13. Canal Outlets & other Head Regulators.pdf

CANAL OUTLETS
CE-402: IRRIGATION ENGINEERING
(CH-03)
EDITED BY
DR. MUHAMMAD AJMAL
LECTURER
AGRI. ENGG. UET PESHAWAR

Introduction to Canal Outlets
 The success of any irrigation enterprise depends on the efficiency of
distributing sufficient supply of water to the irrigator.
 Each irrigator has to receive certain quantity of water proportionate
to his extent in a canal system at the proper time to ensure him a
good crop.
 This distribution of water is carried out by means of outlets
otherwise called modules.
 Proper design of an outlet, is of most importance not only to the
canal engineer but to the irrigator also.

 A canal outlet is a small head regulatory hydraulic structure
constructed at the head of water course which divert water from the
distributary to water course. OR
 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 of the equitable distribution of
water.
 Therefore proper design of outlet is of utmost importance.
Introduction to Canal Outlets

Schematic Diagram of Outlet
▪ Outlet discharge = q
▪ Full supply depth in parent channel = D
▪ Discharge of canal = Q
▪ Hs = distance between canal FSL and lowest
point of the roof block
▪ Width of throat = Bt
▪ Depth of water above crest u/s = G
▪ Flexibility = F
▪ Min. Modular Head = Hm (Working head)

 For the efficient working, an outlet should fulfil the following
requirements.
 It should be simple in design, construction and working.
 It should be strong so that farmer can not tamper with its functioning. If
tampered by farmers can be easily detected.
 It should draw its fair share of sediment carried by channel.
 Simple design: can be fabricated by local masons or technicians.
 From the farmer’s viewpoint, the outlet should give a fairly constant
discharge. However from the canal regulation viewpoint, the outlet should
draw proportionately more or less discharge with the varying supply in the
distribution channel.
Essential Requirements of Canal Outlets

Characteristic of Outlets
• Flexibility: It is defined as the ratio of rate of change of
discharge in outlet to the rate of change
parent channel.
F = (dq/q)/(dQ/Q)
= (m/n)(D/H)
(H/D) is the setting of an outlet
Class Note:
of discharge in
• It is the capacity of an outlet to vary its discharge with the
change in the discharge of the distributary.

 Proportionality
 In a proportional outlet the rate of change of its discharge is equal to
the rate of change of the discharge of the distributing channel.
 For proportionality using F = 1, then H/D = m/n
 The ratio H/D is known as setting.
 From proportionality view point, an outlet is classified into three types
as;
(a) Proportional outlet (F = 1)
(b) Hyper-proportional outlet (F > 1)
(c) Sub-Proportional outlet (F < 1)

 Setting
 It is the ratio of the depth of sill or the crest level of the module
below the full supply of the distributing channel to the full supply
depth of the distributing channel.
 Thus Setting = H/D
 Proportionate outlet
❖ It is equal to the ratio of outlet index to the channel index i.e.
setting = H/D = m/n
 For a channel of trapezoidal shape Q ∞ D5/3 so n= 5/3
 Similarly for orifice type outlet q ∞ H1/2
 Then setting = H/D = m/n = (1/2)/(5/3) = 0.30
 Hence the pipe outlet to be proportional, the outlet is set at 0.30
times the depth below the water surface.

 Hyper-Proportional outlet
❖ The outlet in which the flexibility is greater than one (F >1)
❖ In other words the discharge in the outlet changes by a larger
percentage than the percentage in the discharge of the distributing
channel.
❖ Thus, for a hyper proportional outlet
F > 1
(m/n)(D/H) > 1
Or (H/D) < (m/n)

 Sub-Proportional Outlet
 The outlet in which the flexibility is lesser than one (F < 1)
 In other words the discharge in the outlet changes by a smaller
percentage than the percentage in the discharge of the distributing
channel.
F < 1
(m/n)(D/H) < 1
Or (H/D) > (m/n)

❖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)
• S is the sensitivity of the outlet and G is the gauge reading.
• D depth of water in the distributing channel.
• If q =0 then Set G = 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 = (dq/q)/(dD/D)
•Also,
F = (dq/q)/(dQ/Q) where dQ/Q = n(dD/D)
So F =(dq/q)/(ndD/D)
S = nF

• Efficiency: It is the measure of conservation of head at outlet
and 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. It is
the difference b/w the u/s and d/s water levels which is
absolutely necessary to be maintained to enable the outlet to
pass its design discharge.

• Modular Limits: The upper and lower limits of any one or
more factors beyond which an outlet is incapable of acting as a
module/outlet.
• Modular Range: The range of conditions between the modular
limits within which a module or semi module works as designed.
• Drowning ratio: it is ratio between the depths of water level over
crest on the downstream and upstream of the outlet.

• 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/farmers to draw more than their lawful share of
water by tampering with the outlets. Therefore the outlets must
be tamper proof.

Types of Outlets
1. Non-Modular Outlets
 It is one in which the discharge depends upon the difference of
head in water course and parent channel.
 A variation in either affects the discharge.
 One example of this type of outlets is submerged pipe outlet (shown
in figure).
 The pipes vary from 10 to 30 cm in dia. and is frequently laid on a
light concrete foundation to prevent uneven settlement and
consequent leakage.
 They are generally fixed horizontally at right angles to the flow
direction.

Types of Outlets
 The head loss H through the outlet is given by
H = (Entry loss + Friction loss + Velocity head at the exit)
 Rearranging we have
2 2 2 2
0.5 4 1.5 4
2 2 2 2
V fLV V V fL
H
g gD g g D
 
 

   


1/2
2
1.5 4
D
V gH
D fL
 
  

 
1/2
2
1.5 4
D
q AV A gH
D fL
 
   

 
1/2
2
1.5 4
q CA gH
D
C
D fL

 
  

 
 C is coefficient of discharge
 V flow velocity (m/s)
 f friction factor (0.005 for clean iron
pipe & 0.01 for slightly encrusted
iron pipes
 A is cross section area (m2)
 D is the pipe diameter (m)
 L is the pipe length (m)
 If D is in cm then C will change as
given below
 
1/2
0.05
1.5
400
D
C
D L f
f
 
 
  
 

 

Types of Outlets
 The discharge of an outlet can be increased by digging the head of
watercourse and by lowering the water level in it.
 This will increase the head H and therefore the discharge will be
increased.
 It is common practice to place the pipe at the bed of the distributing
channel to enable the outlet to draw fair share of silt charge.

Assignment No. 3
Write in detail with examples and figures the cross
drainage works

Types of Outlets
-------To be continued------

Types of Outlets
❖ Solved Examples and Problems from book
 Pb#1 what will be the coefficient of discharge for 20 cm diameter
clean iron pipe having 10 m length.
 Solution (Note)
 Pb#2 a submerged pipe outlet has the following data:
F.S.L. of distributary = 1.1 m
F.S.L. of water course = 1.0 m
Length of pipe = 9 m
Dia. of pipe = 20 cm
For clean iron pipe, f = 0.005
Find the discharge through the outlet
 Solution (Note)

Types of Outlets
 Solved Examples and Problems from book
❖ Pb#3 Design a submerge pipe outlet for the following data:
Design discharge through the outlet = 0.04 cumecs
F.S.L. of distributary = 1.1 m
F.S.L. of water course = 1.0 m
Assume C = 0.70
❖ Solution (Note)

Types of Outlets
❖ Semi-Modular Outlets
 In this outlet, the discharge is affected by the change in the
water level of the distributing channel but not with the change in
the water level of the filed channel.
 Following are few examples of the this type of outlet:
a) Pipe outlet discharging freely in the atmosphere
b) Kennedy’s gauge outlet
c) Crump’s Open flume outlet
d) Orifice semi-modules

Types of Outlets
2. Semi Modular Outlet
 Discharge through this module can be found out as following:
 a = Cross section area at the throat
 Ho is mentioned in the figure
𝑞 = 𝐶𝑎 2𝑔𝐻𝑜
 Minimum modular head is 0.22 Ho, where Ho is the depth of water
over the center of the orifice.
 The water discharges at atmospheric pressure from bell mouth
orifice into the truncated cone. The water is led further through
cast iron expansion pipe to a concrete pipe and from there to the
water course.

Crump’s Open FlumeOutlets
• 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.
• This type of structure is built in masonry, but the controlling section
is generally provided with cast iron or steel bed and check plates.

Crump’s Open Flume Outlets
• The discharge formula for the Crump’s open flume outlet is given
as:
Q = C Bt H3/2
• Where: Q is discharge, C is flume constant depends on Bt (width
of the throat) dimensions as given in the table below, and H is
the height of the full supply level of the supply channel above
the crest level of the outlet in meters.
Bt (m)
0.06 – 0.08
0.09 – 0.12
Over 0.12
C
1.60
1.64
1.66

Types of Outlets
❖ Modular Outlets/Rigid Module (Gibb’s Module)
 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.

Types of Outlets

Types of Outlets
 Irrigation water is taken through an inlet pipe to a rising pipe in the
form of a spiral and generally semi-circular.
 The water when flows through it are turned through 180°.
 During the movement in the rising pipe vortex motion* is developed.
* In fluid dynamics, a vortex is a region in a fluid in which the flow
rotates around an axis line, which may be straight or curved.
 As the flow is continuous, angular velocity of the flow is same.
 The eddy chamber is rectangular in section but semi-circular in plan
with horizontal floor.
 It takes back the water in the original direction of flow.

Types of Outlets
 In the eddy chamber the baffles are provided at equal distance to
dissipate excess energy of flow and to maintain a constant discharge.
 After the excess energy of flow is destroyed and discharge is made
constant the water from eddy chamber is taken into a spout.
 The spout is connected to a field channel by means of expansion walls.
 The walls are generally splayed with 1 in 10 (lateral: longitudinal)
expansion.
 The Gibb’s module can be designed to give 0.03 cumec constant
discharge for modular range of 0.3 m.
 The minimum working head required to maintain this discharge is 0.12
m.

Types of Outlets
 It may be recognized that as modular outlets require complicated
arrangement of parts and is quite expensive.
 Secondly, in alluvial tracts the silt trouble is more. Outlet gets choked
up with silt. Hence, this type is not much in practice.
 Discharge from this module can be found out as follows:
m = ro/ri
ro = radius of outer semi-circle
ri = radius of inner semi-circle
di = depth of water at inner circumference
ho = head at outer circumference
2 2
1.5
3 2
1 1 1
2 ( ) log log
2
o i o e e
m m
q r g d h m m
m
m m
 
 
   
 
 

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
necessary working head is available.
distributary and
• 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.

Selection of Type of Outlet
 For channels running with full supply for a certain period
and remaining closed
rotational running, it
for certain
is desirable
other periods, i.e.
to have hyper-
proportional or high flexibility outlets in the head reaches.
 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.

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.

Adjustable Proportional Module (APM)
• The APM is the best type of outlet if the required working
head is available and is the most economical in
adjustment either by raising or lowering the roof block or
crest.
• It is generally costlier than the other types of outlets and also
requires more working head.

Canal Escape
❖ It is a hydraulic structure constructed to remove surplus
water from an irrigation channel (main canal, branch
canal, or distributary etc.) at suitable locations into a
natural drain, river etc.
❖ Conditions that might suddenly lead to accumulation of
excess water in a certain reach of a canal network may
occur due to the following reasons:
✓ Wrong operation of head works in trying to regulate flow in
a long channel resulting in release of excess water than the
total demand in the canal system downstream.
✓ Excessive rainfall in the command area leading to reduced
demand and consequent closure of downstream gates.
✓ Sudden closure of control gates due to a canal bank
breach.

Canal Escape
❖ The excess water in a canal results in the water level
rising above the full supply level which, if allowed to
overtop the canal banks, may cause erosion and
subsequent breaches.
❖ Canal escapes help in releasing the excess water from a
canal at times of emergency.
❖ Also, when a canal is required to be emptied for repair
works, the water may be let off through the escapes.

❖ Escapes are usually of the following three types.
a) Sluice or Surplus Escape
 These are gated escapes with a very low crest height (shown
in Figure).
 These sluices can empty the canal much below its full supply
level and at a very fast rate.
 In some cases, these escapes act as scouring sluices to
facilitate removal of sediment.
 The locations for providing escapes are often determined on
the availability of suitable drains, depressions or rivers with
their bed level at or below the canal bed level so that any
surplus water may be released quickly disposed through these
natural outlets.
 Canal escapes may be provided at intervals of 15 to 20 km for
main canal and at 10 to 15 km intervals for other canals.
Canal Escape

a) Sluice or Surplus Escape
Canal Escape
Figure: Sluice (Surplus) Escape
 This type is now obsolete because of the silt ejector better efficiency.

b) Weir or Surface Escape
 These are constructed in the form of weirs, without any
gate or shutter and spills over when the water level of the
canal goes above its crest level.
Canal Escape

c) Tail Escape
 Tail escape is a structure constructed at the end of a
waterway to evacuate the water to a water body.
 Tail escape consists of a well where its crest level at the
high water level.
 It is also equipped with an orifice at its bottom to
evacuate all the water in 24 hrs if necessary.
 A pipe evacuates the water to the water body under the
road.
Canal Escape

Head Regulator
 Regulators Constructed at the off taking point are called head
regulators.
 When it is constructed at the head of main canal it is known
as canal head regulator.
 When it is constructed at the head of distributary, it is called
distributary head regulator.
❖ Function
 To control the entry of water either from the reservoir or from
the main canal.
 To control the entry of silt into off taking or main canal.
 To serve as a meter for measuring discharge of water.

❖ Construction
✓ The components of head regulator depends upon the
size of canal and location of head regulator.
✓ It consists of one or more gated openings with barrels
running through the bank.
✓ For large canals head regulators are flumed to facilitate
the measurement of discharge.
Head Regulator

 Cross Regulator
✓ A Regulator Constructed in the main canal or parent
canal downstream of an off taking canal is called cross-
regulator.
✓ It is generally constructed at a distance of 9 to 12 km
along the main canal and 6 to 10 km along branch canal.
 Functions
✓ To Control the flow of water in canal system
✓ To feed the off taking Canals
✓ To enable closing of the canal breaches on the d/s
✓ To provide roadway for vehicular traffic
Head Regulator

 Construction
✓ For Cross Regulators abutments with grooves and piers
are constructed parallel to the parent canal.
✓ The sill of regulation is kept little higher than the u/s
bed level of canal across which it is constructed.
✓ Vertical lift gates are fitted in the grooves.
✓ The gates can be operate from the road.
Cross Regulator

13. Canal Outlets & other Head Regulators.pdf

13. Canal Outlets & other Head Regulators.pdf

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