Irrigation Engineering Part 2
Irrigation Engineering Part 2
Irrigation Engineering Part 2
CED-401
CANAL FALLS
INTRODUCTION:
Generally the slope of the natural ground surface is not uniform throughout the alignment.
Sometimes the ground surface may be steep and sometimes it may be very irregular with
abrupt change of grade. In such cases a vertical drop is proved to step down the canal bed and
then it is constituted with permissible slope until another step down is necessary. Such
vertical drops are known as canal falls or simply falls.
PURPOSES:
1. To account for the difference in the natural bed slope of the canal.
2. To save us from cutting and filling.
3. To Increase the velocity of the water in the canal.
4. To control the seepage in the canal.
The canal falls are necessary in case the following conditions occur:
(a) When the slope of the ground suddenly changes to steeper slope, the permissible bed
slope cannot be maintained. In such cases canal falls are provided to avoid excessive earth
work in filling. (Fig. 14.1 (a))
(b) When the slope of the ground is more or less uniform and the slope is greater than the
permissible slope of the canal. In such case also the canal falls are necessary (Fig. 14.1 (b))
(c)In cross drainage works when the difference between bed level of canal and that of
drainage is same or when flood surface level of the canal is above the bed level of drainage
then the canal fall is necessary to carry the canal water below the stream or drainage (i.e. in
case of siphon super passage) (Fig. 14.1 (c))
1. Ogee fall:
In this type fall an ogee (a combination of convex curve and concave curve) is provided for
carrying the canal water from higher level to the lower level. This fall is recommended when
the natural ground surface suddenly changes to a steeper slope along the alignment of the
canal.
2. Rapid fall:
The rapid fall is suitable when the slope of the natural ground surface is even and long. It
consists of a long sloping glacis with longitudinal slope which varies from 1 in 10 to 1 in 20.
3. Stepped Fall:
Stepped fall consists of vertical drops in the form of steps. This fall is suitable in places
where the sloping ground is very long and requires long glacis to connect the higher bed with
the lower bed level. This fall is practically a modification of the rapid fall. Here the sloping
glacis is divided into a number of drops so that the flowing water may not cause any damage
to the canal bed.
In this type of fall a body wall is constructed across the canal. The body wall consists of
several trapezoidal notches between the side piers and the intermediate pier or piers. The sills
of the notches are kept at the upstream bed level of the canal. The size and number of notches
depends upon the full supply discharge of the canal. (Fig. 14.5)
It consists of a vertical drop wall which is constructed with masonry work. The water flows
over the crest of the wall. A water cistern is provided on the downstream side which acts as a
water cushion to dissipate the energy of falling water.Hence it is sometimes known as sarda
fall.(Fig.14.6)
6. Glacis Fall:
It consists of a straight sloping glacis provided with a crest. A water cushion is provided on
the downstream side to dissipate the energy of flowing water. Curtain walls and toe walls are
provided on the upstream and downstream side. This type of fall is suitable for drops up to
1.5 m (Fig.14.7).
In this type of fall the straight sloping glacis is modified by giving parabolic shape which is
known as Montague profile.
In this type of fall the glacis is straight and sloping but baffle walls are provided on the
downstream floor to dissipate the energy of flowing water . The height of baffle depends on
the head of water on the upstream side.
Step 1:
Total fall (Hw)=U/S (Full supply level) – D/S (Full supply level)
Step 2:
Crest length of fall L= Width of channel
Crest width of fall B=0.55 H+W For Rectangular fall
Crest width of fall B=0.45 H+W For Trapezoidal fall
Where,
Step 3:
Step 4:
Velocity of reach (V)=QA
Step 5:
Velocity Head (ha)=V22g
Step 6:
Total energy level (U/S) =(U/S) Water level + V22g
Step 7:
Crest level =Total energy level (U/S) – H
Step 8:
Height of crest above (U/S) bed level (hc) =Crest level – (U/S) Bed level
Step 9:
Hydraulic Head (Hh)=Crest level – (D/S) Bed level
Step 10:
Step 11:
Length of cistern (L)=5 × Hw× He
Step 12:
Depth of cistern (x)=0.25 × (Hw× He)23
Step 13:
Cistern level =(D/S) Bed level – Depth of cistern (x)
Step 14:
(U/S) Pakka floor length=Total Length (LT) – Creep Length (Lc)
Step 15:
(D/S) Stone pitching length=10 He + 2 Hw
Step 16:
The (U/S) wing walls are provided in a circular shape with Radius
R=6 × He
Step 17:
Length of (D/S) Wing wall=8 × H× Hw
Example:
Design a Sarda type fall for the following set of data
Full supply discharge=14 m3sec
Bed width=18 m
(U/S)Bed level=99.5 m
(D/S)Bed level=98.5 m
Solution:
Step 1:
Total fall Hw=USFull supply level–DSFull supply level
=101 – 100 = 1m
Step 2:
=18 m
Crest width of fall (B)=0.55 H+W(For Rectangular fall)
Where,
H = Head above the crest < Total fall (Hw)
D = Normal water depth (Full supply)
Step 3:
Discharge (Q)=0.415 2g × L × H 32 × ( HB )16
14=0.415 2×9.81 × 18 × H 32 × ( H1 )16
14 = 33.09 H 106
H=0.6 m
Step 4:
Velocity of reach (V)=QA
Step 5:
Velocity Head (ha)=V22g = 0.4822 ×9.81 = 0.012 m
Step 6:
Total energy level (U/S)=(U/S) Water level + V22g
Step 7:
Crest level =Total energy level (U/S) – H
Step 8:
Height of crest above (U/S) bed level (hc) =Crest level – (U/S) Bed level
Step 9:
Hydraulic Head (Hh)=Crest level – (D/S) Bed level
Step 10:
=1.91 × 8 = 15.28 m
Step 11:
Length of cistern (L)=5 × Hw× He
=5 × 1× 0.612 = 3.91 m
Step 12:
Depth of cistern (x)=0.25 × (Hw× He)23
Step 13:
Cistern level =(D/S) Bed level – Depth of cistern (x)
Step 14:
(U/S) Pakka floor length=Total Length (LT) – Creep Length (Lc)
Step 15:
(D/S) Stone pitching length=10 He + 2 Hw
Step 16:
Step 17:
=8 × 0.6× 1 = 6.2 m
CANAL OUTLETS:
An outlet is a hydraulic structure convening irrigation water from a state owned channel i.e
(distributary, minor etc) to a privately owned water course.
It is basically the last hydraulic structure at the end of irrigation system.
TYPES OF OUTLETS:
A non-modular outlet is the one in which the discharge depends upon the difference in level
between the water level in the distributory channel and water course. Common examples are
orifices and wooden shoots etc.
A flexible outlet or semi-modular is the one in which the discharge is affected by the
fluctuations in the water level of the distributory channel only. Common examples are, pipe
outlet, kennedy,s gauge outlet etc.
3. Modular/Rigid Outlet:
A Rigid modular is the one which maintain s it s discharge constant, within limits,
irrespective of the fluctuations in the water level of the distributor channel and or field
channel. Common examples are Gibbs Rigid Module, Khanna Module, Ghafoors module.
CHARACTERISTICS OF AN OUTLET:
The design of an outlet depends upon its performance while the performance depends upon
the characteristics of outlet. Following are the characteristics of an outlet,
It is the ration of the rate of change of discharge of outlet (dq /q) to the rate of change of
discharge in the parent channel i.e. distributory (dQ /Q).
70 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
Mathematically,
It is the ratio of change of discharge in the outlet to the rate of change of water level in the
parent channel.
Mathematically,
The necessary minimum difference of water level or pressure between supply and delivery
sides to enabler a module or semi-module to work as designed.
I t is the minimum head required for normal functioning of outlet. It is taken as 10 -20 % of
water head in parent channel i.e.
Hmin = (10-20%) G
It is vital that outlets draw their fair share of silt. This avoids silting or scouring and
consequently remodeling of the distributory.
In a distributory system absorption 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
distributory to enable them to draw their proportional share .It depends on two factors,
1. Transition curve
2. Setting back
There is a tendency on the part of the cultivators to draw more than their lawful share of
water by tempering with the outlets. Therefore the outlets must be tamper proof. Most of the
semi-modules depending upon the formation of hydraulic jump are quite tamper proof.
6. Adjustability:
Readjustments of outlets are required in view of the revision of areas under command and
because of the changed conditions in distributory.
7. Coefficient of discharge:
In order to use the outlet as measuring device the coefficient of discharge should remain
constant in the full modular range.
8. Modular limits:
Modular limits are the limits beyond which an outlet is incapable of acting as a module or
semi-module.
9. Modular Range:
It is the range of various factors between the modular limits within which a module or semi
module works as designed.
It is the ratio between the depths of water level over crest on the downstream and upstream of
the module .i.e. Water level above crest level downstream / Water level above crest
upstream/
11. Efficiency:
It is the ratio of the head acting on the outlet to the full supply depth of the distributory
channel, where the head acting on the outlet is equal to the crest level of module below the
full supply of the distributory channel.
Mathematically,
Setting =HD
When the discharge and the water levels are likely to change the following points must be
noted in selecting the type of outlet to be use,
3. For channel running with full supply for a certain period and remaining closed for a certain
other period (rotational running) it is desirable to have a high flexibility, i.e. hyper-
proportional .
4. The silt drawing capacity of outlet must be 110-115 % assuming a 10-15 % loss in the
parent channel. According to Sharma it’s obtained in following cases,
(i) OSM with a setting of 0.9 D.
(ii) OSM with sharma’s approach at setting of 0.8 D.
(iii) Crump’s or standard design open flume outlet set at bed level.
5. In general rigid module is desirable in the following cases,
(i) Direct outlets on a branch canal subject to variation in supply.
(ii) Above stop dams where heading up is created to feed other canals.
(iii) In channels which sometimes carry extra discharge for specific reasons like leaching.
6. For other cases than those mentioned above semi module outlets are desirable.
In the choice of selecting the type of outlet the available working head is an important
criterion.
DESIGN OF OUTLET:
Step1: Flexibility
L=0.25G
Step8: Efficiency
Solution:
Step1: Flexibility
Hmin=0.2G
Hmin=0.2×3.15=0.63 ft<1 ft O.K
The value of Hmin must be less than available working head otherwise revise the value of G.
L=2.5G
L=2.5×3.15=7.875 ft
Step8: Efficiency
Efficiency,η=G-HminG×100
=3.15-13.15×100
=68.25%
BACK GROUND:
The barrages, head works and allied structures, main canals, branches, distributaries and
minor channels are being maintained by the Provincial Irrigation Departments. The
responsibility of construction and maintenance of the water courses rests with the On Farm
Management Directorate and the water users. Mutual disputes regarding use of canal water
among the users of water course are resolved by the Department under the provisions of the
Canal and Drainage Act 1873.
PRESENT SITUATION:
The last type can be termed as PIM. This concept of PIM is quite different from
“Privatization” in that we are talking about transferring management not to a third party
“Owner” who would purchase the irrigation system from the government and then hire out
irrigation services to farmers. Rather, the PIM concept is similar to an employee owned
business that gives equal share.
For participation to work, the government, the major “stakeholder” in national irrigation
sector, must be willing and interested. At least three sector of the government must be willing
to support PIM:
1. Political Leadership
2. Administrative Leadership
3. Irrigation agency Leadership
The legal framework for the establishment of WUAs, and for enabling operate and maintain
such parts of irrigation system, consist basically sets of legal instruments, namely:
For PIM to be established as a legal entity there has to be a law authorizing its establishment.
Whether established under a separate law or under an umbrella enabling law, the authority
would normally be required to prepare and agree on its bylaws before it can be registered as a
legal entity.
The transfer agreement between the authority and the irrigation agency in which the irrigation
agency agrees to transfer the responsibilities for operation and maintenance of certain parts of
the irrigation system, including the drainage system and the collection and remitting of water
charges and the WUA to carry out such responsibilities.
CONCLUSION
There are certain essentials and pre-requisites for the successful implementation of PIM that
include the following.
• Keep the users intimately involved at all levels and give them ample/real
opportunities to agree or disagree and to suggest amendments.
• Try to build on the existing community structures. Ensure that the institutional
changes are compatible with the local setting and implementable in the context of
socio-political environment and management capabilities of the farming community.
• Develop a well defined implementation mechanism along with enabling legislation,
appropriate bylaws and an effective regulatory framework.
• Training of the WUAs/WUFs and the agency staff in order to enable them to
appreciate their new roles and to efficiently perform the designated functions.
• Setting up of appropriate financial controls and accountability mechanism is
particularly relevant in the backdrop of low literacy rates, back of FOs experience and
concerns of financial mismanagement related to the local bodies and cooperative in
the past.
• The pilot projects must be designed cautiously. This pilot design should ensure
representativeness, replicability and sustainability. An independent monitoring and
evaluation mechanism needs to be set up in order to learn from the implementation of
pilot projects and to make necessary adjustments in the overall plans.
• To perform all functions and duties of the Irrigation Wing of I & P Department;
• To plan, design, construct, operate and maintain the irrigation, drainage and the flood
control infrastructure located within its territorial jurisdiction;
• To improve effective and efficient utilization of irrigation water and its disposal;
• To introduce the concept of participatory management through the pilot AWB and
FOs; Also to adopt and implement policies aimed at promoting formation, growth and
development of FOs and monitoring of its performance;
70 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
WARABANDI
MEANING OF WARABANDI:
The warabandi system in Pakistan includes the following functions and characteristics,
among other things,
1. The main canal distributing points operate at supply levels that would allow distributory
canals to operate at no less than 75 percent of full supply level.
2. Only authorized outlets draw their allotted share of water from the distributary at the same
time
3. Outlets are ungated and deliver a flow of water proportional to the area command.
Today, two types of warabandi are frequently mentioned in Pakistan. The warabandi which
has been decided by the farmers solely on their mutual agreement, without formal
involvement of any government agency is known as kachcha ( ordinary or unregulated)
warabandi , whereas , the warabandi decided after field investigation and public inquiry by
the irrigation department when disputes occurred and issued in officially recognized
warabandi schedules, is called pucca warabandi.
OBJECTIVES OF WARABANDI:
Water use efficiency is to be achieved through the imposition of water scarcity on each and
every user. And equity in distributing through enforces equal share of scarce water per unit
area among all users.
Theoretically in calculating the duration of warabandi turn given to a particular farm plot,
some allowance is added to compensate for the time taken by the flow to fill that part of the
water course leading to the farm plot. This is called water course khal bharai (filling time).
Similarly, in some cases, a farm plot may continue to receive water from a filled portion of
the watercourse even when it is blocked upstream to divert water to another farm or another
part of the water course command. This is called nikal (draining time), and is deducted from
the turn duration of that farm plot.
The calculation for a warabandi schedule starts with determining by observation, the total of
such filling times (TF) and the total of such draining times (TD). Then, for a weekly
warabandi rotation, the unit irrigation time (TU) in hours per hectare can be given by:
TU = (168 – TF + TD)C
69 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
Where,
C = Culturable command area of the watercourse
The value of TU should be the same for all the farmers in the watercourse. A farmer’s
warabandi turn time Tt is given by:
Tt = TU×A + Tf – Td
Where,
A= farm area
Tf = filling time for the farm area
Td= draining time for the farm area
Only some of the farms in a watercourse may be entitled to either filling time or draining
time or both.
Following are the methods of irrigation that are applied in the field,
In this method water is spread or flooded on a rather smooth flat land without much control
or prior preparation. It is of two types controlled flood irrigation in which water is controlled
and uncontrolled in which water is not controlled. This method is practiced largely where
irrigation water is abundant and inexpensive.
In this method only one half to one fifth of the surface is wetted and thus evaporation losses
are less. A furrow consists of a narrow ditch between rows of plants. The lengths of furrows
vary from 3.0 m for gardens up till 500 m for field crops. The general slopes provided for
furrows may vary from 0.2 to 5 %.
In this method the water is applied to the root zone of the crops by
underground network of pipes. The perforated pipes allow the water
to drip out slowly and thus the soil below the root zone of the crops
absorb water continuously. This method is suitable for permeable
soil like sandy soil. It is also known as trickle method of irrigation.
3. SPRINKLER METHOD:
In this method the water is applied to the land in the form of spray like rain. The greatest
advantage of sprinkler irrigation is its adaptabilities to use under conditions where surface
irrigation methods are not efficient.
The following are different kinds of sprinklers,
In this type the lateral pipes are perforated along the top and sides. The
water comes out through the perforations in all directions in the form
of spray. The lateral pipes are supported on pillars.
In this type a series of nozzles are fixed along the lateral pipes. The
lateral pipes are supported on pillars. When the water is forced under
pressure through the network of pipes, it comes out as fountain
through the nozzles and spreads over the land.
In this type the riser pipes are fixed on the lateral pipes at regular
intervals. On the top of the riser pipe are two arms which can rotate
about a vertical axis. When the water is forced under pressure through
the pipes it rises up and comes through the nozzles in the form of
spray. As the arms rotate a circular area is covered by each riser.
INTRODUCTION:
WATER LOGGING:
It is the presence of water in the zone of roots of plants up to 5 ft depth by raising the water
table due to which growth of crops stop. Its best example in Southern Punjab the land in the
surrounding of Indus River at Ghazi Ghat which is nearly up to 0.5 km from the banks of the
Indus River.
Following are the main reasons of losses in irrigation due to water logging.
• Over irrigation
• Seepage from canals
• Inadequate surface drainage
• Obstruction in natural water course
• Obstruction in sub-soil drainage
• Nature of soil
• Incorrect method of cultivation
• Seepage from reservoir
• Poor irrigation management
69 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
1. Over Irrigation:
It means more water is provided then required due to which proper growth of crop is not
possible.
Through seepage from canal on the banks, water level came to rise and the land becomes not
suitable for irrigation purpose.
It means two types of canals, one of which is a “lined canal”. It will not affect the land in
surrounding. On the other hand “an unlined canal” will fully affect the land in surrounding in
the form of seepage due to which this effected land provides fewer crops.
If obstruction in natural water course (rivers) is present, blockage condition of rain water or
other water created are produced and water store on the surface in the form of pounds. It
affects the human health along with cultivation of crops on this land.
Sometime in sub-soil, a hard impervious layer acts as an obstacle due to which water table
rises up to roots of the crops. As a result water logging conditions are generated.
6. Nature of Soil:
A pervious soil provides more water to the crops due to which water logging conditions are
generated. On the other hand impervious soil strata provide more friction in the way of water
flow due to less openings or voids.
It means a farmer must have full knowledge about the crop which is to be cultivated. As a
result of which he irrigates his crop according to the requirement. On the other hand a non-
expert farmer provide more water or not according to the requirement which results in water
logging.
Due to storage of water with high head seepage conditions are created not usually in hilly
areas.
It means management of irrigation supplies a large quantity of water for irrigation to a small
area.
It is another reason of water logging. In area which is fully flat and has no proper drainage
system for excessive rain fall, water starts to store on the surface. As a result water logging
conditions are generated.
Some areas have very less slope (flat) due to which flowing speed of water becomes very
slow. As a result a large quantity of water penetrates into soil and water table rises which
creates water logging conditions.
Topography of land also affects the water table. In hilly areas conditions of water table are
different as compared to the planes areas.
PREVENTION OF WATER LOGGING:
Following are the methods through which we can prevent our lands from water logging.
• Canal Closures
• Lowering Full Supply
• Lining of Canal & Water Courses
• Provision of Intercepting Drainage
• Provision of Surface Drainage
• Pumping
• Plantation
• Restricted Irrigation
• Crop Rotation
• Methods of Irrigation with Less Water
1. Canal Closures:
Water logging can be controlled by stopping supply of water for certain day for irrigation
purpose from the canals.
By lowering of water level from full supply to reasonable head in canals, seepage can be
controlled.
By providing concrete or brick lining, “water logging due to seepage” can also be controlled.
By providing intercepting drains on the banks parallel to canal, water logging due to seepage
can be controlled.
Surface drains are provided in open flat areas to drain off extra irrigation water and rain water
into stream or nala.
6. Pumping:
By pumping we can also lower the water table with pumping pipe at a certain distance from
water logged area.
7. Plantation:
By providing trees and other plants on the banks of the canal or river a large quantity of
seepage water can be controlled. This results in growth of crops in surrounding area.
8. Restricted Irrigation:
Due to large supply or more then required supply of water to the area to be irrigated, water
logging conditions are created.
9. Crop Rotation:
In different seasons, different types of crops should be planted to control the water logging.
To prevent the land from water logging, those methods of irrigation should be adopted in
which less water is used for proper growth of crops. For this purpose Drip Irrigation &
Sprinkling Irrigation method are used.
SALINITY:
Storage of salts in the root zone and on the earth surface is known as Salinity. In simple
words it is called as “Salt Efflorescence”. These salts are NaCl, Na2 SO4, Na2 CO3. NaCl
and Na2 SO4 are called White Alkalis. These shows white color efflorescence. On the other
hand Na2 CO3 is called Black Alkalis and very dangerous for irrigation land.
REASONS OF SALINITY:
Due to different reasons the level of underground water rises and different salts present in sub
soil resolve in that water. These salts flow with water and come in contact with roots of plants
and surface, results in salinity.
Salts come to the top surface when soil is wet. When the surface water evaporates or dries a
salt layer is generated on the surface which is called salinity.
If water is provided more than required to an irrigated land, it can also be resulted into
salinity.
If water is supplied to an irrigated land which has salt particles in it, it will result into salinity.
5. Faulty Condition:
Sometimes more and sometimes less water is provided to a crop by non-expert farmers. As a
result salts of soil mix with water and cause salinity.
6. Low Rainfall:
By normal rain fall salt particles present on the surface, are mixed with rain water and
gradually move downward with rain water. Whereas in case of low rainfall small amount of
salt particles move downward with the rain water. This results in salinity of irrigated land.
PREVENTION OF SALINITY:
Prevention of salinity of an irrigated land is done by reclamation of saline soil. This is done
by the following two methods.
• Temporary Method
• Permanent Method
These two methods are briefly discussed below.
1. TEMPORARY METHOD:
In temporary method following four methods are adopted for the prevention of salinity.
• Ploughing Deeply
• Removing Surface
• Mixing of Good Soil
• Neutralizing the Salts
i. Ploughing Deeply:
In this process 30 cm to 50 cm deep ploughing is done. By using this method salts move to
the 50 cm deep layer and do not disturb the growth of crops.
By mixing good irrigated soil with the saline soil, effect of salts can also be minimized on the
soil. For this purpose sandy clay (silt) is the most suitable.
iv. Neutralizing the Salts:
To remove the salinity effects on the soil, we can also use different types of acids and other
chemicals. These chemicals are sprayed on soil surface to control salinity.
1. PERMANENT METHOD:
When water first comes in contact with surface soil, it causes water logging and salinity. To
lower water table, network of drains is made by filter pipes. When surface water comes to
these drains, this is exposed off to stream without affecting the surface layer.
ii. Leaching:
To remove salinity, those types of crops should be cultivated which remove saline from soil.
Rice Crop is the most suitable for this purpose. It decreases the quantity of Nitrogen from
soil.
By growing special plants salinity can be prevented. For this purpose Australian Grass and
Argemona etc. type of plants are cultivated. These special plants produce acidity due to
which PH value of that land is decreased.
v. Use of Chemicals:
Sulphuric Acid, Gypsum and Calcium Chloride can be used to remove salinity.
By using coal PH value of cultivated land can also be decreased. Its quantity is used
according to salinity effects.
It is a very costly method for a country like Pakistan. In this process electricity is passed
through effected land to control salinity.
Some special plants which produce green fertilizer are cultivated on the effected land to
remove the salinity of that land. For this purpose Patsun and Berseen etc. plants are used.
Waste solution from sugar mill, Lime solution and Bork of Peanut etc. are also used to
decrease the PH value of land.
x. By Earth Worms:
Some types of insects used salts as their food. So by this method we can also control the
salinity.
The irrigation canal prepared by using different materials of construction i.e. bricks, P.C.C or
R.C.C, is called lined irrigation canal.
Lining is indispensable when passing through the porous or sandy tracks. However in
Pakistan majority of the canals are unlined whether or not they pass through the sandy soils.
They were built during the last few decades and resulting a rise of ground water table thus
creating water logging and salinity problems. Recently the irrigation canals are built with
lining. Both types of canal are designed for uniform steady flow.
The design of unlined canal which will remain stable is an important challenge for the
hydraulic & irrigation engineer. The solution of this problem consists in determining the
depth, bed width, side slope and longitudinal slope of the channel so as to produce a non-
silting and non-scouring velocity for the given discharge and sediment load.
Design of an irrigation canal implies a section which is stable that neither silts nor scour with
the given dischargeQ, water surface slope (S) and silt charge. When a channel is stable, it
means the flow in the channel is uniform steady flow.
Stable Channel:
When an artificial channel is used to carry the silty water, both the bed and banks scour or fill
by changing the depth, gradient and width of the channel until a state of balance is attained at
which channel is said to be stable channel. Stable channel is also called as non-silting & non-
scouring channel.
70 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
A velocity which will keep the silt in suspension without scouring the channel is called
critical velocity. It is a standard velocity. It is also called as non-silting & non-scouring
velocity. It is denoted by Vo.
m=VoV
The value of ‘m’ ranges b/w(0.9-1.1). In irrigation canal system the value of ‘m’ decreases
from head to tail.
It is the distance from the near/relative headwork and known as reduced distance.
1 R.D=1000 ft &
In case of canal one mile is equal to 5000 feet. Reduced distance is taken along the canal.
It is the bed slope of the channel due to which water flows under gravity. For most of the
channels in Punjab, the longitudinal bed slope is taken as 1 ft/mile. Mathematically,
There are two main theories being developed for the designing of unlined irrigation canal.
R.G. Kennedy (Exective Engineer Punjab irrigation of the Upper Bari Doab Canal)
published in 1895. His main conclusions from the observations of the 20 sites, in the middle
reach of the UBDC which was built in 1850 and had remained stable, were as follows
The eddies, produced from the bed, support the silt in suspension and therefore the silt
supporting power of the stream is proportional to the bed width but not the perimeter of the
channel.
The value of ‘m’ usually ranges b/w(0.9 to 1.1). If it is not mentioned, then for Kennedy’s
theory
m=1
& D=depth of channel
if Vact > Vo scouring will occur
Step1:
Assume depth
Step2:
Calculate critical velocity
Vo=0.84mD0.64 In F.P.S
Vo=0.55mD0.64 In S.I
Step3:
Calculate area using equation of continuity i.e. Q=AV
Step4:
Calculate bed width from the area.
Step5:
Calculate hydraulic radius.
R=AP
Step6:
Calculate Chezy’s coefficient using kutter’s equation.
C=23+0.00155S+1n1+23+0.00155S×nR In S.I
C=41.6+0.00281S+1.811n1+41.6+0.00281S×nR In F.P.S
Where C is Kutter’s C and in this case it is known as “Flow Resistance
Factor” or “ Chezy’s coefficient”
Step7:
70 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
Step8:
Now find out the critical velocity ratio, i.e. “m”
m=VVo
If, m=(0.9-1.1) then design is O.K otherwise revise the section assuming
different depth.
SOLUTION:
Step1:
Assume depth
D=2 ft
Step2:
Calculate critical velocity
Vo=0.84mD0.64
=0.84×1×20.64
=1.31 ft/sec
Step3:
Calculate area using equation of continuity i.e. Q=AV
A=501.31
=38.168 ft2
Step4:
Calculate bed width from the area.
Area, A=B+DD
38.168=B+2×2
B=17.084 ft
Step5:
Calculate hydraulic radius.
R=AP
Wetted perimeter of cross section,P=17.084+22.828=22.74 ft
R=38.16822.74
=1.678 ft
Step6:
69 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
C=41.6+0.00281S+1.811n1+41.6+0.00281S×nR In F.P.S
C=41.6+0.0028115000+1.8110.021+41.6+0.0028115000×0.021.678
C=78.634
Step7:
Now calculate Actual velocity of flow using Chezy’s equation, i.e.
V=CRS
V=78.634×1.678×15000
V=1.44 ft/sec
Step8:
Now find out the critical velocity ratio, i.e. “m”
m=VVo
m=1.441.31
m=1.09 O.K
If, m=(0.9-1.1) then design is O.K otherwise revise the section assuming
different depth.
SOLUTION:
Step1:
Assume depth
D=2 m
Step2:
Calculate critical velocity
Vo=0.55mD0.64
=0.55×1×20.64
=0.8571 m/sec
69 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
Step3:
Calculate area using equation of continuity i.e. Q=AV
A=350.8571
=40.8354 m2
Step4:
Calculate bed width from the area.
Area, A=B+DD
40.8354=B+2×2
B=18.418 m
Step5:
Calculate hydraulic radius.
R=AP
Wetted perimeter of cross section,P=18.418+22.828=24.074 m
R=40.835424.074=1.7 m
Step6:
Calculate Chezy’s coefficient using kutter’s equation.
C=23+0.00155S+1n1+23+0.00155S×nR In S.I
C=23+0.0015515000+10.0251+23+0.0015515000×0.0251.7
C=44.508
Step7:
Now calculate Actual velocity of flow using Chezy’s equation, i.e.
V=CRS
V=44.5081.7×15000
V=0.8207 m/sec
Step8:
Now find out the critical velocity ratio, i.e. “m”
m=VVo
m=0.82070.8571
m=0.9575 O.K
If, m=(0.9-1.1) then design is O.K otherwise revise the section assuming
different depth.
In 1929 Lacey put forward his theory. He made a systematic study of the observed data and
derived some empirical relations. Then he gave a concept of a regime theory for unlined
channels.
He proposed the following conditions for the zero net erosion or deposition over a
hydrological cycle i.e.
1. Dimensions such as width, depth and slope of a regime channel to carry a given
discharge loaded with given silt charge are all fixed by nature.
2. He also stated that silt is kept in suspension due to the force of vertical eddies.
According to him eddies are generated from bed and sides, both normal to the surface
of generation. Hence the vertical component of eddies generated from sides will also
support the silt.
Regime Channel:
Lacey defined the regime channel as a stable channel transporting a regime silt charge. A
channel will be regime if it flows in incoherent unlimited alluvium of the same character as
that of transported and the silt grade as well as silt charge are constant.
1. The channel is flowing in incoherent unlimited alluvium of the same character as that
of transported.
2. Silt grade as well as silt charge are constant.
3. Discharge is constant.
If all the conditions are fulfilled then the channel is said to be the “true regime”. However it
is very rare that all the above three conditions are realized in the field. So Lacey gave the idea
of initial and final regime for actual channels.
Initial Regime:
It is the state of the channel which has formed its section only and yet not secured the
longitudinal slope. Lacey silt theory is not applicable to the channels in initial regime.
Final Regime:
The channel after attaining its section as well as its longitudinal slope is said to be in final
regime. Unlined channels will be either in initial regime or in final regime.
Permanent Regime:
A channel is said to be in permanent regime when its section is provided with a lining to
protect against scouring of the section thereby imparting the permanency to the channel
section and longitudinal slope. Lacey regime theory is not applicable to such channels. This
case can only exit in lined channels.
Incoherent Alluvium:
It is the soil composed of loose granular graded material which can be scoured with the same
ease with which it is deposited.
This indicates the gradation between the small and big particles and should not be taken to
mean the average mean diameter of a particle.
Lacey’s worked on the channels which were at the final regime conditions. He gave the
following formulae to determine the regime section and the bed slope of channels.
f=1.76m
Where,
m=Average particlesilt sizediameterin mm
Or Following Relation between f and Critical velocity ratio can be used,
f=VVo2
Step2:Velocity Of Flow
V=Qf214016
A=QV
SOLUTION:
B=28.1-2.828D (5)
28.1-2.828D+D×D=42.68
28.1D-1.828D2=42.68
1.828D2- 28.1D+42.68=0
D=13.663 m , 1.7088 m
for D=13.663 m
This value is same as in step 4 so it is O.K. so the required channel section is,
Example 2:
Design an irrigation channel by the following data using Lacey’s regime theory.
Solution:
Step 1:
f=0.9
70 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
Step 2:
V= Qf214016 = 43 ×(0.9)214016=0.8 ms
Step 3:
A=QV=430.8=53.75 m2
Step 4:
Step 5:
Step 6:
P=B+ 5 D ii
As we have the values of Area (A) and wetted perimeter (P) so we can write eq i& iias
53.75 =0.5D2+ BD iv
31.15=B+ 5 D v
From eq v
B=31.15- 5 D vi
D=1.934 m , 16.01 m
for D=1.934 m
B=31.15- 5 ×1.934=26.82 m
for D=16.01 m
This value is almost equal to the value as it was in step 4 so this value is O.K. now we will
find out the value of bed slope as
Example 3:
Design an irrigation channel by the following data using Lacey’s regime theory.
Solution:
Step 1:
Step 2:
V= Qf214016 = 15 ×(1.011)214016=0.6917 ms
Step 3:
A=QV=150.6917=21.6857 m2
Step 4:
Step 5:
Step 6:
P=B+ 5 D ii
18.3976=B+ 5 D iv
B=18.3967- 5 D v
D=1.3568 m , 9.2397 m
for D=1.3568 m
B=18.3967- 5 ×1.3568=15.3628 m
for D=9.2397 m
This value is equal to the value as it was in step 4 so this value is O.K. now we will find out
the value of bed slope as
INTRODUCTION:
Water entering from river to canal has a high silt charge in it. This silt charge makes the canal
shallow and decreases its capacity. So canal cannot draw its authorized discharge from river.
High silt charge in irrigation channels result in deterioration of smooth lining surface and
power generation equipment. The success of irrigation projects depend upon the control of
silt entering the channels from reservoir or river.
SEDIMENT TRANSPORT:
If the channel is not properly designed then the erosion will take place from the bed or silt
will start to deposit i.e. silting & scouring.
• River approach
• Orientation of head works
69 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
1. River approach:
• Straight channel
• Meandering channel
• Bramdiel channels
Proper river approach conditions play a very vital role to control the sediment entry into the
canal. Straight river approach channel is preferable where it is not available. The natural river
curvature can be used as a silt exclusion device by locating the barrage on the bend and the
channel regulator on the outer edge of the curve. Water with heavy concentration of silt,
flows along with the inside edge of the curve.
A proper silt distribution b/w the main channel and the off-take is achieved by selecting a
suitable angle of diversion given to the off-take. Silting of the off-take is caused by the fact
that lower layer of water in main stream have a lower velocity and lower momentum.
Therefore this layer can be deflected more easily into the off-take as compared to the upper
layer that has greater velocity and momentum. So a greater force is required to turn the upper
layer towards off-take. Experiences show that it is better to provide the canal head regulator
at 90°-110° to the river channel for better silt control.
The best design guide for fixing the angle of diversion depends upon the particular situations
such as,
• Discharge ratio
• Sediment charge into channel
• Position of off-take or canal head regulator
The conditions to control silt become better by increasing the crest level of head regulator.
The crest of the head regulator is recommended to the 1/3 depth of water in river.
2. Divide wall:
It is the wall which separates the under sluice from the main weir. Length of divide wall, for
one off-taking canal, is usually one half or two third the length of head regulator. In case of
two off-taking canals its length is up to the end of head regulator. The optimum width of the
pocket (space b/w divide wall, under sluice and canal head regulator) can be determined by
69 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
model studies. It is effective to control the silt entry into the off-take. Long divide wall does
not act as a trap for coarse bed material rather lessens the effect of regulation on curvature of
flow.
Control of silt into the off-take is secured by a suitable approach to the canal pocket through
proper shape and alignment of guide banks. Bottleneck and converging guide banks do not
provide effective silt control as large islands are formed up-stream of the pocket. If
convergence is provided then it should not more than 1/10.
4. Canal closure:
During very high flood period canals are closed. Because flood water contains vary high
concentration of silt. So to dispose off this water, gates of under sluice are opened. It helps to
wash out the sediments deposited in pocket.
Better silt exclusion in still pond system secured by opening the barrage gates adjacent to the
divide wall more than those far away. Opening of the gates is minimum at the center,
increasing gradually towards divide wall, in case canal takes off from banks of the barrage.
6. Submerged vanes:
A submerged curved vane of one third the depth of water, pointing d/s and projecting from a
bank, produce a local concave curvature and helps deflect bed material away from the bank
into the mid stream. It entails very little obstruction to the natural water way of river. Its
demerits are,
• Vanes are not effective in case a deep channel is not formed or maintained along or
near a bank at high river discharges and through out the flood season.
• A single vane on one bank is liable to draw too large a share of top water to feed the
canal and may possibly pull the whole rivers towards it necessitating counter
measures to adverse the conditions thus created.
• Unless a vane is constructed sufficiently u/s, bed sediments thrown up in suspension
may find its way into the canal.
• A vane designed foe low discharge may not be effective during higher or intermediate
discharges.
• Accretion in the river may render the vane partially or completely ineffective.
1. Silt excluder:
SILT EXCLUDERS:
70 Muhammad Farooq Zia (2005-CE-
Compiled
44)
By:
Muhammad Sajid Nazir (2005-CE-
WATER RESOURCES AND IRRIGATION ENGINEERING
CED-401
Silt Excluder is a device by which silt is excluded from water entering the canal. It is
constructed in the bed in front of head regulator.
The fundamental principle, on which a silt control device acts, lies in the fact that a flowing
stream carrying silt in suspension. Concentration of silt particles is greater in lower layer as
compared to upper layer. Hence device is designed to separate upper and lower layer without
any disturbance. The top water is then led towards the canal while the bottom water
containing high silt charge is wasted. This is achieved by silt excluder. Rivers and streams
carry a large amount of silt every year and if it is not controlled then it reduces the capacity of
the hydraulic structures.
The silt excluder consists of a number of under tunnels resting on the floor of the pocket. The
top level of the R.C roof of the tunnels is kept the same as the sill level of the head regulator.
The various tunnels are made of different lengths i.e. the one near to head regulator is of
length as the width of head regulator. The length of the successive tunnels is decreased
towards divide wall as shown in figure. This arrangement separates the water into two clear
layers i.e.
The top layer, above the roof of the under tunnel, enters the head regulator.
The bottom layer containing relatively heavier silt charge which goes to under tunnels and it
is discharged to the d/s of the river through under sluices. The capacity of under tunnel is kept
above 10% of the canal discharge and it is designed so that a minimum velocity of (2-3)
m/sec is maintained. Knowing discharge and discharge velocity the total water way required
for the under tunnels can be determined.
The following points should be kept in mind while designing a silt excluder.
1. The tunnel discharge through under sluice is kept more then 10% and usually 20% of
the canal discharge.
2. The silt excluder should cover only two bays of under sluice as it gives more efficient
results.
3. The approach canal needs not to be lined.
4. The divide wall should be (1.2 – 1.4) times length of head regulator.
5. The top of the slab of silt excluder should be flushed with the head regulator crest. i.e.
the clear height of the tunnels would be = 13 of depth of the water – slab thickness
6. The roof slab should be designed to carry a full water load in case the tunnels are
empty.
7. The first tunnel should completely cover the head regulator length. While other
tunnels should be of shorter lengths.
8. The discharge through tunnels will depend upon the head measured above the center
line of the tunnel.
Tunnels can be treated as culverts, for which the discharge formula is,
Q=CA 2gH
C = coefficient of discharge
A = tunnel area
H = head of water
E= If- IvIf
This indicates the reduction of silt intensity in the canal water as compared with that of
approach canal.
Where,
E = efficiency
If = silt intensity in the approach canal in ppm
Iv = silt intensity in the canal.