Water and irrigation

1. Canal Outlets
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2. Introduction
• An outlet is a hydraulic structure conveying irrigation water
from a state owned distributary to privately owned water
course.
• The outlets are large in number as compared to other
irrigation structures in an irrigation system and hence their
design and type has maximum bearing on the equitable
distribution of water.
• Therefore proper design of outlet is of utmost importance.
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3. Tail cluster
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4. Canal Outlet
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5. Pakka nakka
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6. Essential Requirements of an outlet
• An outlet should be strong and be without movable parts to
minimize tempering
• Tempering by cultivators should be readily detectable
• The outlet must carry its fair share of silt from parent channel
• It should be able to work with small working heads
• It should be simple so that construction is easy
• The total cost of installation and maintenance should be
minimum
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7. Types of Outlets
• Non-Modular Outlets
– It is one in which the discharge depends upon the difference of
head in water course and parent channel.
– Hence, a variation in either affects the discharge.
• Semi-Modular
– It is one in which the discharge depends upon the water level in
distributary only and is independent of water level in water
course.
– This is achieved by producing hydraulic jump within the flume
length.
• Modular Outlets
– It is one in which the discharge is independent of the water level
in water course and parent channel.
– It can be fixed for any discharge value. This is achieved by
creating a free vortex and destroying any extra head more than
allowed for in the designed discharge.
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8. Types of Outlets
Non-Modular
Semi-Modular
(Flexible)
Modular
(Rigid)
Pipe or Barrel
Type
Scratchley
outlet
Weir-Type Orifice-Type With moving parts
Without moving parts
Harvey
Stoddard
outlet
Crumps
open flume
outlet
Jamrao type
open flume
Kennedy
Gauge
Outlet
Crumps
adjustable
proportional
module
Adjustable
orifice semi
module
Gibbs module
Khanna module
Ghafoor rigid
flume module
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9. Characteristic of Outlets
• Flexibility: It is defined as the ratio of rate of change of
discharge in outlet to the rate of change of discharge in
parent channel.
F = (dq/q)/(dQ/Q)
= (m/n)(D/H)
(H/D) is the setting of an outlet
• It is the capacity of an outlet to vary its discharge with the
change in the discharge of the distributary.
• If F=1 Proportional
• If F>1 Hyper-proportional
• If F<1 Sub-proportional
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10. Characteristic of Outlets
• Sensitivity: It is the ratio of rate of change of discharge of
an outlet to the rate of change in the level of distributary
water surface, i.e. normal depth of channel
S = (dq/q)/(dG/D)
• Here, S is the sensitivity and G is the gauge reading of a
gauge which is so set that G = 0 corresponds to the condition
of no discharge through the outlet (i.e., Q0 = 0).
• Sensitivity can also be defined as the ratio of the rate of
change of discharge of an outlet to the rate of change of
depth of flow in the distributary channel.
S = nF
n = 5/3 for wide trapezoidal channel with side slope ½:1
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11. Characteristic of Outlets
• Efficiency: It is equal to the ratio of the head recovered (or
the residual head after the losses in the outlet) to the input
head of the water flowing through the outlet.
• Minimum Modular Head: it is the minimum head required
for the proper functioning of the outlet as per its design.
• Modular Limits: The extreme values of any parameter at
which a module or a semi module ceases to be capable of
acting as such.
• Modular Range: The range of conditions between the modular
limits within which a module or semi module works as designed.
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12. Characteristic of Outlets
• Coefficient of Discharge: In order to use the outlet as a
measuring device the coefficient of discharge should remain
constant in the full modular range.
• Silt Drawing Capacity: It is vital that the outlets should draw
their fair share of silt. This avoids silting or scouring and
consequently remodeling of distributary.
• In a distributary system the absorportion losses are generally taken
as 10-15% and therefore the silt conducting power of outlets should
be around 110-115% as compared to 100% of distributary to
enable them to draw their proportional share.
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13. Characteristic of Outlets
• Adjustability: The adjustment of module may range from
complete reconstruction to the provision of some mechanical
arrangement by which readjustment can be made at little cost.
Readjustments are required in view of the revision of areas under
command and because of change conditions in the distributary.
• Immunity from Tempering: There is tendency on the parts of
cultivators to draw more than their lawful share of water by
tampering with the outlets. Therefore the outlets must be tamper
proof.
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14. Selection of Type of Outlet
• A module (Modular) or semi-module with a constant
coefficient of discharge is the best selection if the discharge
and the water levels are constant in the distributary and
necessary working head is available.
• But the problem of choice becomes quite complex when both
the discharge and levels are likely to change.
• The following points may be noted;
– For a temporary discharge variation a proportional semi
module is desirable to distribute both excess or deficiency
in the parent channel.
– Seasonal variation in the slope require the use of outlets of
low flexibility, i.e., sub-proportional.
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15. Selection of Type of Outlet
– For channels running with full supply for a certain period
and remaining closed for certain other periods, i.e.
rotational running, it is desirable to have hyper-
proportional or high flexibility outlets in the head reaches.
– The silt drawing capacity of outlet must be 110-115% assuming
a 10-15% loss in parent channel.
– In general rigid modules are desirable in the following
circumstances
• Direct outlets on a branch canal subject to variation in supply
• In channels which sometimes carry extra discharge for
specific reasons like leaching.
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16. Open Flume Outlets
• This is a smooth weir with a throat constricted sufficiently long to
ensure that the controlling section remains within the parallel throat
for all discharges up to the maximum
• Since a hydraulic jump forms at the control section, the water level
of the watercourse does not affect the discharge through this type
of outlet. Hence this is a semi-modular outlet.
• This type of structure is built in masonry, but the controlling section
is generally provided with cast iron or steel bed and check plates.
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17. Open Flume Outlets
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18. Open Flume Outlets
• The discharge formula for the open flume outlet is given as:
Q = C Bt H3/2
• Where: Q is related to the coefficient of discharge, C, as given
in the table below; Bt is the width of the throat; and H is the
height of the full supply level of the supply channel above the
crest level of the outlet in ft.
Bt C
0.2 ft – 0.29 ft, 2.90
0.3 ft – 0.39 ft 2.95
Over 0.4ft 3.00
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19. Adjustable Proportional Module (APM)
• In this type of outlet, a cast iron base, a cast iron roof block
and check plates on either side are used to adjust the flow
and is set in a masonry structure
• This outlet works as a semi-module since it does not depend
upon the level of water in the watercourse.
• The roof block is fixed to the check plates by bolts which can
be removed and depth of the outlet adjusted after the
masonry is dismantled.
• This type of outlet cannot be easily tampered with and at the
same time be conveniently adjusted at a small cost.
• The APM is the best type of outlet if the required working
head (MMH) is available and is the most economical in
adjustment either by raising or lowering the roof block or
crest. However, it is generally costlier than the other types of
outlets and also requires more working head.
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20. 20

21. Schematic Diagram of Outlet
D G
Hs
V
Y
J
Hm
FSL
Roof
Block
Outlet discharge = q
Full supply depth in parent
channel = D
Working head = Hw
Discharge of canal = Q
Width of throat = Bt
Depth of water above crest u/s =
G
Flexibility = F
Min. Modular Head = Hm
Distance from tip of roof block to
FSL = Hs
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22. Crump’s Open Flume Outlet Design
• Data:
• Outlet Discharge = q = 4 cfs
• Full Supply Depth = D = 3.5 ft
• Working Head = Hw = 1.0 ft
• Discharge of Distributory = Q = 60 cfs
• Design
– 1. Canal Section
– According to Lacey’s theory, design of distributory comes out to
be
B=38’
D=3.5’
Side Slope
1:0.5
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23. Crump’s Open Flume Outlet Design
• Setting:
G= Setting of outlet = 0.9 D
= 3.15 ft
Head above crest of outlet = 3.15 ft
• Throat Width:
• q= CdBtG3/2
• q= 2.9BtG3/2 Assuming Cd = 2.9
• 4=2.9Bt(3.15)3/2
• Bt= 0.2462 = 0.25 ft
• Note: The value of Bt lies in between 0.2 to 0.29 ft.
D G
Hs
V
Y
J
Hm
FS
L
R = 2 G
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24. Crump’s Open Flume Outlet Design
• Length of Crest:
Length of Crest = 2.5 G
= 2.5x3.15= 7.875=7.9 ft
• Radius of Transition:
R = 2 G=6.30 ft
• Setting Back: This distance by which the wall parallel to the
distributary axis is to be set back bears the same ratio to the width
of distributary as the discharge of the outlet to that of the
distributary.
Setback/width of distributary = q/Q
Setting Back = 2.53 ft
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25. Crump’s Open Flume Outlet Design
• Transition in bed:
The transition in bed is given by curve of radius= 2 G
= 2x3.15=6.3 ft
• D/S Transition:
The slope of d/s glacis is not defined as it depend upon the bed
level of the water course.
• Minimum Modular Head:
MMH = 0.2G =0.2x3.15
= 0.63 ft < working head=1.0 ft (OK)
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26. Crump’s Open Flume Outlet Design
• Flexibility: (m=3/2, n=5/3)
F=(m/n) (D/G)
=(3/2/5/3)(1/0.9)
=1
• Sensitivity:
S = nF
= 5/3 F = 5/3
• Efficiency:
=100xHead recovered/Head put in
=100x(Head put in-Working head)/Head put in
=100 x (3.15-1)/3.15= 68.25%
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27. Tail Cluster
• When the discharge of a secondary, tertiary or quaternary canal
diminishes below 150 l/s, it is desirable to construct structures to
end the canal and distribute the water through two or more outlets,
which is called a tail cluster. Each of these outlets is generally
constructed as an open flume outlet
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