WATER RESOURCES AND IRRIGATION ENGINEERING

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					                   WATER RESOURCES AND IRRIGATION ENGINEERING
                                            CED-401



Irrigation:

The process of artificial application of water to the soil for the growth of agricultural crops is
known as irrigation.

Introduction about Irrigation:

It is practically a science of planning and designing of the water supply system for the
agricultural land to protect the crops from bad effects of drought or low rainfall. It includes
the construction of weirs or dams, barrages or canal system for the regular supply of water to
the cultivable lands.

There are three essential requirements of plant growth,
1. Heat
2. Light
3. Moisture

In England the one of three essential requirements is available that is moisture, which means
irrigation is not required due to sufficient rainfall. But in Pakistan the first two of the three
essential requirements of plant growth that is light and heat is present in large amount but the
third (moisture) is required due to insufficient rainfall . Hence Irrigation is supplementary to
rainfall, when the rainfall is either deficient or comes irregularly or at unseasonable times as
in Pakistan.


Components of the Irrigation System or Irrigation System Network:

The main components of an irrigation system are listed below,
1. Dam /Barrage
2. Canal Head regulator
3. Main canal
4. Branch Canal
5. Distributory canal
6. Minor canal
7. Water course (W.C)


The layout of irrigation network is shown on next page.




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          LAYOUT OF IRRIGATION NETWORK




                  DAM



       RIVER                                BARRAGE


                         MAIN CANAL

                                                                         IRRIGATION
                                                                           SYSTEM
                                      BRANCH CANAL
                                            DISTRIBUTORY



                         W.C
                                                                 MINOR




                                                           W.C


                 FIELD                                                   OUTLET



                                                                 W.C
                                                                          OFWM




    OFWM = ON FARM WATER MANAGEMENT

    W.C = WATER COURSE




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Phases of Irrigation Engineering:

There are four stages of Irrigation Engineering,
   1.           Storage (Dams, Reservoirs) or Diversion (Barrages)
   2.           Conveyance of irrigation water (Canals)
   3.           Distribution (water courses ) and Application (Irrigation methods ) of
       irrigation
   4.           Drainage of excessive water through drains


Importance of Irrigation:

Question: Why irrigation is required?

Answer: We know the water requirements for crops vary from place to place depending upon
the nature of crops and site. In some areas there is no need of irrigation because the
conditions are fulfilled by the natural resources (rainfall etc).

Most of the areas of the earth are situated in arid zones (less than 15'' of mean annual rainfall)
and even in humid zones (greater than 30'' of mean annual rainfall). The rainfall is not
distributed evenly. So it is only possible to use artificial means (supplying channel) to
provide water for more cultivation.

In Pakistan the annual rainfall ranges from 75 mm to 800 mm. So irrigation becomes a
necessity to provide,

   (i)         Sufficient amount of water in desirable seasons
   (ii)        Period of requirement ( Time when water is required)
   (iii)       Desirable amount of water for sufficient type of crops

Thus the necessity of irrigation can be summarized in the following four points,
   1.          Less rainfall (Need fulfill through artificial supply )
   2.          Non-uniform rainfall (through dam , requirement is fulfilled at the need time
       crop)
   3.          Commercial crops with additional water ( more water is required for cash
       crops e.g. Sugar cane , tobacco, rice )
   4.          Controlled water supply (By the construction of proper distribution system the
       yield of crop may be increased)



Necessity of Irrigation:

Question: What are the factors which govern the necessity of irrigation?

Answer: Throughout the crop period adequate quantities of water is required near the root
zone of the plants for their growth. At times during the crop period the rainfall may not be
adequate to fulfill the water requirement. The intensity of rainfall is practically uncertain and
beyond the control of human power and it may not be well distributed throughout the crop



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season or the culturable area. So, irrigation becomes absolutely necessary to fulfill the water
requirement of crops. The following are the factors which govern the necessity of irrigation,

(a)Insufficient Rainfall:

When the seasonal rainfall is less than the minimum requirement for the satisfactory growth
of crops, the irrigation system is essential.

(b)Uneven Distribution of Rainfall:

When the rainfall is not evenly distributed during the crop period or throughout thre culture
able area the irrigation is extremely necessary.

(c)Improvement of Perennial Crops:

Some crops like sugarcane, cotton, etc require water throughout the major part of the year.
But the rainfall may fulfill the water requirement in rainy season only. So for the remaining
part of the year, irrigation becomes necessary.

(d)Development of Agriculture in Desert Area:

In desert area where the rainfall is very scanty, irrigation is required for the development of
agriculture.



Merits and demerits of Irrigation:

The following are the benefits of Irrigation,

Merits of Irrigation

1. Yield of Crops:

In the period of low rainfall or drought the yield of crop may be increased by the irrigation
system.

2. Protection from famine:

The food production of a country can be improved by ensuring the growth of crops by
availing the irrigation facilities. This helps a country to prevent famine situation

3. Improvement in cash crops:

Irrigation helps to improve the cultivation of cash crops like vegetables, fruits, tobacco, etc.

4. Prosperity of farmers:

When the supply of irrigation water is assured, the farmers can grow two or more crops in a
year on the same land. Thus the farmers may earn money and improve their living standard.


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5. Source of revenue:

When, irrigation water is supplied to the cultivators in lieu of some taxes. it helps to earn
revenue which may be spent on other development schemes.

6. Hydroelectric Power generation:

In some river valley projects, multipurpose reservoirs are formed by constructing high dams
where hydroelectric power may be generated along with the irrigation system.

7. Water Supply:

The irrigation canals may be the source of water supply for domestic and industrial purposes.

Demerits of Irrigation

1. Rising of water table:

Due to the excessive of water through the bed and banks of the canals, the water table in the
surrounding area may be raised which may constantly saturate the root zone of the crops and
the soil may develop alkaline property which is harmful to the crops.

2. Formation of marshy land:

Excessive seepage and leakage of water from the irrigation canals may lead to formation of
marshy lands along the course of the canals. These marshy lands form the colonies of
mosquitoes which may be responsible for diseases.

3. Formation of marshy land:

The temperature of the commanded area of an irrigation project may be lowered considerably
and the area may become damp. Due to dampness, the people residing around the area may
suffer from cold, cough and other such diseases originating from dampness.

4. Loss of valuable lands:

Valuable land may get submerged when the storage reservoirs are formed by constructing
barrages or dams and it also may be lost, while constructing irrigation canals.



Resources of Irrigation:

There are three resources of irrigation,
1. Rainfall
2. Surface Water
3. Ground Water


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1. Rainfall:

Rainfall can directly help irrigation by precipitation occurring over the crop area or indirectly
by adding its runoff to the rivers. This runoff is then stored by weir, barrage or dam
downstream or it may replenish as an underground reservoir.
Direct rainfall is the most helpful for the plant and crop growth if it occurs in proper amount
at proper time .But it is unreliable as a source of irrigation water. It varies from year to year
and it may fall altogether. It is irregularly distributed throughout the year as well as within the
same season.
In Pakistan, It occurs particularly in the summer season in the form of high showers resulting
in the heavy rainfall. As the temperature is high evaporation rate is also increased. It is a great
booster for agriculture. For canal irrigated areas the rains compliment the irrigation water.
In Pakistan the mean annual rainfall ranges from 4 to 30 inches in the lower Indus region to
the northern foot hills. Only a small proportion of this annual rainfall makes any direct or
useful contribution to irrigation water supplies. According to World Bank consultants report
the figure ranges from 1 to 17 inches. The rest is either converted to Direct Runoff or
becomes a part of the ground water. While a small proportion is lost by evaporation.
According to estimation the present direct contribution to the crops is 6 MAF / Annual.

2. Surface Water:

Surface water include water diverted from the stream and stored into dams and barrages and
then applied to the land through canals or pumped from rivers, lakes and canals .
In dry months melting snow adds a great amount of water to the river discharge. Snow
remaining on ground provides storage greater than any man made reservoir for 1 foot snow
holds 1-4 inches of water. Snowfall usually occurs over many square miles on the
mountainous terrain providing a surface reservoir which is then released in the summer
months. The most important thing for irrigation engineer is when and how fast this vast
quantity is released. In Pakistan, the rivers carry the melting snow and rains from the northern
hills to the areas where they can be used for irrigation purpose.
River water available in PAKISTAN for irrigation is,

Average flow of River Indus = 90 MAF
Average flow of River Jhelum = 23 MAF
Average flow of River Chenab = 27 MAF
Average flow of River Ravi   = 3 MAF
Average flow of River Sutlej = 2 MAF

Total Surface Flow               = 145 MAF
                                 TOTAL SURFACE FLOW
                                      (145 MAF)




               CANAL DIVERSION                           WASTAGE TO SEA
                  (105 MAF)                                 (40 MAF)

                              TOTAL WATER AVAILABLE FOR FIELD CROPS
                                            (72 MAF)
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3. Ground Water:

Along with the rainfall and surface water the ground water is an important source of
irrigation. In Pakistan we have enormous ground water reservoirs. In rainy season, due to
rain, most of water seeps into the earth thus raising the water table in ground. This water is
then taken out with the help of pumps and tube wells for irrigation purpose. The areas for
which there is no access of canals, there we can get water for irrigation from underground
sources of water.
Ground water can cause water logging sometimes due to rise in water table and this can be
avoided by pumping out water from the ground using several techniques i.e. pumps, tube-
wells etc. In the underground water, there are less chances of the presence of impurities but it
does not contain silt which is helpful for crop production acting as a fertilizing agent.
In Pakistan we normally use all three sources of irrigation. But based on quality, sometimes it
may be desired to use single source of water or it may be necessary to mix the ground water
with the surface water so that the combined salts of both sources mixed in any quantity may
not cause any damage to the crops.

FLOW CHART OF RAINFALL FROM SOURCE TO FIELD:

                                       RAIN FALL


                      AVERAGE FLOW AT FORM-GATES (NAKKAS)
                                    (13 MAF)




  WATER AVAILABLE FOR CROPS                            FIELD APPLICATION LOSSES
         72% (9 MAF)                                          28% (4MAF)



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     FLOW CHART OF SURFACE WATER FROM SOURCE TO FIELD:

                                   GROUND WATER




       GOVERNMENT TUBE WELLS                                 PRIVATE TUBE WELLS
              (9MAF)                                               (35MAF)




CONVEYANCE LOSSES     AVERAGE FLOW AT           CONVEYANCE LOSSES            AVERAGE FLOW AT
   15% (1.35 MAF)         NAKKAS                   5% (1.75 MAF)                 NAKKAS
                        85% (7.65 MAF)                                        95% (33.25 MAF)




                                 FIELD APPLICATION LOSSES
                                        28% (12 MAF)

                                WATER AVAILABLE FOR CROPS
                                       72% (29 MAF)


     FLOW CHART OF GROUNDWATER FROM SOURCE TO FIELD:
                      AVERAGE ANNUAL FLOW AVAILABLE IN RIVERS
                                     (146 MAF)




    DIVERSION TO CANAL IRRIGATION SYSTEM                      FLOW TOWARDS SEA
                75% (109.5 MAF)                                 25% (36.25 MAF)




   HEAD AT WATER COURSES OUTLETS                        CONVEYANCE LOSSES
           75% (82.125 MAF)                               25% (27.375 MAF)




       FLOW TO FORM-GATES                     CONVEYANCE LOSSES
         55% (45.16875 MAF)                    45% (36.95625 MAF)




   WATER AVAILABLE FOR CROPS                         FIELD APPLICATION LOSSES
        72% (32.5215 MAF)                                28% (12.64725 MAF)




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Types Of irrigation System:

There are three types of irrigation,

(i)Gravity Flow or Surface Water Flow Irrigation
(ii)Tank or Reservoir Irrigation
(iii) Lift Irrigation

(i) Gravity Flow Irrigation:

Gravity flow irrigation is providing water where water flow is due to gravity, not under any
mechanical means. Due to gravity water flows from higher areas to the lower areas. After
which it is distributed in the fields.
Silt in the canal water has a manurel value (fertilizing agent).The whole canal irrigation in
our country is gravity irrigation. The gravity flow is cheaper and the quality of water is very
good because of the presence of silt content.

(ii) Tank irrigation:

If the runoff is more than the required amount then headwork and barrages are constructed to
store the water.
A head work consists of a weir, canal head regulator, gate structure (barrage). So, a headwork
is a complete system of structures. Whereas barrage is a part of a headwork, it is constructed
in the path of the river to obstruct water.
The flow of a river is seasonal flow. Sometimes more water is required like in December but
source is scanty and sometimes less water is required like in March but the source is high. So
in order to regulate the flow the reservoirs are constructed in order to

1. Fulfill the irrigation requirements
2. Generate the hydraulic power
3. Regulate the river flow so as to avoid flood

In some areas small dams are constructed for the irrigation purposes. As the topography of
these areas, does not allow the possibility of constructing a canal.

(iii) Lift Irrigation:

When the main source is at the lower level than the supply level then we try to supply water
by using some mechanical means, such type of irrigation is known as Lift Irrigation. This can
be done by the following methods,

(a) Lift from canals
(b)Open Wells
(c) Tube wells

(a)Lift From canals (Rivers):

Pumps are used to lift the water from canals or rivers at lower level to the area at higher level
for irrigation purpose.


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(b)Open Wells:

In villages there are some open holes whose depth intercepts the water table. So the water is
taken out from lower level to the surface for irrigation purpose by adopting different manual
and mechanical methods.

(c)Tube Wells:

It is the lifting of water by pumping from underground reservoir. Extensive surface irrigation
results in an increase in the ground water level due to percolation and seepage which causes
water logging in large areas. Irrigation by this method will reduce the yield. Tube well
irrigation offers a remedial measure by providing sub-surface drainage.
Tube well irrigation can be obtained more quickly than from surface water project. Large
capital costs involve in canal irrigation for the construction of dam, canal and headwork
system but tube well construction cost is very less.




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Flow Irrigation:

The irrigation system in which the water flows under gravity from the source to the
agricultural land/field.
It is also called canal irrigation.

Canal

It is an artificial channel constructed on the ground to carry water to the field either from a
reservoir tank or river.

Classification of Canals:

The canals are classified on the bases of,

1. Based on the nature of source of supply

(a) Perennial Canals
(b) Non-Perennial Canals
(c) Inundation Canals

                 CANAL CLASSIFICATION BASED ON THE NATURE OF SOURCE




        Non-Perennial Canals          Perennial Canal               Inundation Canals



(a) Perennial Canals

These are the canals which get continuous supplies by permanent source of supply like a river
or reservoir are called as permanent canals or perennial canals. These irrigate the field
throughout all the year with equitable rate of flow.

(b) Non-Perennial Canals

These are the canals which irrigate the field for only one part of the year usually during
summer season or at the beginning and end of winter season, called as non-perennial canals.
These canals take-off from rivers which do not have assured supply throughout the year.

(c) Inundation Canals

These are the canals in which the supply depends upon the periodical rise in the river from
where these take off. When the water level rises above the bed level of the canal the water
starts flowing through the canal. As the water level fall below the bed level of the canal. The
flow of water through the canal stops. No regulator is provided at the head of such canal. This
draws lot of quantity of silt which is really beneficial for the crops.




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2. Based on the function of canals

(a) Navigation Canals
(b) Irrigation Canals
(c) Power Canals
(d) Link Canals
(e) Feeder Canals

                    CLASSIFICATION BASED ON THE FUNCTION OF CANAL




Navigation Canals         Irrigation Canals     Power Canals      Link Canals        Feeder Canals




(a) Navigation Canals

The canal which is constructed to carry water from the source to the agriculture land for the
purpose of irrigation is known as irrigation canal. In this canal the velocity of flow is kept
high so that the water may carry silt in suspension for good command areas.
(b) Irrigation Canals

These are the canals which are used for providing transportation and voyage facilities
nationwide and internationally. Sometimes these are also used for irrigation purposes.

(c) Power Canals

The canal which is constructed to supply water with very high force to the hydroelectric
power station for the purpose of moving turbine to generate electric power is known as power
canal.

(d) Link Canals

These are the canals which are constructed to transfer water to the other conveyance structure
which contain in-sufficient quantity of water. These transfer water from river to canal system.
e.g. Sidhnai Mailsi Link Canal

(e) Feeder Canals

These are constructed to provide water to other conveyance structures. These are not used for
irrigation. These canals feed two or more canals. e.g. Lower Chenab canal Feeder

3. Based on the discharge:

(a) Main Canals
(b) Branch Canals
(c) Distributory Canals
(d) Field Canals


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                   CLASSIFICATION BASED ON THE DISCHARGE OF CANAL




    Main Canals           Branch Canals           Distributory Canals        Field Canals




(a) Main Canals

The main canal carries discharge directly from river therefore it carries large supply of water
and cannot be used for direct irrigation. In main canals the discharge is greater than 10
cumecs. The water is taken to the field through the branch canal, distributory channel and
field channel.

(b) Branch Canals

The branch canals are taken from either side of the main canal at suitable points so that whole
command area can be covered by the network. The discharge varies from 5 to 10 cumecs.

(c) Distributory Canals

These take-off from branch canals. The discharge capacity of these channels varies from 0.25
to 3 cumecs. These are divided as,

(i) Major Distributory
(ii) Minor Distributory

(i) Major Distributory

These take-off from branch canals. Sometimes they may also take-off from main canals but
their discharge is always less than the branch canals. These are real irrigation channels
because they supply water to the field directly through outlets. The capacity varied from 0.25
to 3 cumecs.

(ii) Minor Distributory

These distributaries take-off from major distributaries or sometimes from branch canals. They
also provide water to the water courses through outlets provided along with them. The
discharge capacity in this type of canals is 0.25 to 3 cumecs.

(d) Field Canals

These channels are taken from the outlets of the Distributory channels by the cultivators to
supply water to their own lands. These channels are maintained by the cultivators.




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Object of Canal Lining:

The following are the main objects of canal lining,

(a)To Control Seepage

The seepage loss is the maximum loss in unlined canals. Due to seepage the duty of canal
water is much reduced which involves enhancement of storage capacity of a reservoir by
constructing high dam. So, to control seepage loss through the bed and sides of the canal, the
lining of the canal is necessary.

(b) To Prevent Water-logging
Along the course of the canal, there may be low lying areas on one side or both sides of the
canal. Due to the seepage of water through the sides of the canal, these areas may get
converted into marshy lands. This water logging makes the land alkaline which is unsuitable
for agriculture. This water logging area may become the breeding place of mosquitoes which
are responsible for many infectious diseases.

(c) To Increase the Capacity Of Canal

In unlined canal, the velocity of flow should be fixed such that the silting and scouring is
avoided. In practice, the velocity should always be kept below 1 m/s. Due to this low
velocity, the discharge capacity of the canal becomes low. In unlined canal, if the capacity of
the canal is to be increased the cross sectional area has to be increased which involves more
land width. So, the lining of the canal should be such that the velocity and the discharge of
the canal are more with minimum cross-sectional area.

(d) To Increase the Command Area

If the lining is provided in the canals the various losses can be controlled and ultimately the
command area of the project may be enhanced.

(e) To Protect the Canal From The Damage By Flood

The unlined canals may be severely damaged by scouring and erosion caused due to high
velocity of flood water at the time of heavy rainfall. So, to protect the canals from the
damage, the lining should be provided.

(f) To Control the Growth of Weeds

The growth of various types of weeds along the sides of the canals is a common problem.
Again, some types of weeds are found to grow along the bed of the canals. These weeds
reduce the velocity of flow and the capacity of the canals. So, the unlined canals require
excessive maintenance works for clearing the weeds. If lining is provided in the canal, the
growth of weeds can be stopped and velocity and the capacity of the canal may be increased.

Types of Lining:




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There are varieties of linings that are available today but we will be discussing the following
three,
1. Plain Cement Concrete Lining
2. Reinforced Cement Concrete Lining
3. Brick Lining

1. Plain Cement Concrete Lining

This lining is recommended for the canal in full banking. The cement concrete lining is
widely accepted. It can resist the effect of scouring and erosion very efficiently. The velocity
of flow may be kept above 2.5 m/s. It can eliminate completely growth of weeds.
The lining is done by the following steps;

(a) Preparation of sub-grade

The sub grade is prepared by ramming the surface properly with a layer of sand (about 15
cm). Then slurry of cement and sand (1:3) is spread uniformly over the prepared bed.

(b) Laying of concrete

The cement concrete of grade M15 is spread uniformly according to the desired thickness,
(generally the thickness varies from 100mm to 150 mm). After laying, the concrete is tapped
gently until the slurry comes on the top. The curing is done for two weeks. As the concrete is
liable to get damaged by the change of temperature, the expansion joints are provided at
appropriate places.

2. Brick Lining

This lining is prepared by the double layer brick flat soling laid with cement mortar (1:6) over
the compacted sub-grade. The first class bricks should be recommended for the work. The
surface of the lining is finished with cement plaster (1:3). The curing should be done
perfectly.
This lining is always preferred for the following reasons,

(a)This lining is economical.
(b)Work can be done very quickly.
(c) Expansion joints are not required.
(d) Repair works can be done easily.
(e)Bricks can be manufactured from the excavated earth near the site.

However this lining has certain disadvantages,

(a) It is not completely impervious.
(b)It has low resistance against erosion.
(c)It is not so much durable.

3. Reinforced Cement Concrete Lining

Sometimes reinforcement is required to increase the resistance against cracks and shrinkage
cracks. The reduction in the cracks results in less seepage losses. However this reinforcement


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does not increase the structural strength of the lining. This reinforcement adds 10 to 15
percent to the cost and for this reason steel reinforcement is usually omitted except for very
particular situations.

Advantages of Canal Lining

1. It reduces the loss of water due to seepage and hence the duty is enhanced.
2. It controls the water logging and hence the bad effects of water-logging are eliminated.
3. It provides smooth surface and hence the velocity of flow can be increased.
4. Due to the increased velocity the discharge capacity of a canal is also increased.
5. Due to the increased velocity, the evaporation loss also can be reduced.
6. It eliminates the effect of scouring in the canal bed
7. The increased velocity eliminates the possibility of silting in the canal bed.
8. It controls the growth of weeds along the canal sides and bed.
9. It provides the stable section of the canal.
10. It reduces the requirements of land width for the canal, because smaller section of the
canal can be used to produce greater discharge.
11. It prevents the sub-soil salt to come in contact with the canal water.
12. It reduces the maintenance cost for the canals.

Disadvantages of Canal Lining

1. The initial cost of the canal lining is very high. So, it makes the project very expensive
with respect to the output.
2. It involves many difficulties for repairing the damaged section of lining.
3. It takes too much time to complete the project work.
4. It becomes difficult, if the outlets are required to be shifted or new outlets are required to
be provided, because the dismantling of the lined section is difficult.



FACTORS AFFECTING TYPE OF LINING

Question: What are the factors which affect the type of canal lining?

Answer: The selection of type of lining depends on the following factors,

(1) Imperviousness

When the canal passes through the sandy soil the seepage loss is at maximum and the canal is
unstable. So, to make the canal perfectly impervious and reasonably stable, the most
impervious types of linings should be recommended such as cement concrete etc.

(2) Smoothness

The smoothness of the canal bed and sides increases the velocity of flow which further
increases the discharge of the canal. Due to the increased discharge, the duty of water will be
more. So, to increase the duty, the canal surface should be made smooth. The lining like
cement concrete, pre-cast cement concrete etc gives smooth surface to the canal.



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(3) Durability

The ultimate benefit of any project depends on the durability of the hydraulic structures,
canals, etc. So, to make the canal section more durable against all adverse effects like
scouring, erosion, weather action, etc. the most strong and impervious types of lining should
be recommended.

(4) Economy

The lining should be economically viable with the benefits that may be accrued from the
expected revenue, yield of crop, etc. So, by studying the overall benefits the type of lining
should be recommended.

(5) Site Condition

The canal may pass through the marshy land, loose sandy soil, alluvial soil, black clayey soil,
hard soil, etc. So, according to the soil and site condition the type of lining should be
recommended.

(6) Life of Project

Every project should be designed to serve the future three or four decades successfully. The
type of lining should be recommended keeping in mind the life of the project.

(7) Availability of Construction Materials

The expenditure of lining depends on the availability of construction materials, carriage
charges, etc. To reduce the expenditure of lining, the materials which are available in the
vicinity of the project should be utilized.


MANNING‟S ROUGHNESS COEFFICIENTS

Question: What are the Manning’s roughness coefficients for brick, earth, R.C.C and P.C.C?

Answer:
It is denoted by “n”.
Manning‟s Roughness coefficients are:

Brick: 0.014-0.017 (usually taken as 0.015)

Earth: 0.02

P.C.C: 0.014

R.C.C: 0.012




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Cross Drainage Works:

A “cross drainage work” is a hydraulic structure which needs to be constructed at the
crossing of a natural stream and an irrigation canal flowing normally at right angles
underneath or over the natural stream.
It is generally a very costly item and should be avoided by,
   i.            Diverting one stream into another.
  ii.            Changing the alignment of the canal so that it crosses below the junction of
         two streams.

Necessity of Cross- Drainage Works:

The following factors justify the necessity of cross drainage works,

1. The water shed canals do not cross natural drainages. But in actual orientation of the canal
network, this ideal condition may not be available and the obstacles like natural drainages
may be present across the canal. So, the cross drainage works must be provided for running
the irrigation system.
2. At the crossing point, the water of the canal and the drainage get intermixed. So, for the
smooth running of the canal with its design discharge the cross drainage works are required.
3. The site condition of the crossing point may be such that without any suitable structure, the
water of the canal and drainage cannot be diverted to their natural directions. So, the cross
drainage works must be provided to maintain their natural direction of flow.

Types Of Cross Drainage Works:

Depending upon the relative bed levels, maximum water levels and relative discharges of
canals and drainages the cross drainage works may be of following types,

   1. Type1-Irrigation Canal passes over the drainage:

       In this type of C.D work, an irrigation canal is taken over the drainage




       This condition involves construction of following,

   a) Aqueduct:

       The hydraulic structure in which irrigation canal is
       passing over the drainage is known as aqueduct. This
       structure is suitable when bed of canal is above the



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   highest flood level of drainage. In this case, the
   drainage water passes clearly below the canal.
b) Siphon Aqueduct:

   The hydraulic structure in which irrigation canal is
   passing over the drainage, but the drainage water
   cannot pass clearly below the canal is known as
   siphon aqueduct. It flows under siphoned action. This
   structure is suitable when the bed level of canal is
   below the highest flood level of the drainage.

   Advantages of Type1:

          The canal running perennially is above ground and is open to inspection.
          Damage done by floods is rare.

   Disadvantages of Type1:

          During high floods, the foundation can be scoured or the water way of the
           drain may be chocked with trees.

2. Type2-Drainage passes over the irrigation canal:

   In this type of cross drainage work, drainage is taken over the canal.




This condition involves the construction of the following,

a) Super Passage:

   The hydraulic structure in which the drainage is passing
   over the irrigation canal is known as super passage. This
   structure is suitable when the bed level of drainage is
   above the flood surface level of the canal. The water of the
   canal passes clearly below the drainage.

b) Siphon Super passage:

   The hydraulic structure in which the drainage is taken
   over the irrigation canal, but the canal water passes below
   the drainage under siphonic action is known as siphon
   super passage. This structure is suitable when the bed


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   level of drainage is below the full supply level of the
   canal.
c) Canal Siphon:

   If two canals cross each other and one of the canals is
   siphoned under the other, then the hydraulic structure
   at    crossing      is    called   “canal     siphon”.
   For example, lower Jhelum canal is siphoned under
   the Rasul-Qadirabad link canal and the crossing
   structure is called “L.J.C siphon”

   Advantages of Type2:

       C.D works are less liable to damage then the earthwork of canal.
   Disadvantages of Type2:

          Perennial canal is not open to inspection.
          It is difficult to clear the silt deposited in the barrels of the C.D.work.

3. Type3-Drainage and Canal intersection at the same level:

   In this type of work, the canal water and drainage water are permitted to intermingle.




a) Level Crossing:

   When the beds of the drainage and canal are
   practically at the same level, then a hydraulic structure
   is constructed which is known as level crossing. This
   is suitable for the crossing of large drainage with main
   canal.

The level crossing consists of the following components.
1. Crest Wall: It is provided across the drainage just at
   the upstream side of the crossing point. The top level of the crest wall is kept at the
   full supply level of the canal.
2. Drainage Regulator: it is provided across the drainage just at downstream side of the
   crossing point. The regulator consists of adjustable shutters at different tiers.




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   3. Canal Regulator: it is provided across the canal just
      at downstream side of the crossing point. The regulator
      consists of adjustable shutters at different tiers.

   b) Inlet and Outlet:

       In the crossing of small drainage with small channel
       no hydraulic structure is constructed. Simple openings
       are provided for the flow of water in their respective
       directions. It is not necessary for the number of inlets
       and outlets to be same. There may be one outlet for two or three inlets.
       A canal inlet is constructed when the cross drainage flow is small and its water may
       be absorbed into the canal without causing appreciable rise.


       Advantages of Type3:

              Low initial cost

       Disadvantages of Type3:

              Regulation of such work is difficult & requires additional staff
              The canal has to be designed to carry the increased flood discharge of drain.
              The faulty regulation of the gates may damage the canal.
              There is additional expenditure of silt clearance.


Suitability of Cross-Drainage Works:

The factors which affect the selection of the suitable type of cross drainage works are:

1. Relative bed levels and water levels of the canal and drainage
2. Size of the canal and the drainage

The following considerations are important,

1. When the bed level of the canal is much above the highest flood level (H.F.L) of the
drainage, so that sufficient headway is available for floating rubbish etc and also for the
structural elements of the work. An ‘aqueduct’ is the obvious choice. Similarly, if the bed
level of the drain is well above the Flood surface level (F.S.L) of the canal, ‘Super-passage’
is provided.

2. The necessary headway between the canal bed level and the drain H.F.L can be increased
by shifting the crossing to the downstream of the drainage. If, however, it is not possible to
change the canal alignment or if such a shifting does not give sufficient headway between the
two levels, a ‘siphon aqueduct’ may be provided. Thus in case of siphon aqueduct, the H.F.L
of the drain is above channel (canal) bed.




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3. When the canal bed level is much lower but the F.S.L of the canal is higher than the bed
level of drainage, a ‘canal siphon’ is preferred.

4. When the drainage and the canal cross each other practically at the same level a ‘level
crossing’ may be preferred. This type of work is avoided as far as possible.

The considerations governing the choice between aqueduct and siphon aqueduct (or a super
passage and siphon-super passage) are,

(i)Suitable canal alignment
(ii)Suitable soil available for bank connections and
(iii) Nature of available foundation

As discussed earlier, the relative difference between the bed level of the canal and the H.F.L
of the drainage can be suitable altered by changing the canal alignment so that the point of
crossing is shifted upstream or Downstream of the drainage.

For example, if the canal alignment is such that headway is not available between the H.F.L
of the drain and the bed of the canal, a siphon aqueduct is to be constructed at the crossing.
But if the other conditions are not favorable for the construction of the siphon aqueduct, the
canal alignment may be changed so that the crossing is shifted to the downstream and
sufficient headway required for the construction of an aqueduct is available.


PROPER SITE FOR DRAINGE CROSSING:

The site selected for the cross drainage works should have the following main characteristics,
1. It should be such that it requires minimum disturbance regarding the approach and tail
reaches of the drainage channel.
2. Suitable foundation soil should be available at reasonable depth.
3. Sufficient headway is available for the super structure of the aqueduct over the H.F.L of
the natural stream.
4. Suitable existing topography, geological and hydraulic conditions for the cross drainage
works at reasonable costs.




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WATER REQUIREMENTS FOR CROPS

It is defined as, “The quantity of water required by a crop in a given period of time for normal
growth under field conditions.” It includes evaporation and other unavoidable wastes.
Usually water requirement for crop is expressed in water depth per unit area.
Mathematically,




Water losses produced after passing from Nakka is called application losses.

FACTORS AFFECTING THE WATER REQUIREMENT

The following are the factors which affect on the water requirements of the crops,

1. Water table:

If the water table is nearer to the ground surface, the water requirement will be less & vice
versa.

2. Climate:

In hot climate the evaporation loss is more and hence the water requirement will be more and
vice versa.

3. Ground Slope:

If the slope of the ground is steep the water requirement will be more due to less absorption
time for the soil.

4. Intensity of Irrigation:

It is directly related to water requirement, the more the intensity greater will be the water
required for a particular crop.

5. Type of Soil:

In sandy soil water percolates easily so water required is more. While in clayey soils water
requirement is less.

6. Method of Application of water:

In sprinkler method less water is required as it just moist the soil like rainwater whereas in
flood more water is required.



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7. Method of Ploughing:

In deep ploughing less water is required and vice versa.
CONSUMPTIVE USE OF WATER

It is the quantity of water used by the vegetation growth of a given area.

Mathematically,



It is expressed in terms of depth of water.

FACTORS AFFECTIING THE CONSUMPTIVE USE OF WATER

Consumptive use varies with,
1. Evaporation which depends on humidity.
2. Mean Monthly temperature.
3. Growing season of crops and cropping pattern.
4. Monthly precipitation in area.
5. Wind velocity in locality.
6. Soil and topography.
7. Irrigation practices and method of irrigation.
8. Sunlight hours.

TYPES OF CONSUMPTIVE USE

Following are the types of consumptive use,
1. Optimum Consumptive Use
2. Potential Consumptive Use
3. Seasonal Consumptive Use

1. Optimum Consumptive Use:

It is the consumptive use which produces a maximum crop yield.

2. Potential Consumptive Use:

If sufficient moisture is always available to completely meet the needs of vegetation fully
covering the entire area then resulting evapotranspiration is known as Potential Consumptive
Use.

3. Seasonal Consumptive Use:

The total amount of water used in the evapotranspiration by a cropped area during the entire
growing season.

METHODS OF ESTIMATION OF CONSUMPTIVE USE:

For the estimation of consumptive use there are two main methods,


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1. Direct Methods/Field Methods
2. Empirical Methods

1. Direct Methods:

In this method field observations are made and physical model is used for this purpose. This
includes,
(i) Vapour Transfer Method/Soil Moisture Studies
(ii) Field Plot Method
(iii) Tanks and Lysimeter
(iv) Integration Method/Summation Method
(v) Irrigation Method
(vi) Inflow Outflow Method

(i) Vapour Transfer Method:

In this method, soil moisture measurements are taken before and after each irrigation. The
quantity of water extracted per day from soil is computed for each period. A curve is drawn
by plotting the rate of use against time and from this curve, the seasonal use can be estimated.
This method is suitable in those areas where soil is fairly uniform and ground water is deep
enough so that it does not affect the fluctuations in the soil moisture within the root zone of
the soil.
It is expressed in terms of volume i.e. Acre-feet or Hectare-meter

(ii) Field Plot Method:

We select a representative plot of area and the accuracy depends upon the representativeness
of plot (cropping intensity, exposure etc).It replicates the conditions of an actual sample field
(field plot). Less seepage should be there.



The drawback in this method is that lateral movement of water takes place although more
representative to field condition. Also some correction has to be applied for deep percolation
as it cannot be ascertained in the field.

(iii) Tanks and Lysimeter:

In this method, a watertight tank of cylindrical shape having diameter 2m and depth about 3m
is placed vertically on the ground. The tank is filled with sample of soil. The bottom of the
tank consists of a sand layer and a pan for collecting the surplus water. The plants grown in
the Lysimeter should be the same as in the surrounding field. The consumptive use of water
is estimated by measuring the amount of water required for the satisfactory growth of the
plants within the tanks. Consumptive use of water is given by,

Where,




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Lysimeter studies are time consuming and expensive. Methods 1 and 2 are the more reliable
methods as compare to this method.


(iv) Integration Method:

In this method, it is necessary to know the division of total area, i.e. under irrigated crops,
natural native vegetation area, water surface area and bare land area.
In this method, annual consumptive use for the whole area is found in terms of volume. It is
expressed in Acre feet or Hectare meter.
Mathematically,


Where,




(v) Irrigation Method:

In this method, unit consumption is multiplied by some factor. The multiplication values
depend upon the type of crops in certain area.
This method requires an Engineer judgment as these factors are to be investigated by the
Engineers of certain area.

(vi) Inflow Outflow Method:

In this method annual consumptive use is found for large areas. If               is the valley
consumptive use its value is given by,


Where,




All the above volumes are measured in acre-feet or hectare-meter.

2. Empirical Methods:

Empirical equations are given for the estimation of water requirement. These are,

(i) Lowry Johnson Method:

The equation for this method is,


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(ii) Penman Equation:

According to this method,




ET = Evapotranspiration or consumptive use in mm
Ea = Evaporation (mm/day)
H = Daily head budget at surface (mm/day)
H is a function of radiation, sunshine hours, wind speed, vapour pressure and other climatic
factors.
Δ = Slope of saturated vapour pressure curve of air at absolute temperature in °F

(iii) Hargreave‟s Method:

It is a very simple method. According to this method,


Where,




DEFINITONS

Gross Command Area (G.C.A):

The whole area enclosed between an imaginary boundary lines which can be included in an
irrigation project for supplying water to agricultural land by the network of canals is known
as Gross command Area (G.C.A).It includes both the culturable and unculturable areas.
Mathematically,


Unculturable Command Area (Un-C.C.A):

The area where the agriculture cannot be done and crops cannot be grown is known as
unculturable area. The marshy lands, lakes, ponds, forests, villages etc are considered as
unculturable.

Culturable Command Area (C.C.A):

The total area within an irrigation project where the cultivation can be done and crops can be
grown.




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Mathematically,


In formation of projects and schemes, C.C.A is roughly taken as                  of G.C.A
depending upon the configuration of land.
Intensity of Irrigation:

It is defined as the ratio of cultivated land for a particular crop to the total C.C.A.
It is expressed as a %age of C.C.A.
For example, if the total C.C.A is 1000 hectares where wheat is cultivated in 250 hectares
Then,



Area to Be Irrigated:

It is the product of C.C.A and the intensity of irrigation.

Mathematically,


Crop Ratio:

It is defined as the ratio of the areas of the two main crop seasons, e.g. Kharif and Rabi.
For example, if the area under Kharif crop is 2500 hectares and the area under Rabi crop is
5000 hectares then, crop ratio of kharif to Rabi is 1:2 (i.e.

Crop Season:

The period during which some particular types of crops can be grown every year on the same
land is known as crop season.
    i. Kharif Season: This season ranges from June to October. The crops are sown in the
           very beginning of monsoon and harvested at the end of autumn.
           The major Kharif crops are---- Rice, Millet, Maize, Jute, and Groundnut.
    ii. Rabi Season: This season ranges from October to March. The crops are sown in the
           very beginning of winter and harvested at the end of spring.
           The major Rabi crops are-----Wheat, Gram, Mustard, Rapeseed, Linseed, Pulses,
           Onion etc.

Consumptive Use of “Rabi Crops”

                    CROP                                      CONSUMPTIVE USE “Δ”
                    Wheat                                           37 cm
                    Gram                                            30 cm
                    Barley                                          30 cm
                   Potatoes                                        60-90 cm
                  Sugarcane                                         90 cm




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Consumptive Use of “Kharif Crops”

                    CROP                                     CONSUMPTIVE USE
                    Cotton                                       25-40 cm
                     Rice                                       125-150 cm
                    Maize                                         45 cm
Crop Rotation:

The process of changing the type of crop for the cultivation on the same land is known as
crop rotation.
It is found that if same crop is cultivated on the same land every year, the fertility of the land
gets reduced and the yield of crop also gradually reduces. This is due to the reason that
necessary salts required for the growth of a particular crop get exhausted.
Few crop rotations possible are,
     i. Rice----Gram
     ii. Wheat----Millet----Gram
     iii. Rice---Gram---Wheat

Time Factor:

The ratio of the number of days the canal has actually been kept open to the number of days
the canal was designed to remain open during the base period is known as time factor.
Mathematically,


For example, a canal was designed to kept open for 15 days, but it was practically kept open
for 10 days for supplying water to the culturable area, then the time factor is

Capacity Factor:

It is the ratio of the average discharge to the maximum discharge (design discharge).
Mathematically,


For example, a canal was designed or the maximum discharge of 50 cumecs, but the average
discharge is 40 cumecs, then the capacity factor is

Number of Watering:

The total depth of water required by a crop is not applied at one time but it is supplied over
the base period by stages depending upon requirement, these numbers of stages are known as
“Number of Watering”

Paleo:

 The initial watering which is done on the land to provide moisture to the soil just before
sowing any crop is known as paleo or paleva.




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Kor Watering:

The first watering which is done when the crop has grown to about three centimeters is called
Kor Watering.


Kor Period:

The portion of the base period in which Kor watering is needed is called “Kor Period”

Cumec Day:

The quantity of water flowing continuously for one day at the rate of one cumec is known as
cumec day.




Crop Period:

It is defined as the total time period from the time of sowing of a crop to the time of
harvesting it.
It is the period in which crop remain in the field. It is expressed in number of days.

Base Period:

It is the period from the first to the last watering of the crop just before its maturity. It is
denoted by “B” and expressed in number of days.

Delta:

It is the total depth of water required by a crop during entire base period. It is also called
consumptive use. It lies in base period. It is expressed in terms of depth and denoted by “Δ‟.

Field Capacity:

It is defined as the amount of water held in the soil after the excess gravitational water has
been drained.

Permanent Wilting Point: (Wilting Coefficient)

It is the water content at which plants can no longer extract sufficient water from soil for its
growth.



Water Allowance:


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It is the total cusecs required at the outlet to irrigate 1000 acres of C.C.A.




Duty:

The duty of water is defined as number of hectares that can be irrigated by constant supply of
water at the rate of one cumec throughout the base period. It is expressed in hectares/cumec
and is denoted by “D”. For example if 3 cumecs of water is required for the crop sown in, an
area 5100 hectares, the duty of the irrigation will be                                 and the
discharge of 3 cumecs is required throughout the base period.

RELATION BETWEEN DUTY, DELTA AND BASE PERIOD:

In M.K.S System:
Let



By definition,




In F.P.S System:
Let



By definition,




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FACTORS AFFECTING DUTY:

The factors that affect the duty are described below,

1. Soil Characteristics:

If the soil of the canal bed is porous and coarse grained, it leads to more seepage loss and
consequently low duty. If it consists of alluvial soil, the percolation loss will be less and the
soil retains the moisture for longer period and consequently the duty will be high.

2. Climatic Condition:

When the temperature of the command area is high the evaporation loss is more and the duty
becomes low and vice versa.

3. Rainfall:

If rainfall is sufficient during the crop period, the duty will be more and vice versa.

4. Base Period:

When the base period is longer, the water requirement will be more and the duty will be low
and vice versa.

5. Type of Crop:

The water requirement for various crops is different. So the duty varies from crop to crop.

6. Topography of Agricultural Land:

If the land is uneven the duty will be low. As the ground slope increases the duty decreases
because there is wastage of water.

7. Method of Ploughing:

Proper deep ploughing which is done by tractors requires overall less quantity of water and
hence the duty is high.

8. Methods of Irrigation:

The duty of water is high in case of perennial irrigation system as compared to that in
inundation irrigation system.



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9. Water Tax:

If some tax is imposed the farmer will use the water economically thus increasing the duty.




METHODS OF IMPROVING DUTY:

Various methods of improving duty are:

(1) Proper Ploughing:

Ploughing should be done properly and deeply so that the moisture retaining capacity of soil
is increased.

(2) Methods of supplying water:

The method of supplying water to the agriculture land should be decided according to the
field and soil conditions. For example,

Furrow method…………For crops sown ion rows
Contour method………..For hilly areas
Basin……………………For orchards
Flooding………………..For plain lands

(3) Canal Lining:

It is provided to reduce percolation loss and evaporation loss due to high velocity.

(4) Minimum idle length of irrigation Canals:

The canal should be nearest to the command area so that idle length of the canal is minimum
and hence reduced transmission losses.

(5) Quality of water:

Good quality of water should be used for irrigation. Pollution en route the canal should be
avoided.

(6) Crop rotation:

The principle of crop rotation should be adopted to increase the moisture retaining capacity
and fertility of the soil.

(7) Method of Assessment of water:

Particularly, the volumetric assessment would encourage the farmer to use the water
carefully.



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(8) Implementation of Tax:

The water tax should be imposed on the basis of volume of water consumption.




IRRIGATION EFFICIENCY:

The ratio of the amount of water available (output) to the amount of water supplied (input) is
known as Irrigation Efficiency. It is expressed in percentage.

The following are the various types of irrigation efficiencies,

(a) Water Conveyance Efficiency: (        )
It is the ratio of the amount of water applied, to the land to the amount of water supplied from
the reservoir. It is obtained by the expression,




Where,




(b) Water Application Efficiency: (       )

It is the ratio of the water stored in root zone of plants to the water applied to the land. It is
obtained by the expression,




Where,




(c) Water Use Efficiency: (     )
It is the ratio of the amount of water used to the amount of water applied. It is obtained by the
expression,




Where,


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(d) Consumptive use Efficiency: (        )
It is the ration of the consumptive use of water to the amount of water depleted from the root
zone. It is obtained by the expression,



Where,




ASSESSMENT OF IRRIGATION WATER

The water which has been supplied to the farmer is at government expenses. Some nominal
charge must, therefore be leaved on the farmer for using this water. This is called
“Assessment of irrigation water”. Therefore the knowledge of the same is very essential to
engineers.

WHY ASSESSMENT OF WATER IS NEEDED:

The charges must be leaved on the farmers for the following reasons,

(a) To recover the cost of construction of the project by which it has been possible to supply
water.
(b) To recover the maintenance cost of the various works staff for certain improvement.
(c) To check the cultivators against uneconomical and careless use of water.

METHODS OF ASSESSMENT OF IRRIGATION WATER:

There are five methods of the assessment,
1. Assessment on area basis or crop rate basis
2. Volumetric assessment
3. Assessment on seasonal basis
4. Composite rate basis
5. Permanent assessment or Betterment levy basis

1. Assessment on Area Basis or Crop Rate Basis:

There are fixed charges for different types of crops on area basis. It is a very old system.
The rates for different crops have been fixed on the basis of,
(a) Cash value of the crops and hence the paying capacity
(b) Water requirements of the crop


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(c) Time of demand of water

2. Volumetric assessment:

In this method water rates are charged on the basis of actual volume of water supplied at
outlet head. It is very difficult to maintain as measurement of water is difficult.
3. Assessment on seasonal basis:

In this method rate of assessment is based on the type of crop grown in a particular area
during certain crop season.

4. Composite rate basis:

It is a combination of water charges and land revenue i.e Malia and Abiana .

5. Permanent assessment or Betterment levy basis:

In this system farmers have their own sources of supply but they use the irrigation water only
in the instance of a drought. In such a case the farmers are levied at a fixed rate every year
this levy is known as betterment levy. And the farmers are authorized to use the water in a
drought.

METHODS OF APPLICATION OR DISTRIBUTION OF WATER

There are various methods of application of water some of which are listed as under,

1. Surface method
(a) Furrow method
(b) Flooding method
2. Sub- Surface method
3. Sprinkler method

1. Surface method:

It includes the furrow method and the flooding method. In this method water is distributed
through the small channels which flood the area.

(a) Furrow method:

The irrigation water is supplied to the land by digging narrow channels known as furrows at
regular intervals. This method is best suited for potatoes, tobacco, sugarcane etc.

(b) Flooding method:

In this method the field is flooded with water with the help of field channels. It may be
controlled and uncontrolled.

2. Sub- Surface method:




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In this method water is applied to the root zone of the crops by underground networks of
pipes. This method is also called as drip method or trickle method of irrigation.
3. Sprinkler method:

In this method the water is applied to land in the form of spray like rain. It is done by the
network of main pipes, sub main pipes and lateral pipes.

PROBLEMS FROM IRRIGATION ENGINEERING BY NN BASAK

Problem No. 01

A channel is to be designed for irrigating 5000 hectares in kharif crop and 4000 hectares in
Rabi crop. The water requirement for kharif and Rabi are 60 cm and 25cm respectively. The
Kor period for kharif is 3 weeks and for Rabi is 4 weeks. Determine the discharge of the
channel for which it is to be designed.

Solution:

                                          Consumptive
 Crop         Area         Base Period                   Duty
                                              Use
            Hectares       week    days    cm     M      Hectares/cumecs         cumecs
 Kharif          5000       3       21     60      0.6                            16.53
 Rabi            4000       4       28     25      025                             4.13

Problem No. 02

The gross command area of an irrigation project is 1.5 lakh hectares, where 7,500 hectares
are unculturable. The area of kharif crop is 60,000 hectares and that of Rabi crop is 40,000
hectares. The duty of Kharif is 3000 hectares/cumec and the duty of Rabi is 4000
hectares/cumec.
Find
(a) The design discharge of channel assuming 10% transmission loss.
(b) Intensity of irrigation for Kharif and Rabi.

Solution:

Given that,




Now,

Discharge for Kharif crops,



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Discharge for Rabi crops,




    a)
    b)




Problem No. 03

The gross command area of an irrigation project is 1 lakh hectares. The culturable command
area is 75% of G.C.A. The intensities of irrigation for Kharif and Rabi are 50% and 55%
respectively. If the duties for kharif and Rabi are 1200 hectare/cumec and 1400
hectares/cumecs respectively, determine the discharge at the head of the canal considering
20% provisions for the transmission loss, overlap allowance, evaporation loss etc.

Solution:

Given that,




For kharif crops,




For Rabi crops,




So, to meet up the actual water requirement of the crops, the discharge of the canal at the head of the
field should be 31.25 cumecs (as it maximum). Now considering 20% provision for losses,


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Problem No. 04

Determine the head discharge of a canal from the following data. The value of time factor
may be assumed as 0.75.
                                                                              Duty In
       Crop            Base Period In Days        Area In Hectare
                                                                         Hectares/Cumecs
        Rice                    120                    4000                     1500
       Wheat                    120                    3500                     2000
     Sugarcane                  310                    3000                     1200


Solution:

                       Base Period In                                Duty In
       Crop                                 Area In Hectare                      Discharge=
                           Days                                  Hectares/Cumecs
      Rice                  120                   4000                1500            2.667
     Wheat                  120                   3500                2000            1.750
    Sugarcane               310                   3000                1200            2.500

As, the base period of sugarcane is 310 days, it will require water both in Kharif and Rabi seasons.




So, the required head discharge of the canal is 6.889 cumecs


Problem No. 05

Find out the capacity of a reservoir from the following data. The culturable command area is
80,000 hectares.
                                                                              Intensity Of
        Crop             Base Period In Days        Area In Hectare
                                                                            Irrigation In %
        Rice                      120                    1800                      25
       Wheat                      120                    2000                      30
     Sugarcane                    320                    2500                      20



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Assume that canal and reservoir losses are 5% and 10% respectively.

Solution:

Calculating Delta for each crop,




Calculation of Area for each crop,




Volume of water Required for each crop,




Considering canal loss of 5%,


Considering reservoir loss of 10%,



Problem No. 06:

The command area of a channel is 4000 hectares. The intensity of irrigation of a crop is 70%.
The crop requires 60cm of water in 15 days, when the effective rainfall is recorded as 15cm
during that period.
    (a) The duty at the head of field
    (b) The duty at the head of channel
    (c) The head discharge at the head of channel

Assume total losses as 15%.

Solution:

Given that,




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   a)

   b)




   c)

PROBLEMS FROM IRRIGATION & WATER POWER ENGINEERING BY B.C. PUNMIA
Example no. 3.2:

A crop requires a total depth of 92 cm of water for a base period of 120 days. Find the duty of
water.

Solution:

Given that,
Base period, B=120 days
Depth of water,



Example 3.3:

An irrigation canal has gross commanded area of 80,000 hectares out of which 85% is
culturable irrigable. The intensity of irrigation for Kharif season is 30% and for Rabi season
is 60%. Find the design discharge at the head of the canal if the duty at its head is 800
hectares/cumecs for Kharif season and 1700 hectares/cumecs for Rabi season.

Solution:

Given that,




Now,



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Example 3.4:

A water course has a culturable commanded area of 2600 hectares, out of which the
intensities of irrigation of perennial sugar-cane and rice crops are 20% and 40% respectively.
The duty for these crops at the head of water course is 750 hectares/cumecs and 1800
hectares/cumecs respectively. Find the discharge required at the head of water course if the
peak demand is 20% of the average requirement.

Solution:

Given that,




                                                                =1.52 cumecs

Example 3.5:

The left branch canal carrying a discharge of 20 cumecs has culturable commanded area of
20,000 hectares. The intensity of Rabi crop is 80% and the base period is 120 days. The right
branch canal carrying a discharge of 8 cumecs has culturable commanded area of 12,000
hectares, intensity of irrigation of Rabi crop is 50%, and the base period is 120 days.
Compare the efficiencies of the two canal systems.

Solution:

   a) For Left canal




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   b) For Right canal




Example 3.6

A water course has a culturable commanded area of 1200 hectares. The intensity of irrigation
for crop A is 40% and for B is 35%, both the crops being Rabi crops. Crop A has a kor period
of 20 days and crop B has kor period of 15 days. Calculate the discharge of the water course
if the depth for A crop is 10 cm and for B it is 16 cm.

Solution:

   a) For crop A




   b) For crop B




Example 3.7:

A water course commands an irrigated area of 600 hectares. The intensity of irrigation of rice
in this area is 60%. The rice transplantation of Rice crop takes 12 days and total water depth
of water required by the crop is 50 cm on the field during the transplantation period. During
the transplantation period, the useful rain falling on the field is 10 cm. the duty of the
irrigation water for the crop on the field during transplantation, at the head of the field and



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also at the head of the distributory, assuming losses of water to be 20% in the water course.
Also, calculate the discharge required in the water course.

Solution:




Numerical Solved in Class:

Find out the discharge, base time period is 5 months.

                                        CONSUMPTIVE      CONSUMPTIVE        TOTAL WATER
                             AREA
       CROP                               USE “Δ”          USE “Δ”            DEMAND
                             (Acres)
                                           (Inches)          (ft)               (Acre ft)
      Cotton                 45156.55         18              1.5              67734.825
       Maize                 22608.18         20             1.67               37680.3
     Sugarcane               13561.92         40             3.33               45206.4
      Fooder                  5427.76         18              1.5               8141.64
     Vegetable                5233.37         18              1.5              7850.055
       Rice                  22608.18         60               5               113040.9
                                                                              Σ=279654.12




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Head work:

A hydraulic structure which supplies water to the off-taking canal is called as a „Head work‟.
Head work may be divided into two classes:
   1. Storage Headwork
   2. Diversion Headwork

Storage Head work:

When dam is constructed across a river to form a storage reservoir, it is known as storage
head work. It stores water during the period of excess supplies in the river and releases it
when demand overtakes the available supplies.

Diversion Head work:

When a weir or barrage is constructed across a river to raise the water level and to divert the water to
the canal, then it is known as diversion head work. The flow in the canal is controlled by canal head
regulator.

It serves the following purposes:
           i.   It raises the water level in the river so that the command area can be increased.
          ii.   It regulates the intake of water into the canal.
         iii.   It controls the silt entry into the canal.
         iv.    It reduces fluctuations in the level of supply in the river.
          v.    It stores water for tiding over small periods of short supplies.

A diversion headwork can further be sub-divided into two principal classes:
   1. Temporary spurs or bunds
   2. Permanent weirs and barrages.

Temporary spurs or bunds are those which are temporary and are constructed every year after
floods, however, for important works, weirs or barrages are constructed since they are of
permanent nature if properly designed.

Weirs:

The weir is a solid obstruction put across the river to raise its water level and divert the water
into the canal.


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If a weir also stores water for tiding over small periods of short supplies, it is called as
„storage weir‟. The main difference between the storage weir and dam is only in height and
duration for which the supply is stored. A dam stores the supply for a comparatively longer
duration.

Barrage:

The function of barrage is similar to that of weir; but the heading up of water is affected by
the gates alone. No solid obstruction is put across the river. The crest level in the barrage is
kept at a low level. During the floods, the gates are raised to clear off the high flood level,
enabling the high flood to pass downstream to mix afflux. When the flood recedes, the gates
are lowered and the flow is obstructed, thus raising the water level to upstream of the barrage.
Due to this, there is less silting and better control over the levels. However, barrages are
much more costly than weirs.

Comparison Between Barrage And Weir:

    WEIR                                           BARRAGE
                                                         1. It is relatively costly due to
        1. It is cheaper because of Simple
                                                            complicated            structure
           construction.
                                                            construction.
        2. A weir has high crest level.
                                                         2. It is with lower crest level.
        3. It provides more afflux.                      3. It provides less afflux
        4. It does not have control all Over             4. It provides effective control over
           the river water.                                 the entire river flow.
        5. It causes more silting on Up-                 5. It provides less silting on Up-
           stream.                                          stream bed.
        6. Over the weir, bridge cannot be               6. Bridge for transportation Purpose
           constructed.                                     can be constructed
        7. Crest shutter is difficult to operate
           i.e. more time and labour is
                                                         7. Gates are convenient to operate.
           required.




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Components Of Barrage:




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1. Main Barrage Portion:

   This is the main body of the barrage normally of R.C.C. slabs which support the steel
   gates. In cross-section it consist of;
    U/S concrete floor to lengthen the path of sewage and to protect the middle portion
       where the piers, gates and bridge are located.
    A crest at the required height above the floor on which the gates rest in there closed
       position.
    U/S glacis having the necessary slope to join the u/s floor level to highest point, the
       crest.
    D/S glacis of suitable shape & slope. This joins the crest to the d/s floor level (which
       may be at river bed level or below). The hydraulic jump forms on the glacis since it is
       more stable than on the horizontal floor and this reduces the length of concrete work
       requires on d/s.
    D/S floor is built of concrete and is constructed so as to contain the hydraulic jump.
       Thus it takes care of turbulence which would otherwise cause erosion. It is also
       provided with friction blocks of suitable and at distances determined by hydraulic
       model experiments in order to increase friction and destroy residual K.E.

2. Guide Bank:

   Guide Bank are earthen embankments with stone pitching in the slopes facing water, to
   guide the river through the barrage, These river training works are provided for rivers
   flowing in planes, upstream and downstream of the hydraulic structures or bridges built
   on the river. Guide banks guide the river water flow through the barrage.

3. River Training Works:

   It includes guide banks, marginal bunds, spurs etc. Functions are as follows,

             To provide and non-tortuous approach to weir.
             To prevent the river from out-flanking the weir.
             To prevent additional area to be submerged due to afflux.


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               To prevent erosion of the river banks (protective works).

4. Marginal Bunds:

   Marginal bunds are flood embankments in continuation of guide banks designed to
   contain the floods within the flood plain of the river. Both height and length vary
   according to back water effect caused by the barrage. They are not provided with stone
   pitching and fully cover the back- water length.

5. Spurs:

   Marginal bunds are also called as „Spurs‟.

6. Canal Head Regulator:

   These are the structure constructed at the head (off take) of the canal adjacent to the under
   sluices. Its function is

             To admit water into the off taking canal.
             To regulate the supplies into the canal.
             To indicate the discharge passed into the canal from design discharge formula
              and observed head of water on the crest.
             To control the silt entry into the canal.

   During heavy floods, it should be closed otherwise high silt quantity will leave to the
   canal.


7. Divide Wall:

   It is a long wall constructed at right angle to the weir axis. It is extended up to the
   upstream end of the canal head regulator. In case of one canal off-taking from each bank
   of the river, one divide-wall is provided on front of each of the head regulators of the off
   takes. Similarly on the d/s side it should extend to cover the hydraulic hump and the
   resulting turbulence. The main functions are as follows;
        To generate a parallel flow and thereby avoid damage to the flexible protection
           area of the undersluice portion.
        To keep the cross-section, if any, away from the canal.
        To serve as a trap for coarser bed material.
        To serve as a side-wall of the fish ladder.
        To separate canal head regulator from main weir.

8. Fish Ladder:

   It is a narrow trough opening along the divide wall towards weir side provided with
   baffles (screen to control the flow of the liquid, sand etc.), so as to cut down the velocity
   of flowing water from u/s to d/s. location of fish ladder adjacent to divide wall is


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   preferred because there is always some water in the river d/s of the under sluice only. It
   may be built within the divide wall.
   A fish ladder built along the divide wall is a device designed to allow the fish to negotiate
   the artificial barrier in either direction. In the fish ladder, the optimum velocity is (6-8)
   ft/sec. this can be at Maralam Qadirabad & Chashma barrages.
   Fish move from u/s to d/s in search of relatively warm water in the beginning of water
   and return u/s for clear water before the onset of monsoon.

9. Main Weir And Impervious Floor:

   The obstruction constructed across the river is weir. Impervious layer consist of u/s apron,
   u/s glacis, crest, d/s glacis and downstream apron.
   Its function is to raise water level locally and divert supplies into the canal. Main
   concentration is on raising the water level and taking care of the disturbances causes by
   the hydraulic jump.

10. Flexible Apron:

   A flexible apron is placed d/s of the filter of the filter and consists of boulders large
   enough not to be washed away by the highest likely water velocity. The protection is
   enough as to cover the slope of scour depth i.e. (        depth of scour on u/s side) and
   (    scour       depth    on       the     d/s      side)     at     a      slope      of   .

11. Crest:

   Crest is the weir surface at the required height above the floor at which gates rest at its
   closed position.

12. Pervious Floor:

   (Upstream Talus and Downstream Talus)
   It prevents scouring under the impervious floor.
            To serve as inverted filter.
            To check scour downstream.
            To withstand high velocities d/s of the hydraulic jump.

13. Under sluice:

   Under sluice is the opening at low level in the part of barrage which is adjacent to the off
   takes. These openings are controlled by gates. They form the d/s end of the still ponds
   bounded on two sides of divide-wall and canal head regulator.
   They perform the following functions:
            To control silt entry into the canal.
            To protect d/s floor from hydraulic jump.
            To lower the highest flood level.
            To scour the silt deposits in the pockets periodically.



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              To maintain a clear and well-defined river channel approaching the canal
               head-regulator.

       A number of bays at the extreme ends of the barrage adjacent to the canal regulator
       have a lower crest level than the rest of the bays. The main function is to draw water
       in low river flow conditions due to formation of a deep channel under sluice portion.
       This also helps to reduce the flow of silt into the canal due to drop in velocity of river
       water in deep channel in front of canal regulator. Accumulated silt can be washed
       away easily by opening the under sluice gates due to high velocity currents generated
       by lower crest levels or a high differential head.

14. Inverted Filter:

    An inverted filter is provided between the d/s sheet piles and the flexible protection.
    It typically consists of 6” sand, 9‟‟ coarse sand and 9” gravel. The filter material may vary
    with the size of the particles forming river bed. It is protected by placing concrete blocks
    of sufficient weigh and size, over it. Slits (jhiries) are left b/w the blocks to allow the
    water to escape. The length of the filter should be (            d/s depth of sheet pile). It
    performs following functions,
          It checks the escape of fine soil particles in the seepage water.
          In the case of scour, it provides adequate cover for the d/s sheet piles against the
            steepening of exit gradient.
15. Sheet Piles:

   There are generally three or four sheet piles. These are made of mild steal, each portion
   being           ) ft in width and in thickness of the required length and having groove to
   link with the other sheet piles. From the functional point view, in a barrage, these are
   classified into three types i.e.
            u/s sheet piles
            intermediate sheet piles
            d/s sheet piles


   (a) Upstream Sheet Piles:

       U/S sheet piles are located at the U/S
       end of the U/S concrete floor. These
       piles are driven into the soil beyond
       the maximum possible scour that
       may occur. Their functions are:
            To protect the barrage
               structure from scour.
            To reduce uplift pressure in
               the barrage floor.
            To hold the sand compacted and densified between two sheet piles in order to
               increase the bearing capacity when the barrage floor is defined as raft.



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   (b) Intermediate Sheet Piles:

          They are situated at the end of U/S and D/S glacis and serve as second line of




          defense. In case of the u/s or d/s sheet piles collapse due to advancing scour or
          undermining, then these sheet piles give protection to the main structure of the
          barrage. The intermediate sheet piles also help to lengthen the seepage path and
          reduce up-lift pressure.

   (c) Down Stream Piles:

          These are placed at the end of the d/s concrete floor and their main function is to
          check the exit gradient. Their depth should be greater than the maximum possible
          scour.

16. Shutters or Gates:

       Weirs are provided either with shutters or counter balanced gates to maintain pond
       level. A shuttered weir is relatively cheaper but locks in speed. Better control is
       possible in a gated weir (barrage). Their function is
                  To maintain pond level.
                  To raise the water level during low supplies.

          In case of higher floods, shutters are dropped down and overflow takes place
          while in case of gated weir, gates are raised during floods.



Site Selection of a Barrage:




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The following considerations should be kept in mind while selecting the site for a barrage.
 The site must have a good command over the area to be irrigated and also must not be at
   too far distance to avoid long feeder channels.
 The width of the river at the site should be preferably minimum with a well-defined and
   stable river approach.
 A good land approach to the site will reduce expenses of the transportation and the
   ultimate cost of the project.
 There must be easy diversion of the river after construction
 Existence of central approach of the river to the barrage after diversion, this is essential
   for proper silt control.
 If it is intended to convert the existing in-undation canals into the perennial canals, site
   selection is limited by the position of the head-regulator and the alignment of the existing
   in-undation canals.
 A rock foundation is the best but in the alluvial planes the bed is invariably sandy.

The common practice in Pakistan has been to build the barrage on dry land in a bye river and
after completion to divert the river through it.
 This gave an oblique approach and created many problems. The following guidelines
    have now been proposed by the Irrigation Research Institute, Lahore. Their
    recommendations are based on extensive hydraulic model experiments for each individual
    case.
 Where the angle b/w the headwork axis and the river axis exceeds 10 degree, the problem
    arises of concentration of flow on one side and island formation within the guide banks
    on the other side occurs due to heavy silting as in case of Islam, Sidhnai and Balloki
    barrages in Pakistan.
 If the river axis is to the right of the headwork axis, the concentration of flow is generally
    on the left side with the consequent tendency to form an island on the right and vice
    versa.
 When a barrage is located below the confluence of two rivers, it should be located
    sufficiently far below the confluence and consideration must be given as to which of the
    rivers dominate the confluence.



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   The barrage should be located as far as possible in the centre of the flood plain.
    Asymmetry of location increases the likelihood shoal forming and calls for expensive
    training works.
   The most suitable site for a barrage when constructed on dry land, is below the outer side
    of the convex bund which is followed by the straight reach of the river.

SOME IMPORTANT TERMS:

       Khadir:
       It is the flood plain of the river.
       Khadir Axis: (Flood Plain Axis)
       It is the line passing through the centre of the river course, b/w the two banks up to
       the back water effect.
       Weir Axis: (Barrage Axis)
       It is the line along which the crest of the weir is laid.
       River Axis:
       It is a line parallel to the khadir axis at the centre of the barrage axis b/w the
       abutments.
       Headwork Axis:
       The headwork axis is a line perpendicular to the barrage axis at the centre of the weir
       abutments.


Retrogression:

It is a temporary phenomenon which occurs after the construction of barrage in the river
flowing through alluvial soil. As a result of back water effect and increase in the depth, the
velocity of water decreases resulting in deposition of sedimentation load. The water flowing
through the barrage have less silt, so water picks up silt from downstream bed. This results in
lowering d/s river bed to a few miles. This is known as retrogression.
It may occur for the first few years and bed levels often recover their previous level. Within a
few years, water flowing over the weir has a normal silt load and this cycle reverses. Then
due to greater depth, silt is deposited and d/s bed recovers to equilibrium. Retrogression value
is minimum for flood discharge and maximum for low discharge. The values vary
ft.

Accretion:

It is the reverse of retrogression and normally occurs u/s, although it may occur d/s after the
retrogression cycle is complete.
There is no accurate method for calculating the values of retrogression and accretion but the
values which have been calculated from different barrages can be used as a guideline.

Subsoil Flow Considerations:

The subsoil flow or the foundation seepage may cause harm in two ways
   1.           Uplift Pressure
   2.           Piping (undermining)

1. Uplift Pressure:


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    Barrages or diversion dams are normally built on the porous subsoil, normally occurring
    in river beds such as silt, fine sand or gravel. These are low head structures not requiring
    rock foundations and they can be built on porous soil, with the provision of necessary
    safeguards against uplift pressure. This is defined as residual pressure of the seeping
    water acting vertically with the effect of trying to lift up the body of barrage. Therefore,
    in case of gravity floors, the thickness of aprons or the glacis must be of greater weight
    than the uplift pressure. The problem is the exact determination of the uplift.

2. Piping /Undermining:

    There is a second way in which the seepage flow underneath the structure may danger its
    stability and is called as piping phenomenon. When the seepage velocity in microscopic
    flow channels in the subsoil under the apron is such that the seepage force at the exit
    point becomes greater than the submerged weight and friction of the soil, very fine soil
    particles become displaced. This can be observed as muddy water emerging from the soil
    surface. With this continuing process and a subsoil consisting of fine particles
    surrounding large particles, the removal fine particles causes unequal settlement of the
    subsoil and ultimately the collapse of the structure due piping. The river discharge over
    the barrage further aggravates the situation by washing away the loosened soil due to
    excessive exit gradient.

    The problem consists, therefore, in controlling the seepage force so that it cannot carry
    away the foundation material. Various theories are developed to solve these two
    problems.

       Bligh‟s Creep Theory
       Lane‟s Weighted Creep Theory
       Khosla‟s Method of Independent Variable




Bligh‟s Creep Theory:
The percolating water creeps along the profile of the bottom of hydraulic structure which is in-contact with the
soil.
Creep Length:
The path traced by the percolating water is known as creep length.
Hydraulic Gradient:
The loss of the head per unit creep length is known as hydraulic gradient and it is constant throughout its
passage. The head loss is inversely proportional to the creep length.



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Let,



Then according to the Bligh‟s creep theory,




Depth of every cutoff (sheet pile) is multiplied by a coefficient 2.
According to the Bligh‟s creep theory,


Where,



                                                     So,
Therefore,
The reciprocal of hydraulic gradient        is known as Bligh‟s coefficient which is denoted by C.


Safety Against Piping:
The creep length should be sufficient to provide the safe hydraulic gradient according to the type of soil.
According to the Bligh‟s creep theory, if          then there will be no danger of piping. The values of Bligh‟s
coefficient C for different type of soils as suggested by the Bligh‟s are,

                       Type of Soil                                         Bligh‟s coefficient „C‟
                   Very fine sand / Silt                                              18
                        Fine Sand                                                     15
                       Coarse Sand                                                    12
                      Gravel + Sand                                                    9
                  Boulder + Gravel + Sand                                           (4 – 6)
                         Clay Soil                                                 (1.6 – 3)
                         Soft Clay                                                     3
                       Medium Clay                                                     2
                        Hard Clay                                                     1.8
                      Very Hard Clay                                                  1.6


Practical Application (Safety Against Piping):
If,                   ,
Then check the stability for fine sand, coarse sand and clay soil?

Solution:
For fine sand,




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For fine sand value of


                                                                         (Not Ok)
For Coarse Sand,


For coarse sand value of


                                                                         (Not Ok)
For Clay soil,


For clay soil value of


                                                                         (Ok)
Exit Velocity:
The percolating water emerges at d/s end of the flow with the velocity is called exit velocity.

Safety against uplift pressure:
Computation of the uplift pressure:
Underneath the floor, uplift pressure at various points is of primary importance. The uplift pressure exerted by
the water at any point is,
                                                                 ------------ (1)
Where,




Where,




                                                                                      ------------- (2)
Comparing equation (1) & (2),


Subtracting from both sides,


                                                                                      [As:                ]


Where,



           is applied on above expression therefore,


This equation gives the floor thickness ‟t‟ at any point of the floor where the uplift pressure ordinate measure
above the top of the floor is „h‟.

Practical application (safety against uplift pressure):



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The figure shows the section of hydraulic structure founded on sand. Calculate the average hydraulic gradient.
Also find the uplift pressure at 6m, 12m and 18m from the upstream end of the floor. Find the thickness of the
floor.
Given data:



Solution:




Uplift pressure:
    1. At point A,




    2.   At point B,




    3.   At point C,




Thickness of floor:




    1.   Thickness of floor at A,



    2.   Thickness of floor at B,



    3.   Thickness of floor at C,



LANE‟S CREEP THEORY:

According to lane weighted creep theory the effective creep length is calculated by
multiplying the vertical offsets by 3 and adding it to the horizontal length. Lane suggested the
following formula for computing the affective creep length is,



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Where,
S = the sum of all the horizontal contacts and of all the sloping contacts less than 45 degrees
V= the sum of all the vertical contacts plus the sloping contacts greater than 45 degrees

Sr. No.                      Type of Soil                                              c
  1.                    Very fine Sand                                               8.5
  2.                      Fine Sand                                                  7.0
  3.                     Coarse Sand                                                 5.0
  4.                   Gravel and Sand                                              3 to 4
  5.               Boulders + Gravel + Sand                                        2.5 to 3
  6.                         Clay                                                  1.6 to 3

Safety against piping:

To ensure safety against piping the average hydraulic gradient H/L must not exceed 1/c.




Khosla‟s Theory:
Structure stability of Dam against piping and uplift pressure, is checked by Bligh‟s theory and Lane‟s theory. If
structure is failed by any of the above mentioned theories then the data is adjusted by the Khosla‟s theory to
make it safe.
Calculation of creep length by Bligh‟s creep theory:




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Safety against piping:
For fine sand,




So structure is unsafe against piping.
Calculation of creep length by Lane‟s weighted creep theory:


Safety against piping:
For fine sand,




So structure is safe against piping.
Now to make structure safe against piping in accordance with Bligh‟s creep theory, we will apply Khosla‟s
theory. To adjust the data for this we will increase the creep length to reduce the value of (H/L). The desired
creep length for the safety of structure against piping in accordance with Bligh‟s creep theory can be found as,




To adjust this increment in length, we increase the creep length by adding one more sheet pile of depth 12m. By
this change of 24m will be accommodated. But still an increment of                               is required. This
further desired increment is achieved by increasing the horizontal floor length. Let we increase u/s floor length
by 7m and d/s floor length by 6.4265m.
 Exit Gradient:
Exit gradient at d/s end of an impervious floor length     and the cut-off    is given by,


Where,




According to Khosla‟s theory the range of exit gradient is




DESIGN OF A BARRAGE:

There are two aspects of the design of a barrage i.e.

1. Surface flow / Overflow consideration


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2. Safety against subsoil flow i.e. (by Bligh’s creep theory, Lane’s weighted creep theory and
Khosla’s theory)

1. SURFACE FLOW / OVERFLOW CONSIDERATION:

Following items have to be estimated / designed in case of overflow considerations,

1. Estimation of design flood.
2. Length of barrage i.e. (Width between abutments)
3. Retrogression.
4. Barrage profile. i.e. U/S floor level, D/S floor level, crest level

1. Estimation of design flood:

The design flood (maximum flood) is estimated for which the barrage is to be designed
depending upon the life of structure. The design flood estimation may be for 50 years, 100
years etc.

2. Length of Barrage (Width b/w Abutments):

Lacey‟s formula can be used for fixing the length of barrage

i.e.

Where,
Pw = Wetted perimeter
Q= Maximum flood discharge

From t the length of barrage can be evaluated as,

Length of barrage =

Where,
L.L.C = Lacey‟s looseness coefficient
Take L.L.C = 1.8, if not mentioned

3. Retrogression:

It is a temporary phenomenon which occurs after the construction of barrage in the river
flowing through alluvial soil. As a result of back water effect and increase in the depth, the
velocity of water decreases resulting in deposition of sedimentation load. The water flowing
through the barrage have less silt, so water picks up silt from downstream bed. This results in
lowering d/s river bed to a few miles. This is known as retrogression.
It may occur for the first few years and bed levels often recover their previous level. Within a
few years, water flowing over the weir has a normal silt load and this cycle reverses. Then
due to greater depth, silt is deposited and d/s bed recovers to equilibrium. Retrogression value
is minimum for flood discharge and maximum for low discharge. The values vary
ft.




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4. Accretion:

It is the reverse of retrogression and normally occurs upstream, although it may occur d/s
after the retrogression cycle is complete.
There is no accurate method for calculating the values of retrogression and accretion but the
values which have been calculated from different barrages can be used as a guideline.

5. Barrage profile:

    Crest level:
   The crest level is fixed by the total head required to pass the design flood over the crest.
   The pond level is taken as the H.F.L.
   Maximum scour depth can be calculated from Lacey‟s scour formula,

                      (M.K.S)

   R=0.9         (F.P.S)




                                       Taken as 2.03(M.K.S)




    Scour Protection:
   u/s scour protection
   d/s scour protection




EXAMPLE No. 01:



SOLUTION:



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According to Laceys formula,




EXAMPLE NO.02:

Calculate the crest level of main weir and under sluice for a gated diversion structure for the
following data,




SOLUTION:

Discharge per unit length of barrage,



Maximum scour depth,




Approach Velocity,


Velocity Head,



Total Energy Head,




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Now from figure,




The crest level of undersluice is usually kept 1m below than that of main weir, i.e.



Hence the required result.




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