal section are first changed to
vertical walls keeping the same canal bed width (BB c). Beyond this, the vertical
section is reduced gradually to form a reduced sized flume of width (Bf B ). Various
formulae have been proposed for deciding the intermediate curve, that is, an
equation deciding the width (BB x) at any distance x from the start of the fluming,
assuming a length L for the transition. One formula that is commonly used for this
kind of transition is the UPIRI method, commonly known as Mitra’s transition and
is given as follows:
The length L of the transition is assumed to be equal to 2 *(Bc – Bf). In another
type of transition, the vertical curved walls of a normal canal section is both
transformed in to vertical walls of a flume as well as its section is reduced
gradually, as shown in Figure 20. This results in reduction of the canal bed width
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from BB c to Bf B and the side slopes from M0 to O. The values for the bed width Bx
at any length X from the start of the transition and the corresponding side slope mx
are given by the following expression
Water Conveyance Structures
It is necessary that the flow of irrigation water in the water conveyance system is
always under control. Water control structures are therefore required for water
conveyance system to control the flow of water and dispose at safer velocity. The
different types of flow control structures used to regulate water flow are presented
in this lesson
5.3.1 Drop Structures
Drop structure is used for conveying water in the channel from higher elevation to
lower elevation while controlling the energy and velocity of the water as it passes
over. These structures are needed in canals and ditches to convey water down steep
slopes at non-erosive velocities. Drop structure is constructed at end of each reach to
lower water head abruptly in to the next reach by subdividing the slope in to several
reaches with relatively flat slopes. Water is conveyed down the slope in the stepwise
manner. The components of drop structure include an inlet section, a vertical or
inclined drop, a stilling pool or other means of dissipating energy, and an outlet
section for discharging water into the next reach. Kruse et al., (1980) recommend
that drop heights in conveyance canals and ditches be limited to maximum of 0.6 m
to 1 m and that drop height in distribution laterals be less than 15 to 30 cm. Fig.
13.1 shows series of drop structures on a steep sloping land.
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Fig. 5.1. A view of Drop structures in a canal on steep sloping land.
(Source: http://www.fao.org/docrep/R4082E/r4082e06.htm)
5.3.2 Chute Spillways
These are used to convey water from steep slopes. Chutes are lined, high-velocity
open channels (Fig. 13.2 and 13.3). Chute structures are constructed with concrete,
bricks or cement. They have an inlet, a steep-sloped section of lined canal where the
elevation change occurs, a stilling pool or other energy dissipation device, and an
outlet section. Chutes may be made to control flow for elevation changes up to 6m. A
straight apron is used for small structure used in small irrigation channel. Fig 5.2
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Fig. 5.2. Chute spillway.
(Source: http://www.splash.com.my/images/spillway.jpg)
Fig. 5.3. Section of Chute spillway.
(Source:http://www.hydrology.bee.cornell.edu/BEE473Homework_files/ChutesWeirs
.pdf)
5.3.3 Pipe Drop Spillways
Pipe drop structure (Fig. 13.4) is used where a channel has to cross an
embankment. In such cases water can be safely discharged from a higher to a lower
one by providing a pipe drop. This type of structure allows the discharge of water
through a pipeline, without disturbing the existing bunds or embankment. The
components of structures are gated pipe, stilling basin with end sill. Stilling basin is
provided for dissipation of energy of water flow. A stilling basin is made up of brick
or stone masonry, or concrete. A masonry or concrete apron is provided at the inlet
end of the pipe to prevent seepage around it. The discharge capacity of the pipe
drop structure may be determined by the relationship
Q=AV
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in which,
Q = discharge (m3s-1)
A= area of cross-section of the pipe (m2)
V = velocity of flow (m/sec)
In designing the pipe size, head loss due to friction in the pipe line, entrance losses
and loss at the bends are considered.
Fig. 5.4. A drop-inlet pipe spillway with drain pipe.
(Source: www. aqua.ucdavis.edu/Database Root/pdf/USDA590C.pdf)
Example 13.1: Determine the capacity of 3.5 m long (l) pipe of pipe drop spillway to
be used for effective drop in head (H) as 1.2 m. The diameter of pipe (d) is 100 mm
and friction coefficient (f) is 0.012.
Solution:
The applicable formula for the total head in pipe drop spillway is
where
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v = velocity of flow and g = acceleration due to gravity
Substituting the values is above equation
References
http://www.fao.org/docrep/R4082E/r4082e06.htm.FarmWaterDistribution System
http://www.splash.com.my/images/spillway.jpg.
http://www.hydrology.bee.cornell.edu/BEE473Homework_files/ChutesWeirs.pdf
Kruse, E. G., Humpherys, and Pope, E. J. (1980). Farm Water Distribution Systems
(In Design and Operation of Farm Irrigation Systems Edited by Jensen, M.E) An
ASAE Monograph Number.3 American Society of Agricultural Engineers Michigan
USA: 395-446.
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Suggested Reading
Humberto Blanco-Canqui, Rattan Lal. (2008). Principles of Soil Conservation and Management,
The Ohio State University, Columbus, OH, USA Kansas State University, Hays, KS, USA.
James, Larry G. (1988), Principles of Farm Irrigation System Design, John Wiley and Sons, Inc.,
New York.
Michael, A. M. (2010). Irrigation Theory and Practice, Vikas Publishing House PVT Ltd, Nodia,
India: 313-318.
CHAPTER 5 - IRRIGATION SYSTEM
5.1 Main intake structure and pumping station
5.2 Conveyance and distribution system
5.3 Field application systems
5.4 Drainage system
The irrigation system consists of a (main) intake structure or (main) pumping station, a
conveyance system, a distribution system, a field application system, and a drainage system
(see Fig. 69).
Fig. 69. An irrigation system
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The (main) intake structure, or (main) pumping station, directs water from the source of supply,
such as a reservoir or a river, into the irrigation system.
The conveyance system assures the transport of water from the main intake structure or main
pumping station up to the field ditches.
The distribution system assures the transport of water through field ditches to the irrigated
fields.
The field application system assures the transport of water within the fields.
The drainage system removes the excess water (caused by rainfall and/or irrigation) from the
fields.
5.1 Main intake structure and pumping station
5.1.1 Main intake structure
5.1.2 Pumping station
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5.1.1 Main intake structure
The intake structure is built at the entry to the irrigation system (see Fig. 70). Its purpose is to
direct water from the original source of supply (lake, river, reservoir etc.) into the irrigation
system.
Fig. 70. An intake structure
5.1.2 Pumping station
In some cases, the irrigation water source lies below the level of the irrigated fields. Then a
pump must be used to supply water to the irrigation system (see Fig. 71).
Fig. 71. A pumping station
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There are several types of pumps, but the most commonly used in irrigation is the centrifugal
pump.
The centrifugal pump (see Fig. 72a) consists of a case in which an element, called an impeller,
rotates driven by a motor (see Fig. 72b). Water enters the case at the center, through the
suction pipe. The water is immediately caught by the rapidly rotating impeller and expelled
through the discharge pipe.
Fig. 72a. Diagram of a centrifugal pump
Fig. 72b. Centrifugal pump and motor
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The centrifugal pump will only operate when the case is completely filled with water.
5.2 Conveyance and distribution system
5.2.1 Open canals
5.2.2 Canal structures
The conveyance and distribution systems consist of canals transporting the water through the
whole irrigation system. Canal structures are required for the control and measurement of the
water flow.
5.2.1 Open canals
An open canal, channel, or ditch, is an open waterway whose purpose is to carry water from
one place to another. Channels and canals refer to main waterways supplying water to one or
more farms. Field ditches have smaller dimensions and convey water from the farm entrance to
the irrigated fields.
i. Canal characteristics
According to the shape of their cross-section, canals are called rectangular (a), triangular (b),
trapezoidal (c), circular (d), parabolic (e), and irregular or natural (f) (see Fig. 73).
Fig. 73. Some examples of canal cross-sections
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The most commonly used canal cross-section in irrigation and drainage, is the trapezoidal
cross-section. For the purposes of this publication, only this type of canal will be considered.
The typical cross-section of a trapezoidal canal is shown in Figure 74.
Fig. 74. A trapezoidal canal cross-section
The freeboard of the canal is the height of the bank above the highest water level anticipated. It
is required to guard against overtopping by waves or unexpected rises in the water level.
The side slope of the canal is expressed as ratio, namely the vertical distance or height to the
horizontal distance or width. For example, if the side slope of the canal has a ratio of 1:2 (one to
two), this means that the horizontal distance (w) is two times the vertical distance (h) (see Fig.
75).
Fig. 75. A side slope of 1:2 (one to two)
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The bottom slope of the canal does not appear on the drawing of the cross-section but on the
longitudinal section (see Fig. 76). It is commonly expressed in percent or per mil.
Fig. 76. A bottom slope of a canal
An example of the calculation of the bottom slope of a canal is given below (see also Fig. 76):
or
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ii. Earthen Canals
Earthen canals are simply dug in the ground and the bank is made up from the removed earth,
as illustrated in Figure 77a.
Fig. 77a. Construction of an earthen canal
The disadvantages of earthen canals are the risk of the side slopes collapsing and the water
loss due to seepage. They also require continuous maintenance (Fig. 77b) in order to control
weed growth and to repair damage done by livestock and rodents.
Fig. 77b. Maintenance of an earthen canal
iii. Lined Canals
Earthen canals can be lined with impermeable materials to prevent excessive seepage and
growth of weeds (Fig. 78).
Fig. 78. Construction of a canal lined with bricks
Lining canals is also an effective way to control canal bottom and bank erosion. The materials
mostly used for canal lining are concrete (in precast slabs or cast in place), brick or rock
masonry and asphaltic concrete (a mixture of sand, gravel and asphalt).
The construction cost is much higher than for earthen canals. Maintenance is reduced for lined
canals, but skilled labour is required.
5.2.2 Canal structures
The flow of irrigation water in the canals must always be under control. For this purpose, canal
structures are required. They help regulate the flow and deliver the correct amount of water to
the different branches of the system and onward to the irrigated fields.
There are four main types of structures: erosion control structures, distribution control
structures, crossing structures and water measurement structures.
i. Erosion control structures
a. Canal erosion
Canal bottom slope and water velocity are closely related, as the following example will show.
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A cardboard sheet is lifted on one side 2 cm from the ground (see Fig. 79a). A small ball is
placed at the edge of the lifted side of the sheet. It starts rolling downward, following the slope
direction. The sheet edge is now lifted 5 cm from the ground (see Fig. 79b), creating a steeper
slope. The same ball placed on the top edge of the sheet rolls downward, but this time much
faster. The steeper the slope, the higher the velocity of the ball.
Fig. 79. The relationship between slope and velocity
Water poured on the top edge of the sheet reacts exactly the same as the ball. It flows
downward and the steeper the slope, the higher the velocity of the flow.
Water flowing in steep canals can reach very high velocities. Soil particles along the bottom and
banks of an earthen canal are then lifted, carried away by the water flow, and deposited
downstream where they may block the canal and silt up structures. The canal is said to be
under erosion; the banks might eventually collapse.
b. Drop structures and chutes
Drop structures or chutes are required to reduce the bottom slope of canals lying on steeply
sloping land in order to avoid high velocity of the flow and risk of erosion. These structures
permit the canal to be constructed as a series of relatively flat sections, each at a different
elevation (see Fig. 80).
Fig. 80. Longitudinal section of a series of drop structures
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Drop structures take the water abruptly from a higher section of the canal to a lower one. In a
chute, the water does not drop freely but is carried through a steep, lined canal section. Chutes
are used where there are big differences in the elevation of the canal.
ii. Distribution control structures
Distribution control structures are required for easy and accurate water distribution within the
irrigation system and on the farm.
a. Division boxes
Division boxes are used to divide or direct the flow of water between two or more canals or
ditches. Water enters the box through an opening on one side and flows out through openings
on the other sides. These openings are equipped with gates (see Fig. 81).
Fig. 81. A division box with three gates
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b. Turnouts
Turnouts are constructed in the bank of a canal. They divert part of the water from the canal to a
smaller one.
Turnouts can be concrete structures (Fig. 82a), or pipe structures (Fig. 82b).
Fig. 82a. A concrete turnout
Fig. 82b. A pipe turnout
c. Checks
To divert water from the field ditch to the field, it is often necessary to raise the water level in the
ditch. Checks are structures placed across the ditch to block it temporarily and to raise the
upstream water level. Checks can be permanent structures (Fig. 83a) or portable (Fig. 83b).
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Fig. 83a. A permanent concrete check
Fig. 83b. A portable metal check
iii. Crossing structures
It is often necessary to carry irrigation water across roads, hillsides and natural depressions.
Crossing structures, such as flumes, culverts and inverted siphons, are then required.
a. Flumes
Flumes are used to carry irrigation water across gullies, ravines or other natural depressions.
They are open canals made of wood (bamboo), metal or concrete which often need to be
supported by pillars (Fig. 84).
Fig. 84. A concrete flume
b. Culverts
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Culverts are used to carry the water across roads. The structure consists of masonry or
concrete headwalls at the inlet and outlet connected by a buried pipeline (Fig. 85).
Fig. 85. A culvert
c. Inverted siphons
When water has to be carried across a road which is at the same level as or below the canal
bottom, an inverted siphon is used instead of a culvert. The structure consists of an inlet and
outlet connected by a pipeline (Fig. 86). Inverted siphons are also used to carry water across
wide depressions.
Fig. 86. An inverted siphon
iv. Water measurement structures
The principal objective of measuring irrigation water is to permit efficient distribution and
application. By measuring the flow of water, a farmer knows how much water is applied during
each irrigation.
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In irrigation schemes where water costs are charged to the farmer, water measurement
provides a basis for estimating water charges.
The most commonly used water measuring structures are weirs and flumes. In these structures,
the water depth is read on a scale which is part of the structure. Using this reading, the flow-rate
is then computed from standard formulas or obtained from standard tables prepared specially
for the structure.
a. Weirs
In its simplest form, a weir consists of a wall of timber, metal or concrete with an opening with
fixed dimensions cut in its edge (see Fig. 87). The opening, called a notch, may be rectangular,
trapezoidal or triangular.
Fig. 87. Some examples of weirs
A RECTANGULAR WEIR
A TRIANGULAR WEIR
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A TRAPEZOIDAL WEIR
b. Parshall flumes
The Parshall flume consists of a metal or concrete channel structure with three main sections:
(1) a converging section at the upstream end, leading to (2) a constricted or throat section and
(3) a diverging section at the downstream end (Fig. 88).
Fig. 88. A Parshall flume
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Depending on the flow condition (free flow or submerged flow), the water depth readings are
taken on one scale only (the upstream one) or on both scales simultaneously.
c. Cut-throat flume
The cut-throat flume is similar to the Parshall flume, but has no throat section, only converging
and diverging sections (see Fig. 89). Unlike the Parshall flume, the cut-throat flume has a flat
bottom. Because it is easier to construct and install, the cut-throat flume is often preferred to the
Parshall flume.
Fig. 89. A cut-throat flume
5.3 Field application systems
5.3.1 Surface irrigation
5.3.2 Sprinkler irrigation
5.3.3 Drip irrigation
There are many methods of applying water to the field. The simplest one consists of bringing
water from the source of supply, such as a well, to each plant with a bucket or a water-can (see
Fig. 90).
Fig. 90. Watering plants with a bucket
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This is a very time-consuming method and it involves quite heavy work. However, it can be used
successfully to irrigate small plots of land, such as vegetable gardens, that are in the
neighbourhood of a water source.
More sophisticated methods of water application are used in larger irrigation systems. There are
three basic methods: surface irrigation, sprinkler irrigation and drip irrigation.
5.3.1 Surface irrigation
Surface irrigation is the application of water to the fields at ground level. Either the entire field is
flooded or the water is directed into furrows or borders.
i. Furrow irrigation
Furrows are narrow ditches dug on the field between the rows of crops. The water runs along
them as it moves down the slope of the field.
The water flows from the field ditch into the furrows by opening up the bank or dyke of the ditch
(see Fig. 91a) or by means of syphons or spiles. Siphons are small curved pipes that deliver
water over the ditch bank (see Fig. 91b). Spiles are small pipes buried in the ditch bank (see
Fig. 91c).
Fig. 91a. Water flows into the furrows through openings in the bank
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Fig. 91b. The use of siphons
Fig. 91c. The use of spiles
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ii. Border irrigation
In border irrigation, the field to be irrigated is divided into strips (also called borders or
borderstrips) by parallel dykes or border ridges (see Fig. 92).
The water is released from the field ditch onto the border through gate structures called outlets
(see Fig. 92). The water can also be released by means of siphons or spiles. The sheet of
flowing water moves down the slope of the border, guided by the border ridges.
Fig. 92. Border irrigation
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iii. Basin irrigation
Basins are horizontal, flat plots of land, surrounded by small dykes or bunds. The banks prevent
the water from flowing to the surrounding fields. Basin irrigation is commonly used for rice grown
on flat lands or in terraces on hillsides (see Fig. 93a). Trees can also be grown in basins, where
one tree usually is located in the centre of a small basin (see Fig. 93b).
Fig. 93a. Basin irrigation on the hillside
Fig. 93b. Basin irrigation for trees
5.3.2 Sprinkler irrigation
With sprinkler irrigation, artificial rainfall is created. The water is led to the field through a pipe
system in which the water is under pressure. The spraying is accomplished by using several
rotating sprinkler heads or spray nozzles (see Fig. 94a) or a single gun type sprinkler (see Fig.
94b).
Fig. 94a. Sprinkler irrigation using several rotating sprinkler heads or spray nozzles
Fig. 94b. Sprinkler irrigation using a single gun type sprinkler
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5.3.3 Drip irrigation
In drip irrigation, also called trickle irrigation, the water is led to the field through a pipe system.
On the field, next to the row of plants or trees, a tube is installed. At regular intervals, near the
plants or trees, a hole is made in the tube and equipped with an emitter. The water is supplied
slowly, drop by drop, to the plants through these emitters (Fig. 95).
Fig. 95. Drip Irrigation
5.4 Drainage system
A drainage system is necessary to remove excess water from the irrigated land. This excess
water may be e.g. waste water from irrigation or surface runoff from rainfall. It may also include
leakage or seepage water from the distribution system.
Excess surface water is removed through shallow open drains (see Surface drainage, Chapter
6.2.1). Excess groundwater is removed through deep open drains or underground pipes (see
Subsurface drainage, Chapter 6.2.2).
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Irrigation Distribution Systems, Methods of water Measurement and
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