irrigation engineering part 2

Muhammad Farooq Zia (2005-CE-44) Muhammad Sajid Nazir (2005-CE-

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CANAL FALLS

INTRODUCTION:

Generally the slope of the natural ground surface is not uniform throughout the alignment. Sometimes the ground surface may be steep and sometimes it may be very irregular with abrupt change of grade. In such cases a vertical drop is proved to step down the canal bed and then it is constituted with permissible slope until another step down is necessary. Such vertical drops are known as canal falls or simply falls.

PURPOSES:

1. To account for the difference in the natural bed slope of the canal.2. To save us from cutting and filling.3. To Increase the velocity of the water in the canal.4. To control the seepage in the canal.

NECESSITY OF CANAL FALLS:

The canal falls are necessary in case the following conditions occur:

(a) When the slope of the ground suddenly changes to steeper slope, the permissible bed slope cannot be maintained. In such cases canal falls are provided to avoid excessive earth work in filling. (Fig. 14.1 (a))

(b) When the slope of the ground is more or less uniform and the slope is greater than the permissible slope of the canal. In such case also the canal falls are necessary (Fig. 14.1 (b))

(c)In cross drainage works when the difference between bed level of canal and that of drainage is same or when flood surface level of the canal is above the bed level of drainage then the canal fall is necessary to carry the canal water below the stream or drainage (i.e. in case of siphon super passage) (Fig. 14.1 (c))

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TYPES OF CANAL FALLS:

The different types of canal falls are, 1. Ogee fall2. Rapid fall3. Stepped Fall4. Trapezoidal Notch Fall5. Vertical Drop or Sarda Fall6. Glacis Fall

1. Ogee fall:

In this type fall an ogee (a combination of convex curve and concave curve) is provided for carrying the canal water from higher level to the lower level. This fall is recommended when the natural ground surface suddenly changes to a steeper slope along the alignment of the canal.

2. Rapid fall:

The rapid fall is suitable when the slope of the natural ground surface is even and long. It consists of a long sloping glacis with longitudinal slope which varies from 1 in 10 to 1 in 20.

3. Stepped Fall:

Stepped fall consists of vertical drops in the form of steps. This fall is suitable in places where the sloping ground is very long and requires long glacis to connect the higher bed with the lower bed level. This fall is practically a modification of the rapid fall. Here the sloping glacis is divided into a number of drops so that the flowing water may not cause any damage to the canal bed.

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4. Trapezoidal Notch Fall:

In this type of fall a body wall is constructed across the canal. The body wall consists of several trapezoidal notches between the side piers and the intermediate pier or piers. The sills of the notches are kept at the upstream bed level of the canal. The size and number of notches depends upon the full supply discharge of the canal. (Fig. 14.5)

5. Vertical Drop or Sarda Fall:

It consists of a vertical drop wall which is constructed with masonry work. The water flows over the crest of the wall. A water cistern is provided on the downstream side which acts as a water cushion to dissipate the energy of falling water.Hence it is sometimes known as sarda fall.(Fig.14.6)

6. Glacis Fall:

It consists of a straight sloping glacis provided with a crest. A water cushion is provided on the downstream side to dissipate the energy of flowing water. Curtain walls and toe walls are provided on the upstream and downstream side. This type of fall is suitable for drops up to 1.5 m (Fig.14.7).

This type of fall may be of the two types,(i) Montague type(ii) Inglis type fall

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(i) Montague type:

In this type of fall the straight sloping glacis is modified by giving parabolic shape which is known as Montague profile.

(ii) Inglis type fall:

In this type of fall the glacis is straight and sloping but baffle walls are provided on the downstream floor to dissipate the energy of flowing water . The height of baffle depends on the head of water on the upstream side.

TYPES OF CANAL FALLS USED IN PAKISTAN:

Sarda falls are normally used in Pakistan.

Design of Sarda Fall:

Step 1:

Total fall (Hw )=U / S(Full supply level) – D / S(Full supply level)

Step 2:Crest length of fall ( L )=Widthof channel

Crest width of fall (B )=0.55√ H+W (For Rectangular fall )Crest width of fall (B )=0.45√ H+W (For Trapezoidal fall )

Where,

H=Head above the crest<Total fall ( H w )D=Normal water depth(Full supply)

Step 3:

Discharge (Q)=0.415√2 g L× H32 × ¿

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Step 4:

Velocity of reach(V )=QA

Step 5:

Velocity Head (ha)=V 2

2 g

Effective Head (H e)=H + V 2

2 g

Step 6:

Total energy level (U / S)=(U /S )Water level+ V 2

2 g

Step 7:Crest level=Total energy level (U / S)– H

Step 8:

Height of crest above (U / S)bed level (hc)=Crest level – (U /S)Bed level

Step 9:Hydraulic Head (H h)=Crest level – (D /S )Bed level

Step 10:

Total Length(LT )=Hydraulic Head (H h)× Bligh ’ scoefficient (c )

Step 11:Length of cistern(L)=5 ×√Hw × H e

Step 12:Depthof cistern(x)=0.25 ×¿

Step 13:Cistern level=(D /S)Bed level – Depth of cistern( x)

Step 14:(U /S)Pakka floor length=Total Length (LT )– Creep Length (Lc )

Step 15:(D / S)Stone pitching length=10 H e+2 H w

Step 16:

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The (U /S )wingwalls are provided∈acircular shape with Radius

R=6 × H e

Step 17:Length of (D /S )Wing wall=8×√ H × Hw

Example:

Design a Sarda type fall for the following set of data

Full supply discharge=14 m3/ sec

Bed width=18 m

Full supply depth(FSD )=1.5m

Full supply level (U /S)=101.00 m

Full supply level (D /S)=100.00m

(U /S)Bed level=99.5m

(D / S)Bed level=98.5m

Natural surface level=99.5 m(D /S )

Bligh’ sCoefficient (c)=8

Solution:

Step 1:

Total fall ( Hw )=U /S ( Full supply level) – D /S ( Full supply level )¿101 – 100=1m

Step 2:

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Crest length of fall ( L )=Widthof channel

¿18 mCrest width of fall (B)=0.55√ H+W (For Rectangular fall)

Where,

H=Head above the crest<Total fall (Hw)D=Normal water depth(Full supply)

Assumingsuitable value for (H)e .g . 0.7 m<(H w)1m

(U /S) Depth(D)=Full su pply level (U / S)– N . S . LD=101 – 99.5=1.5 m

Crest width of fall (B)=0.55√0.7+1.5=0.815 m≈ 1m

Step 3:

Discharge (Q)=0.415√2 g × L × H32 ×¿14=0.415√2× 9.81 ×18 × H

32 ×¿14=33.09 H

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H=0.6 m

Step 4:

Velocity of reach(V )=QA

A=(18+18+1.5+1.5)

2× 1.5=29.25 m

V= 1429.25

=0.48 m/ sec

Step 5:

Velocity Head (ha)=V 2

2 g= 0.482

2 ×9.81=0.012 m

Effecti ve Head (H e )=H+ V 2

2 g=0.6+0.012=0.612 m

Step 6:

Total energy level (U / S)=(U /S )Water level+ V 2

2 g

¿101+0.012=101.012 m

Step 7:Crest level=Total energy level (U / S)– H

¿101.012 – 0.6=100.41 m

Step 8:Height of crest above (U / S)bed level (hc)=Crest level – (U /S)Bed level

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¿100.41 – 99.5=0.91m

Step 9:Hydraulic Head (H h)=Crest level – (D /S )Bed level

¿100.41 – 98.5=1.91m

Step 10:

Total Length (LT )=Hydraulic Head (H h)× Bligh ’ sconstant (c)

¿1.91 ×8=15.28 m

Step 11:Length of cistern(L)=5 ×√Hw × H e

¿5 ×√1× 0.612=3.91 m

Step 12:Depthof cistern(x)=0.25 ×¿

¿0.25 ×¿

Step 13:Cistern level=(D /S)Bed level – Depth of cistern( x)

¿98.5 – 0.18=98.32 m

Step 14:(U /S)Pakka floor length=Total Length (LT )– Creep Length (Lc )

¿15.28 – (1.2+0.3+1+0.3+4+0.2)=8.28m≈ 8.5m

At end theoretically the impervious floor should be finished butfor safety we extend this distance by x .

Step 15:(D / S)Stone pitching length=10 H e+2 H w

¿(10× 0.612)+(2×1)=8.12≈ 8.5 m

Step 16:

R=6 × H e=6 × 0.612=3.7 m

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Step 17:

Length of (D /S )wingwalls are kept straight up¿adistance of ,

Length of (D /S )Wing wall=8×√ H × Hw

¿8 ×√0.6 × 1=6.2 m

SITE SELECTION / LOCATION OF CANAL FALLS:

The location of falls is decided from the following considerations,(i) For the canals which do not irrigate the area directly the fall should be located from the considerations of economy in cost and excavation of the channel with regard to balancing the depth and the cost of fall itself.(ii)For a canal irrigating the area directly a fall may be provided at a location where F.S.L outstrips the ground level, but before the bed of the canal comes into filling.(iii)The location of the fall may also be decided from the consideration of the possibility of combining it with a regulator or a bridge or any other masonry works.(iv)A relative economy of providing large number of small falls v/s small number of big falls should be worked out. The provision of small number of big falls results in unbalanced earthwork, but there is always some saving in the cost of the fall structure.(v) For a minor or distributory, falls may be located on the downstream of the outlets as this helps in increasing the command area and improving the efficiency of the outlet.

CANAL OUTLETS:

An outlet is a hydraulic structure convening irrigation water from a state owned channel i.e (distributary, minor etc) to a privately owned water course.It is basically the last hydraulic structure at the end of irrigation system.

TYPES OF OUTLETS:

Outlets may be classified as,

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1. The non-Modular Outlet2. Semi-Modular / Flexible Outlet3. Modular/Rigid Outlet

1. The non-Modular Outlet:

A non-modular outlet is the one in which the discharge depends upon the difference in level between the water level in the distributory channel and water course. Common examples are orifices and wooden shoots etc.

2. Semi-Modular / Flexible Outlet:

A flexible outlet or semi-modular is the one in which the discharge is affected by the fluctuations in the water level of the distributory channel only. Common examples are, pipe outlet, kennedy,s gauge outlet etc.

3. Modular/Rigid Outlet:

A Rigid modular is the one which maintain s it s discharge constant, within limits, irrespective of the fluctuations in the water level of the distributor channel and or field channel. Common examples are Gibbs Rigid Module, Khanna Module, Ghafoors module.

CHARACTERISTICS OF AN OUTLET:

The design of an outlet depends upon its performance while the performance depends upon the characteristics of outlet. Following are the characteristics of an outlet,

1. Flexibility of an outlet (F)2. Sensitivity of an outlet (S)3. Minimum Modular Head (H min)4. Silt Drawing Capacity 5. Safety against Tempering

1. Flexibility of an outlet (F):

It is the ration of the rate of change of discharge of outlet (dq /q) to the rate of change of discharge in the parent channel i.e. distributory (dQ /Q).Mathematically,

F=(dq /q )(dQ /Q)

Where, q= discharge in outletQ= discharge in distributory F= FlexibilityIf, F=1 the outlet is called as “Proportional Outlet”.F<1 the outlet is called as “Sub-Proportional Outlet”.F>1 the outlet is called as “Hyper-Proportional Outlet”.

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2. Sensitivity of an outlet (S):

It is the ratio of change of discharge in the outlet to the rate of change of water level in the parent channel. Mathematically,

S=(dq /q)(dG /G)

Where, S= Sensitivityq=Discharge in the outlet G=Water level of parent channel

3. Minimum Modular Head (H min):

The necessary minimum difference of water level or pressure between supply and delivery sides to enabler a module or semi-module to work as designed. I t is the minimum head required for normal functioning of outlet. It is taken as 10 -20 % of water head in parent channel i.e.

Hmin=(10−20 %)G

4. Silt Drawing Capacity:

It is vital that outlets draw their fair share of silt. This avoids silting or scouring and consequently remodeling of the distributory.In a distributory system absorption losses are generally taken as 10-15 % and therefore, the silt conducting power of outlets should be around 110-115 % as compared to 100 % of distributory to enable them to draw their proportional share .It depends on two factors,

1. Transition curve2. Setting back

5. Safety against Tempering:

There is a tendency on the part of the cultivators to draw more than their lawful share of water by tempering with the outlets. Therefore the outlets must be tamper proof. Most of the semi-modules depending upon the formation of hydraulic jump are quite tamper proof.

6. Adjustability:

Readjustments of outlets are required in view of the revision of areas under command and because of the changed conditions in distributory.

7. Coefficient of discharge:

In order to use the outlet as measuring device the coefficient of discharge should remain constant in the full modular range.

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8. Modular limits:

Modular limits are the limits beyond which an outlet is incapable of acting as a module or semi-module.

9. Modular Range:

It is the range of various factors between the modular limits within which a module or semi module works as designed.

10. Drowning Ratio:

It is the ratio between the depths of water level over crest on the downstream and upstream of the module .i.e. Water level above crest level downstream / Water level above crest upstream/

11. Efficiency:

It is defined as the ratio of the head recovered to the head input.

12. Setting of an outlet:

It is the ratio of the head acting on the outlet to the full supply depth of the distributory channel, where the head acting on the outlet is equal to the crest level of module below the full supply of the distributory channel.Mathematically,

Setting=HD

SELECTION OF THE TYPE OF OUTLET:

When the discharge and the water levels are likely to change the following points must be noted in selecting the type of outlet to be use,

1. For a temporary discharge variation a proportional semi-module is desirable to distribute both excess and deficiency in the parent channel.2. Seasonal variation in the slope requires the use of outlets with low flexibility i.e. sub-proportional.3. For channel running with full supply for a certain period and remaining closed for a certain other period (rotational running) it is desirable to have a high flexibility, i.e. hyper-proportional .4. The silt drawing capacity of outlet must be 110-115 % assuming a 10-15 % loss in the parent channel. According to Sharma it’s obtained in following cases,(i) OSM with a setting of 0.9 D.(ii) OSM with sharma’s approach at setting of 0.8 D.(iii) Crump’s or standard design open flume outlet set at bed level.5. In general rigid module is desirable in the following cases, (i) Direct outlets on a branch canal subject to variation in supply.

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(ii) Above stop dams where heading up is created to feed other canals.(iii) In channels which sometimes carry extra discharge for specific reasons like leaching.6. For other cases than those mentioned above semi module outlets are desirable.

In the choice of selecting the type of outlet the available working head is an important criterion.

DESIGN OF OUTLET:

Step1: Flexibility

Suppose outlet is proportional i.e. F=1

Step2: Setting of outlet

Calculate water level in parent channel

F=0.9DG

1=0.9DG

G=0.9 D

Step3: Throat Width “Bt

q=K B t G32

Where

K=Coefficient of outletK=23

Cd √2 g w h ere Cd=0.64

K=3.088 (T h eoratical value )K=2.9↔ 3 ( Practical value )

Value of Bt lies in the range 0.20 ft ↔ 0.29 ft

Step4: Minimum Modular Head, Hmin

Hmin=0.2 GThe value of Hmin must be less than available working head otherwise revise the value ofG.

Step5: Length of the Crest or outlet

L=0.25G

Step6: Radius of transitionR=2G

Step7: Setting back length

Settingback lengt h= qQ

× Widt h of c h annel

If width of channel is not given then assume its value.

Step8: Efficiency

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Efficiency ,η=Head recovered at d /sHead put∈at u /s ¿(G−Hmin

G )×100

Question: Design an outlet for the following data,Q=4 cusecsF . S dept h=3.5 ’Distributory Disc harge ,Q=60 cusecsWorking Head=1 ’S=1 ft /canal mile

Solution:

Step1: Flexibility

Suppose outlet is proportional i.e. F=1

Step2: Setting of outlet

Calculate water level in parent channel,For open flume outlet,

F=0.9DG

1=0.9DG

G=0.9 DG=0.9× 3.5=3.15 ft

Step3: Throat Width “Bt

q=K B t G32

Where

K=Coefficient of outletK=23

Cd √2 g w here Cd=0.64 K=3.088 (T h eoratical value )

K=2.9↔ 3 ( Practical value )

Assume value of K=2.95 for our case

q=K B t G324=2.95 × B t ×3.15

32 Bt=0.2425 ft

Value of Bt lies in the range 0.20 ft ↔ 0.29 ft

Step4: Minimum Modular Head, Hmin

Hmin=0.2 GHmin=0.2 ×3.15=0.63 ft<1 ft O . K

The value of Hmin must be less than available working head otherwise revise the value ofG.

Step5: Length of the Crest or outlet

L=2.5 GL=2.5 × 3.15=7.875 ft

Step6: Radius of transition

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R=2GR=2× 3.15=6.30 ft

Step7: Setting back length

Let t hewidt hof ch annel , B=1 ft

Settingback lengt h= qQ

× Widt h of c h annel¿ 460

×15¿1 ft

If width of channel is not given then assume its value.

Step8: Efficiency

Efficiency ,η=(G−Hmin

G )×100¿( 3.15−13.15 )×100¿68.25 %

PARTICIPATORY IRRIGATION METHOD

BACK GROUND:

The barrages, head works and allied structures, main canals, branches, distributaries and minor channels are being maintained by the Provincial Irrigation Departments. The responsibility of construction and maintenance of the water courses rests with the On Farm Management Directorate and the water users. Mutual disputes regarding use of canal water among the users of water course are resolved by the Department under the provisions of the Canal and Drainage Act 1873.

PRESENT SITUATION:

The major problems are:

1. Insufficient availability of canal water in comparison to its demand. 2. Absence of reliable and equitable distribution of canal water.3. Delays in settlement of disputes on canal water distribution.4. Insufficient up-keep of distributaries, minors, water course leading to physical deterioration of the system.5. Wastage of water on watercourse and distribution channels.

WHAT IS PARTICIPATORY IRRIGATION MANAGEMENT?

Participatory Irrigation Management or PIM refers to the involvement of irrigation users or beneficiaries in all aspects of irrigation management and at all levels. Management approaches in irrigation general fall into three categories:

1. Public Sector Management

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2. Private Sector Management3. User’s Organization

The last type can be termed as PIM. This concept of PIM is quite different from “Privatization” in that we are talking about transferring management not to a third party “Owner” who would purchase the irrigation system from the government and then hire out irrigation services to farmers. Rather, the PIM concept is similar to an employee owned business that gives equal share.

IMPLEMENTATION OF PARTICIPATORY IRRIGATION MANAGEMENT:

These are some common issues to consider, such as:

1. Creating an enabling environment2. The legal framework3. Start-up pilots and expansion phases.

CREATING AN ENABLING ENVIRONMENT:

For participation to work, the government, the major “stakeholder” in national irrigation sector, must be willing and interested. At least three sector of the government must be willing to support PIM:

1. Political Leadership 2. Administrative Leadership 3. Irrigation agency Leadership

THE LEGAL FRAMEWORK FOR PIM:

The legal framework for the establishment of WUAs, and for enabling operate and maintain such parts of irrigation system, consist basically sets of legal instruments, namely:

1. The enabling law2. The bylaws of the WUA and3. The transfer agreement between the irrigation agency and the WUAs.

The enabling law:

For PIM to be established as a legal entity there has to be a law authorizing its establishment.

Bylaws of the water user association/water user federation:

Whether established under a separate law or under an umbrella enabling law, the authority would normally be required to prepare and agree on its bylaws before it can be registered as a legal entity.

The transfer agreement:

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The transfer agreement between the authority and the irrigation agency in which the irrigation agency agrees to transfer the responsibilities for operation and maintenance of certain parts of the irrigation system, including the drainage system and the collection and remitting of water charges and the WUA to carry out such responsibilities.

CONCLUSION

There are certain essentials and pre-requisites for the successful implementation of PIM that include the following.

Keep the users intimately involved at all levels and give them ample/real opportunities to agree or disagree and to suggest amendments.

Try to build on the existing community structures. Ensure that the institutional changes are compatible with the local setting and implementable in the context of socio-political environment and management capabilities of the farming community.

Develop a well defined implementation mechanism along with enabling legislation, appropriate bylaws and an effective regulatory framework.

Training of the WUAs/WUFs and the agency staff in order to enable them to appreciate their new roles and to efficiently perform the designated functions.

Setting up of appropriate financial controls and accountability mechanism is particularly relevant in the backdrop of low literacy rates, back of FOs experience and concerns of financial mismanagement related to the local bodies and cooperative in the past.

The pilot projects must be designed cautiously. This pilot design should ensure representativeness, replicability and sustainability. An independent monitoring and evaluation mechanism needs to be set up in order to learn from the implementation of pilot projects and to make necessary adjustments in the overall plans.

MAJOR FUNCTIONS OF PIDA:

The major functions of the authority would include the following:

To perform all functions and duties of the Irrigation Wing of I & P Department; To plan, design, construct, operate and maintain the irrigation, drainage and the flood

control infrastructure located within its territorial jurisdiction; To improve effective and efficient utilization of irrigation water and its disposal; To introduce the concept of participatory management through the pilot AWB and

FOs; Also to adopt and implement policies aimed at promoting formation, growth and development of FOs and monitoring of its performance;

Measure for reducing O & M expenditure and improving maintenance planning; Measure to improve assessment and collection of water rates and drainage access; To make the authority financially self-sustaining with regard to the O & M cost of

canal irrigation and drainage within a period of 7 to 10 years.

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WARABANDI

MEANING OF WARABANDI:

WARABANDI is a rotational method for equitable distribution of the available water in an irrigation system by turns fixed according to a predetermined schedule specifying year, day, time and duration of supply to each irrigator in proportion to the size of his landholding in the outlet command. The term WARABANDI means turns (Wahr) which are fixed (bandi).The cycle begins at the head and proceeds to the tail of the water course, and during each turn, the farmer has the right to use all of the water flowing in the water course. A central irrigation agency manages the primary main canal system and its secondary level distributary and minor canals and delivers water at the head of the tertiary level water course through an outlet popularly known as a mogha which is designed to provide a quantity of water proportional to the CCA of the water course.

FUNCTIONS AND CHARACTERISTICS:

The warabandi system in Pakistan includes the following functions and characteristics, among other things,1. The main canal distributing points operate at supply levels that would allow distributory canals to operate at no less than 75 percent of full supply level.2. Only authorized outlets draw their allotted share of water from the distributary at the same time 3. Outlets are ungated and deliver a flow of water proportional to the area command.

KACHCHA WARABANDI AND PUCCA WARABANDI :

Today, two types of warabandi are frequently mentioned in Pakistan. The warabandi which has been decided by the farmers solely on their mutual agreement, without formal involvement of any government agency is known as kachcha ( ordinary or unregulated)

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warabandi , whereas , the warabandi decided after field investigation and public inquiry by the irrigation department when disputes occurred and issued in officially recognized warabandi schedules, is called pucca warabandi.

OBJECTIVES OF WARABANDI:

As an integrated water management system, warabandi is expected to achieve two main objectives, 1. High efficiency 2. Equity in water use Water use efficiency is to be achieved through the imposition of water scarcity on each and every user. And equity in distributing through enforces equal share of scarce water per unit area among all users.

Consequently relying on the many virtues of warabandi as theoretically framed, particularly its fairness, the tendency in to believe that warabandi ensures equity in distribution to each farmers field regardless of whether the land is situated at the upper reaches of the outlet or at the tail end, whether the farmer is economically or politically powerful or not and whether he belongs to a low or high caste. In Pakistan too, the equity in water management is commonly perceived as the central operational objective for the management of its large canal system through warabandi. The average water duty or water allowance designed for this system is around 4 cusecs for 1000 acres.

CALCULATION OF WARABANDI SCHEDULES:

Theoretically in calculating the duration of warabandi turn given to a particular farm plot, some allowance is added to compensate for the time taken by the flow to fill that part of the water course leading to the farm plot. This is called water course khal bharai (filling time). Similarly, in some cases, a farm plot may continue to receive water from a filled portion of the watercourse even when it is blocked upstream to divert water to another farm or another part of the water course command. This is called nikal (draining time), and is deducted from the turn duration of that farm plot. The calculation for a warabandi schedule starts with determining by observation, the total of such filling times (T F) and the total of such draining times (T D). Then, for a weekly warabandi rotation, the unit irrigation time (T U) in hours per hectare can be given by:

T U=(168 – T F +T D)

C

Where,C=Culturable command area of the watercourseThe value of T U should be the same for all the farmers in the watercourse. A farmer’s warabandi turn time T t is given by:

T t=T U × A+T f – Td

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Where, A=farm areaT f =filling time for the farmareaT d=draining time for the farmarea

Only some of the farms in a watercourse may be entitled to either filling time or draining time or both.

IRRIGATION METHODS AND PRACTICES

Following are the methods of irrigation that are applied in the field,

1. Surface Irrigation Methods(i)Flood Irrigation(ii) Furrow Method (iii) Basin Irrigation 2. Sub-Surface Method(i)Drip Irrigation3. Sprinkler Method

1. SURFACE IRRIGATION METHODS:

(i) Flood Irrigation:

In this method water is spread or flooded on a rather smooth flat land without much control or prior preparation. It is of two types controlled flood irrigation in which water is controlled and uncontrolled in which water is not controlled. This method is practiced largely where irrigation water is abundant and inexpensive.

(ii) Furrow method:

In this method only one half to one fifth of the surface is wetted and thus evaporation losses are less. A furrow consists of a narrow ditch between rows of plants. The lengths of furrows vary from 3.0 m for gardens up till 500 m for field crops. The general slopes provided for furrows may vary from 0.2 to 5 %.

(iii) Basin method:

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This method is employed for watering orchards. In this method each tree or a group of trees are enclosed by circular channel through which water flows. The circular channel is known as basin. Each basin is connected to field channel. When all the basins are filled with water the supply of water is stopped.

2. SUB SURFACE METHOD

(i) Drip irrigation:

In this method the water is applied to the root zone of the crops by underground network of pipes. The perforated pipes allow the water to drip out slowly and thus the soil below the root zone of the crops absorb water continuously. This method is suitable for permeable soil like sandy soil. It is also known as trickle method of irrigation.

3. SPRINKLER METHOD:

In this method the water is applied to the land in the form of spray like rain. The greatest advantage of sprinkler irrigation is its adaptabilities to use under conditions where surface irrigation methods are not efficient. The following are different kinds of sprinklers,

(a) Perforation on lateral pipes(b) Fixed nozzles on lateral pipes(c) Rotating Sprinklers

(a) Perforation on lateral pipes:

In this type the lateral pipes are perforated along the top and sides. The water comes out through the perforations in all directions in the form of spray. The lateral pipes are supported on pillars.

(b) Fixed nozzles on lateral pipes:

In this type a series of nozzles are fixed along the lateral pipes. The lateral pipes are supported on pillars. When the water is forced under pressure through the network of pipes, it comes out as fountain through the nozzles and spreads over the land.

(c) Rotating Sprinklers:

In this type the riser pipes are fixed on the lateral pipes at regular intervals. On the top of the riser pipe are two arms which can rotate about a vertical axis. When the water is forced under pressure through

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the pipes it rises up and comes through the nozzles in the form of spray. As the arms rotate a circular area is covered by each riser.

WATER LOGGING AND SALINITY CHALLENGES IN PAKISTAN & THEIR POSSIBLE REMEDIAL MEASURES

INTRODUCTION:

Water logging and salinity are a twin menace to agriculture.

WATER LOGGING:

It is the presence of water in the zone of roots of plants up to 5 ft depth by raising the water table due to which growth of crops stop. Its best example in Southern Punjab the land in the surrounding of Indus River at Ghazi Ghat which is nearly up to 0.5 km from the banks of the Indus River.

REASONS OF WATER LOGGING:

Following are the main reasons of losses in irrigation due to water logging.

Over irrigation Seepage from canals Inadequate surface drainage Obstruction in natural water course Obstruction in sub-soil drainage Nature of soil Incorrect method of cultivation Seepage from reservoir Poor irrigation management Excessive rain fall Flat country slope Topography of land

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These reasons are discussed one by one.

1. Over Irrigation:

It means more water is provided then required due to which proper growth of crop is not possible.

2. Seepage from Canal:

Through seepage from canal on the banks, water level came to rise and the land becomes not suitable for irrigation purpose.

3. Inadequate Surface Drainage:

It means two types of canals, one of which is a “lined canal”. It will not affect the land in surrounding. On the other hand “an unlined canal” will fully affect the land in surrounding in the form of seepage due to which this effected land provides fewer crops.

4. Obstruction In Natural Water Course:

If obstruction in natural water course (rivers) is present, blockage condition of rain water or other water created are produced and water store on the surface in the form of pounds. It affects the human health along with cultivation of crops on this land.

5. Obstruction in sub-soil drainage:

Sometime in sub-soil, a hard impervious layer acts as an obstacle due to which water table rises up to roots of the crops. As a result water logging conditions are generated.

6. Nature of Soil:

A pervious soil provides more water to the crops due to which water logging conditions are generated. On the other hand impervious soil strata provide more friction in the way of water flow due to less openings or voids.

7. Incorrect Method of Cultivation:

It means a farmer must have full knowledge about the crop which is to be cultivated. As a result of which he irrigates his crop according to the requirement. On the other hand a non-expert farmer provide more water or not according to the requirement which results in water logging.

8. Seepage from Reservoir:

Due to storage of water with high head seepage conditions are created not usually in hilly areas.

9. Poor Irrigation Management:

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It means management of irrigation supplies a large quantity of water for irrigation to a small area.

10. Excessive Rain Fall:

It is another reason of water logging. In area which is fully flat and has no proper drainage system for excessive rain fall, water starts to store on the surface. As a result water logging conditions are generated.

11. Flat Country Slope:

Some areas have very less slope (flat) due to which flowing speed of water becomes very slow. As a result a large quantity of water penetrates into soil and water table rises which creates water logging conditions.

12. Topography of Land:

Topography of land also affects the water table. In hilly areas conditions of water table are different as compared to the planes areas. PREVENTION OF WATER LOGGING:

Following are the methods through which we can prevent our lands from water logging.

Canal Closures Lowering Full Supply Lining of Canal & Water Courses Provision of Intercepting Drainage Provision of Surface Drainage Pumping Plantation Restricted Irrigation Crop Rotation Methods of Irrigation with Less Water

1. Canal Closures:

Water logging can be controlled by stopping supply of water for certain day for irrigation purpose from the canals.

2. Lowering Full Supply Level:

By lowering of water level from full supply to reasonable head in canals, seepage can be controlled.

3. Lining of Canals & Water Courses:

By providing concrete or brick lining, “water logging due to seepage” can also be controlled.

4. Provision of Intercepting Drains:

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By providing intercepting drains on the banks parallel to canal, water logging due to seepage can be controlled.

5. Provision of Surface Drains:

Surface drains are provided in open flat areas to drain off extra irrigation water and rain water into stream or nala.

6. Pumping:

By pumping we can also lower the water table with pumping pipe at a certain distance from water logged area.

7. Plantation:

By providing trees and other plants on the banks of the canal or river a large quantity of seepage water can be controlled. This results in growth of crops in surrounding area.

8. Restricted Irrigation:

Due to large supply or more then required supply of water to the area to be irrigated, water logging conditions are created.

9. Crop Rotation:

In different seasons, different types of crops should be planted to control the water logging.

10. Methods of Irrigation with Less Water:

To prevent the land from water logging, those methods of irrigation should be adopted in which less water is used for proper growth of crops. For this purpose Drip Irrigation & Sprinkling Irrigation method are used.

SALINITY:

Storage of salts in the root zone and on the earth surface is known as Salinity. In simple words it is called as “Salt Efflorescence”. These salts are NaCl, Na2 SO4, Na2CO3. NaCl and Na2 SO4 are called White Alkalis. These shows white color efflorescence. On the other hand Na2CO3 is called Black Alkalis and very dangerous for irrigation land.

The allowable PH value for irrigation land is 7 to 8.5. The less PH value for irrigation land is 8.5 to 9. The dangerous PH value for irrigation land is 11.0

REASONS OF SALINITY:

Following are the main reasons.

Rising of water table Wet and dry process

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Bad irrigation system Use of faulty water Faulty cultivation Low rainfall

These are briefly discussed below one by one.

1. Rising of Water Table:

Due to different reasons the level of underground water rises and different salts present in sub soil resolve in that water. These salts flow with water and come in contact with roots of plants and surface, results in salinity.

2. Wet & Dry Process:

Salts come to the top surface when soil is wet. When the surface water evaporates or dries a salt layer is generated on the surface which is called salinity.

3. Bad Irrigation System:

If water is provided more than required to an irrigated land, it can also be resulted into salinity.

4. Use of Faulty Water:

If water is supplied to an irrigated land which has salt particles in it, it will result into salinity.

5. Faulty Condition:

Sometimes more and sometimes less water is provided to a crop by non-expert farmers. As a result salts of soil mix with water and cause salinity.

6. Low Rainfall:

By normal rain fall salt particles present on the surface, are mixed with rain water and gradually move downward with rain water. Whereas in case of low rainfall small amount of salt particles move downward with the rain water. This results in salinity of irrigated land.

PREVENTION OF SALINITY:

Prevention of salinity of an irrigated land is done by reclamation of saline soil. This is done by the following two methods.

Temporary Method Permanent Method

These two methods are briefly discussed below.

1. TEMPORARY METHOD:

In temporary method following four methods are adopted for the prevention of salinity.

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Ploughing Deeply Removing Surface Mixing of Good Soil Neutralizing the Salts

i. Ploughing Deeply: In this process 30 cm to 50 cm deep ploughing is done. By using this method salts move to the 50 cm deep layer and do not disturb the growth of crops.

ii. Removing Upper Surface:

Salinity can also be controlled by removing surface soil from 15 cm to 30cm.

iii. Mixing of Good Soil:

By mixing good irrigated soil with the saline soil, effect of salts can also be minimized on the soil. For this purpose sandy clay (silt) is the most suitable.iv. Neutralizing the Salts:

To remove the salinity effects on the soil, we can also use different types of acids and other chemicals. These chemicals are sprayed on soil surface to control salinity.

2. PERMANENT METHOD:

In this method following methods are adopted to prevent salinity.

Lowering Water Table Leaching Crop Rotation Growing Special Plants Use of Chemicals Use of Coal Electro Dialysis Use of Green Manure Use of Waste Products By Earth Worms

These methods are defined below.

i. Lowering of Water Table:

When water first comes in contact with surface soil, it causes water logging and salinity. To lower water table, network of drains is made by filter pipes. When surface water comes to these drains, this is exposed off to stream without affecting the surface layer.

ii. Leaching:

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In this method 15 cm to 25 cm layer of water is stored on the area affected by salinity. As a result salts mix with water and move downward and make the land useful for irrigation. It should keep in mind that stored water must free from salts and other chemicals.

iii. Crop Rotation:

To remove salinity, those types of crops should be cultivated which remove saline from soil. Rice Crop is the most suitable for this purpose. It decreases the quantity of Nitrogen from soil.

iv. Growing Special Plants:

By growing special plants salinity can be prevented. For this purpose Australian Grass and Argemona etc. type of plants are cultivated. These special plants produce acidity due to which PH value of that land is decreased.

v. Use of Chemicals:

Sulphuric Acid, Gypsum and Calcium Chloride can be used to remove salinity.

vi. Use of Coal:

By using coal PH value of cultivated land can also be decreased. Its quantity is used according to salinity effects.

vii. Electro Dialysis:

It is a very costly method for a country like Pakistan. In this process electricity is passed through effected land to control salinity.

viii. Use of Green Manure:

Some special plants which produce green fertilizer are cultivated on the effected land to remove the salinity of that land. For this purpose Patsun and Berseen etc. plants are used.

ix. Use of Waste Products:

Waste solution from sugar mill, Lime solution and Bork of Peanut etc. are also used to decrease the PH value of land.

x. By Earth Worms:

Some types of insects used salts as their food. So by this method we can also control the salinity.

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Design Of Irrigation Canal:

Irrigation canals may be of two types,

Unlined Irrigation Canal:

The irrigation canal prepared by earth is called unlined canal

Lined Irrigation Canal:

The irrigation canal prepared by using different materials of construction i.e. bricks, P.C.C or R.C.C, is called lined irrigation canal.

Lining is indispensable when passing through the porous or sandy tracks. However in Pakistan majority of the canals are unlined whether or not they pass through the sandy soils. They were built during the last few decades and resulting a rise of ground water table thus creating water logging and salinity problems. Recently the irrigation canals are built with lining. Both types of canal are designed for uniform steady flow.

Design Of Unlined Irrigation Canal:

The design of unlined canal which will remain stable is an important challenge for the hydraulic & irrigation engineer. The solution of this problem consists in determining the depth, bed width, side slope and longitudinal slope of the channel so as to produce a non-silting and non-scouring velocity for the given discharge and sediment load.

Design of an irrigation canal implies a section which is stable that neither silts nor scour with the given discharge(Q ), water surface slope (S) and silt charge. When a channel is stable, it means the flow in the channel is uniform steady flow.

Stable Channel:

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When an artificial channel is used to carry the silty water, both the bed and banks scour or fill by changing the depth, gradient and width of the channel until a state of balance is attained at which channel is said to be stable channel. Stable channel is also called as non-silting & non-scouring channel.

Critical Velocity (V o):

A velocity which will keep the silt in suspension without scouring the channel is called critical velocity. It is a standard velocity. It is also called as non-silting & non-scouring velocity. It is denoted by V o .

Critical Velocity Ratio (m ):

It is defined as the mean actual velocity(V ) to the critical velocity(V o). Mathematically,

m=V o

V

The value of ‘m’ ranges b/w(0.9−1.1). In irrigation canal system the value of ‘m’ decreases from head to tail.

Reduced Distance (R . D):

It is the distance from the near/relative headwork and known as reduced distance.

1 R .D=1000 ft∧¿

5 R .D=1 mile ( for canal )

In case of canal one mile is equal to 5000 feet. Reduced distance is taken along the canal.

Bed Slope (S):

It is the bed slope of the channel due to which water flows under gravity. For most of the channels in Punjab, the longitudinal bed slope is taken as 1 ft/mile. Mathematically,

S= 1 ft1 mile(canal)

= 1 ft5000 ft

There are two main theories being developed for the designing of unlined irrigation canal.

1. Kennedy’s silt theory2. Lacey’s regime theory

Kennedy’s Silt Theory(Non−silting∧non−scouring theory ):

R.G. Kennedy (Exective Engineer Punjabirrigation of the Upper Bari Doab Canal) published in 1895. His main conclusions from the observations of the 20 sites, in the middle reach of the UBDC which was built in 1850 and had remained stable, were as follows

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A regime/stable channel is one which neither silts nor scours.

The eddies, produced from the bed, support the silt in suspension and therefore the silt supporting power of the stream is proportional to the bed width but not the perimeter of the channel.

The critical velocity (non−silting∧non−scouring ve locity )V o is given as under,

V o=0.84 m D0.64 (F . P . S )V o=0.55 m D0.64 (S . I )Where,

m=criticalvelocity ratio= VV o

=velocity ¿Chez y ' sformula ¿velocity ¿

Kenned y ' stheory ¿

The value of ‘m’ usually ranges b/w(0.9¿1.1). If it is not mentioned, then for Kennedy’s theorym=1¿ D=depthof channelif V act >V o (scouring will occur )

V act <V o(silting will occur )

V act=V o(nonsilting∧nonscouring will occur )

velocity ¿chez y ' sformula ,V =C ❑√RS

DESIGN PROCEDURE OF UNLINED IRRIGATION CANALS BY KENNEDY’S SILT THEORY:

Kennedy’s silt theory is obsolete now but is of historic importance. Following steps are involved in the design of irrigation canal,

Step1: Assume depth

Step2: Calculate critical velocity

V o=0.84 m D0.64 (¿F . P . S )V o=0.55 m D0.64 (¿ S . I )

Step3:Calculate area using equation of continuity i.e. Q=AV

Step4:Calculate bed width from the area.

Step5:Calculate hydraulic radius.

R= AP

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Step6:Calculate Chezy’s coefficient using kutter’s equation.

C=23+( 0.00155

S+ 1

n )1+(23+ 0.00155

S )× n

√ R

(¿S . I )C=41.6+( 0.00281

S+ 1.811

n )1+(41.6+ 0.00281

S )×n

√R

(¿ F . P . S )

Where C is Kutter’s C and in this case it is known as “Flow Resistance Factor” or “ Chezy’s coefficient”

Step7:Now calculate Actual velocity of flow using Chezy’s equation, i.e.

V=C √RSWhere ,V=Actual Velocity of flowC=Chez y ' sCoefficient R=Hydraulic RadiusS=Bed Slope

Usually ¿1

5000 , if not given.

Step8:Now find out the critical velocity ratio, i.e. “m”

m= VV o

If, m=(0.9−1.1) then design is O.K otherwise revise the section assuming different depth.

DESIGN NO. 01:Design any channel for following data,Q=50 cusecsS=1/5000N=0.02Side slope 1:1(if not given)

SOLUTION:

Step1: Assume depth

D=2 ft

Step2: Calculate critical velocity

V o=0.84 m D0.64¿0.84 × 1× 20.64¿1.31 ft / sec


A= 501.31¿38.168 ft2


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Area, A= (B+D ) D38.168=(B+2 )× 2B=17.084 ft


R= AP

Wetted perimeter of cross section, P=17.084+2 (2.828 )=22.74 ft

R=38.16822.74

¿1.678 ft


C=41.6+( 0.00281

S+ 1.811

n )1+(41.6+ 0.00281

S )×n

√R

(¿ F . P . S )C=

41.6+( 0.002811

5000

+ 1.8110.02 )

1+(41.6+0.00281

15000 )×

0.02

√1.678

C=78.634


V=C √RSV=78.634 ×√1.678 ×1

5000V=1.44 ft /sec


m= VV o

m=1.441.31

m=1.09 O .K


Hence Required Perimeters are,Depth , D=2 ft Basewidth , B=17.084 ftTop width, T=21.084 ftSide slope=1 :1Bed slope=1/5000

DESIGN NO. 02:Design any channel for following data,Q=35 cumecsS=1/5000N=0.025Side slope=1 :1(if not given)

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SOLUTION:

Step1: Assume depth

D=2mStep2: Calculate critical velocity

V o=0.55 m D0.64¿0.55 ×1 ×20.64¿0.8571 m /sec


A= 350.8571 ¿40.8354 m2


Area, A= (B+D ) D40.8354= (B+2 ) ×2B=18.418 m


R= AP

Wetted perimeter of c ross section ,P=18.418+2 (2.828 )=24.074 m

R=40.835424.074

=1.7 m


C=23+( 0.00155

S+ 1

n )1+(23+ 0.00155

S )× n

√ R

(¿S . I )C=

23+( 0.001551

5000

+ 10.025 )

1+(23+0.00155

15000 )× 0.025

√1.7

C=44.508


V=C √RSV=44.508√1.7×1

5000V=0.8207 m /sec


m= VV o

m=0.82070.8571

m=0.9575 O . K


Hence Required Perimeters are,

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Depth , D=2 mBasewidth , B=18.418 mTop width, T=22.418 mSide slope=1 :1Bed slope=1/5000

Design Of Unlined Canal By Lacey’s Regime Theory:

In 1929 Lacey put forward his theory. He made a systematic study of the observed data and derived some empirical relations. Then he gave a concept of a regime theory for unlined channels.

He proposed the following conditions for the zero net erosion or deposition over a hydrological cycle i.e.

1. Discharge should be constant.2. Loose granular alluvial material, which can be scoured out as easily as it can be

deposited, should be of same characteristics. 3. Silt grade and silt discharge are constant.

Assumptions In Lacey’s Theory:

Following are two assumptions in Lacey’s theory,

1. Dimensions such as width, depth and slope of a regime channel to carry a given discharge loaded with given silt charge are all fixed by nature.

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2. He also stated that silt is kept in suspension due to the force of vertical eddies. According to him eddies are generated from bed and sides, both normal to the surface of generation. Hence the vertical component of eddies generated from sides will also support the silt.

Regime Channel:

Lacey defined the regime channel as a stable channel transporting a regime silt charge. A channel will be regime if it flows in incoherent unlimited alluvium of the same character as that of transported and the silt grade as well as silt charge are constant.

Conditions For Regime Channel:

There are three conditions for a regime channel.

1. The channel is flowing in incoherent unlimited alluvium of the same character as that of transported.

2. Silt grade as well as silt charge are constant. 3. Discharge is constant.

If all the conditions are fulfilled then the channel is said to be the “true regime”. However it is very rare that all the above three conditions are realized in the field. So Lacey gave the idea of initial and final regime for actual channels.

Initial Regime:

It is the state of the channel which has formed its section only and yet not secured the longitudinal slope. Lacey silt theory is not applicable to the channels in initial regime.

Final Regime:

The channel after attaining its section as well as its longitudinal slope is said to be in final regime. Unlined channels will be either in initial regime or in final regime.

Permanent Regime:

A channel is said to be in permanent regime when its section is provided with a lining to protect against scouring of the section thereby imparting the permanency to the channel section and longitudinal slope. Lacey regime theory is not applicable to such channels. This case can only exit in lined channels.

Incoherent Alluvium:

It is the soil composed of loose granular graded material which can be scoured with the same ease with which it is deposited.

Regime Silt Charge:

It is the minimum transported load consistent with fully active bed.

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Regime Silt Grade:

This indicates the gradation between the small and big particles and should not be taken to mean the average mean diameter of a particle.

DESIGN FORMULAE FOR UNLINED CANALS BY LACEY’S THEORY:

Lacey’s worked on the channels which were at the final regime conditions. He gave the following formulae to determine the regime section and the bed slope of channels.

Step1: Silt Factor

f =1.76√mWhere,

m=Average particle ( silt ) ¿(diameter¿)∈mmOr Following Relation between f and Critical velocity ratio can be used,

f =( VV o )

2

Step2:Velocity Of Flow

V=(Q f 2

140 )16

Step3: Hydraulic Depth

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R=2.5V 2

fStep4: Area of flow

A=QV

Step5: Wetted PerimeterP=4.75√Q

Step6: Base Width & Depth of Water

R= Ap

Step7:Bed slope

S=0.0003( f53

Q16 )

DESIGN NO. 01:Design an irrigation canal by the following data using Lacey’s Regime TheoryFull Supply Discharge ,Q=35 cumecs

Longitudinal / Bed Slope, S= 15000

Side Slope=1 :1

SOLUTION:

Step1: Silt Factor

S=0.0003( f53

Q16 ) 1

5000=0.0003( f

53

Q16 )f =1.12

Step2:Velocity Of Flow

V=(Q f 2

140 )16¿( 35 ×1.122

140 )16¿0.824 m/ s

Step3: Hydraulic Depth

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R=2.5V 2

f¿2.5 ×

0.8242

1.12¿1.5156 m

Step4: Area of flow

A=QV

A= 350.824 A=42.68 m2

Step5: Wetted Perimeter

P=4.75√QP=4.75√35P=28.1 m

Step6: Base Width & Depth of Water

R= AP

−−−−−−−−−−−(1)R=(B+ D ) D

B+2 (1.414 D )−−−−−(2)A=(B+D ) D−−−−−−−( A )

P=B+2 (1.414 D )−−−−−( H )

Substituting values in equation ( A )∧(H ) respectively,

eq . ( A )⇒ (B+D ) × D=42.68 (3 )

eq . ( H )⇒B+2.828 D=28.1 (4 )

B=28.1−2.828 D(5)

Put value of ‘B’ in eq (3)

(28.1−2.828 D+D )× D=42.6828.1 D−1.828 D2=42.681.828 D2−28.1 D+42.68=0

Solving this quadratic equation we get

D=13.663 m , 1.7088 m

Substituting these values in eq (5)

for D=13.663 m

B=28.1−2.828 × (13.663 )=−10.5389 m (negative value so neglected )for D=1.7088 m

B=28.1−2.828 × (1.7088 )=23.2675 m

So required results are

B=23.6375 m∧D=1.7088 m

Now to check, substitute these values in eq (1)

R= BD+D 2

B+2.828 D=

(23.2675× 1.7088 )+(1.7088 )2

23.2675+(2.828 ×1.7088 )=42.6795

28.0999=1.51 m

This value is same as in step 4 so it is O.K. so the required channel section is,

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B=23 . 2675 m , D=1. 7088 m, S= 15000

,T=27 .2675 m

Example 2:

Design an irrigation channel by the following data using Lacey’s regime theory.

full supply discharge , Q=43 cumecs

silt factor , f =0.9

Solution:

Step 1:f =0.9

Step 2:

V=(Q f 2

140 )16=( 43 ×(0.9)2

140 )16=0.8 m / s

Step 3:

A=QV

= 430.8

=53.75 m2

Step 4:

R=2.5×V 2

f=2.5 ×

(0.8)2

0.9=1.78 m

Step 5:

P=4.75√Q=4.75√43=31.15 m

Step 6:

Let the slope is 1 (V ): 0.5(H ) then

area of flow , A=0.5 D2+BD (i )

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P=B+√5 D (ii )

as R= AP

=0.5D2+BDB+√5D

(iii )

As we have the values of Area (A) and wetted perimeter (P) so we can write eq (i )∧( ii ) as

53.75=0.5 D2+BD (iv )

31.15=B+√5 D (v )

From eq (v )

B=31.15−√5D ( vi )

Put this value of ‘B’ in eq (iv)

0.5 D2+(31.15−√5 D ) × D=53.75

−1.7361 D2+31.15 D−53.75=0

Solving the quadratic equation we will get

D=1.934 m, 16.01 m

Substituting these values of ‘D’ in eq (vi)

for D=1.934 m

B=31.15−√5× 1.934=26.82 m

for D=16.01 m

B=31.15−√5× 16.01=−4.65 m¿

So the result is B=26.82 m∧D=1.934 m

Substituting these values in eq (iii )

R=0.5 D2+BDB+√5 D

=0.5 (1.934)2+26.82× 1.934

26.82+√5 ×1.934= 53.74

31.1445=1.72 m

This value is almost equal to the value as it was in step 4 so this value is O.K. now we will find out the value of bed slope as

S=0.0003( f53

Q16 )=0.0003( (0.9 )

53

( 43 )16 )= 1

7437m /m

Hence the required section is,

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Top width, T=28.614 mBasewidth , B=26.68 mWater depth , D=1.934 mBed slope , S= 17437

Example 3:

Design an irrigation channel by the following data using Lacey’s regime theory.

full supply discharge , Q=15 cumecs

m=0.33 mm∧side slope=12

:1

Solution:

Step 1:

f =1.76√m=1.76√0.33=1.011

Step 2:

V=(Q f 2

140 )16=( 15×(1.011)2

140 )16 =0.6917 m /s

Step 3:

A=QV

= 150.6917

=21.6857 m2

Step 4:

R=2.5×V 2

f=2.5 ×

(0.6917)2

1.011=1.1831m

Step 5:

P=4.75√Q=4.75√15=18.3967 m

Step 6:

We know for side slope( 12

:1), channel is of trapezoidal shape.

area of flow , A=0.5 D2+BD ( i )

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P=B+√5 D (ii )

as R= AP

=0.5D2+BDB+√5D

(iii )

As we have value of ‘P’ so we can write as eq (ii ) as,

18.3976=B+√5 D (iv )

B=18.3967−√5D ( v )

Substituting the values of ‘R’ and ‘B’ in eq (iii )

1.1831=0.5 D2+(18.3967−√5 D) D(18.3967−√5 D)+√5 D

21.7651=18.3967 D−1.7361 D 2

1.7361 D2−31.15 D−21.7651=0

Solving the quadratic equation we will get

D=1.3568 m , 9.2397 m

Substituting these values of ‘D’ in eq (v)

for D=1.3568 m

B=18.3967−√5× 1.3568=15.3628 m

for D=9.2397 m

B=18.3967−√5× 9.2397=−2.2639 m¿

So the result is B=15.3628 m∧D=1.3568 m

Substituting these values in eq (iii )

R=0.5 D2+BDB+√5 D

=0.5 (1.3568)2+15.3628 ×1.3568

15.3628+√5 ×1.3568=21.7646

18.3968=1.1831m

This value is equal to the value as it was in step 4 so this value is O.K. now we will find out the value of bed slope as

S=0.0003( f53

Q16 )=0.0003( (1.011)

53

(15 )16 )= 1

5149m /m

Hence the required section is,

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Top width, T=16.7196 mBasewidth , B=15.3628 mWater depth , D=1.3568 m

Bed slope , S= 15149

SILT CONTROL IN IRRIGATION CANALSILT:

It is the amount of sediments entering in the channels.

INTRODUCTION:

Water entering from river to canal has a high silt charge in it. This silt charge makes the canal shallow and decreases its capacity. So canal cannot draw its authorized discharge from river. High silt charge in irrigation channels result in deterioration of smooth lining surface and power generation equipment. The success of irrigation projects depend upon the control of silt entering the channels from reservoir or river.

SEDIMENT TRANSPORT:

There are two ways of sediment’s transportation.

Suspended load transport Bed load transport

If the channel is not properly designed then the erosion will take place from the bed or silt will start to deposit i.e. silting & scouring. There are four possible methods to solve this problem.

Design an unlined channel to produce the non-silting and non-scouring velocity. Take preventive measuring against silt entering. Make arrangements to eject the silt that has already entered the canal or properly

distribute it to the off taking distributaries. Design the outlets so as to draw an equitable amount of silt.

PREVENTIVE/PRECAUTIONARY MEASURES AGAINST SILT ENTRY:

These are as follows,

River approach Orientation of head works Regular crest level Divide wall

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Shape of guide bank Canal closure Double wedge regulator Submerged vanes Silt excluder

1. River approach:

There are three types of channels.

Straight channel Meandering channel Bramdiel channels

Proper river approach conditions play a very vital role to control the sediment entry into the canal. Straight river approach channel is preferable where it is not available. The natural river curvature can be used as a silt exclusion device by locating the barrage on the bend and the channel regulator on the outer edge of the curve. Water with heavy concentration of silt, flows along with the inside edge of the curve.

2. Orientation of head works:

A proper silt distribution b/w the main channel and the off-take is achieved by selecting a suitable angle of diversion given to the off-take. Silting of the off-take is caused by the fact that lower layer of water in main stream have a lower velocity and lower momentum. Therefore this layer can be deflected more easily into the off-take as compared to the upper layer that has greater velocity and momentum. So a greater force is required to turn the upper layer towards off-take. Experiences show that it is better to provide the canal head regulator at 90°-110° to the river channel for better silt control.

The best design guide for fixing the angle of diversion depends upon the particular situations such as,

Discharge ratio Sediment charge into channel Position of off-take or canal head regulator

3. Regulator crest level:

The conditions to control silt become better by increasing the crest level of head regulator. The crest of the head regulator is recommended to the 1/3 depth of water in river.

4. Divide wall:

It is the wall which separates the under sluice from the main weir. Length of divide wall, for one off-taking canal, is usually one half or two third the length of head regulator. In case of two off-taking canals its length is up to the end of head regulator. The optimum width of the pocket (space b/w divide wall, under sluice and canal head regulator) can be determined by model studies. It is effective to control the silt entry into the off-take. Long divide wall does

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not act as a trap for coarse bed material rather lessens the effect of regulation on curvature of flow.

5. Shape of guide banks:

Control of silt into the off-take is secured by a suitable approach to the canal pocket through proper shape and alignment of guide banks. Bottleneck and converging guide banks do not provide effective silt control as large islands are formed up-stream of the pocket. If convergence is provided then it should not more than 1/10.

6. Canal closure:

During very high flood period canals are closed. Because flood water contains vary high concentration of silt. So to dispose off this water, gates of under sluice are opened. It helps to wash out the sediments deposited in pocket.

7. Double wedge regulation:

Better silt exclusion in still pond system secured by opening the barrage gates adjacent to the divide wall more than those far away. Opening of the gates is minimum at the center, increasing gradually towards divide wall, in case canal takes off from banks of the barrage.

8. Submerged vanes:

A submerged curved vane of one third the depth of water, pointing d/s and projecting from a bank, produce a local concave curvature and helps deflect bed material away from the bank into the mid stream. It entails very little obstruction to the natural water way of river. Its demerits are,

Vanes are not effective in case a deep channel is not formed or maintained along or near a bank at high river discharges and through out the flood season.

A single vane on one bank is liable to draw too large a share of top water to feed the canal and may possibly pull the whole rivers towards it necessitating counter measures to adverse the conditions thus created.

Unless a vane is constructed sufficiently u/s, bed sediments thrown up in suspension may find its way into the canal.

A vane designed foe low discharge may not be effective during higher or intermediate discharges.

Accretion in the river may render the vane partially or completely ineffective.

9. Silt excluder:

This device is used as a remedial measure. It is of two major types,

Tunnel type silt ejector in canal Vortex tube silt ejector in canal

SILT EXCLUDERS:

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Silt Excluder is a device by which silt is excluded from water entering the canal. It is constructed in the bed in front of head regulator.

The fundamental principle, on which a silt control device acts, lies in the fact that a flowing stream carrying silt in suspension. Concentration of silt particles is greater in lower layer as compared to upper layer. Hence device is designed to separate upper and lower layer without any disturbance. The top water is then led towards the canal while the bottom water containing high silt charge is wasted. This is achieved by silt excluder. Rivers and streams carry a large amount of silt every year and if it is not controlled then it reduces the capacity of the hydraulic structures.

River Satlug carries 35 M.ton/year amount of silt. River Indus carries 440 M.ton/year amount of silt. River Jehlum carries 70 M.ton/year amount of silt.

The effect of silt on reservoir can be approximated as,

Capacity of Warsak Dam is reduced from 23,000 AF to 10,000 AF in ten years. Capacity of Tarbela Dam is reduced from 9.3 MAF to 1 MAF in ten years. Capacity of Mangla Dam is reduced about 30% in 50 years due to sedimentation and

silting.

The silt excluder consists of a number of under tunnels resting on the floor of the pocket. The top level of the R.C roof of the tunnels is kept the same as the sill level of the head regulator. The various tunnels are made of different lengths i.e. the one near to head regulator is of length as the width of head regulator. The length of the successive tunnels is decreased towards divide wall as shown in figure. This arrangement separates the water into two clear layers i.e.

The top layer, above the roof of the under tunnel, enters the head regulator. The bottom layer containing relatively heavier silt charge which goes to under tunnels and it is discharged to the d/s of the river through under sluices. The capacity of under tunnel is kept above 10% of the canal discharge and it is designed so that a minimum velocity of (2-3) m/sec is maintained. Knowing discharge and discharge velocity the total water way required for the under tunnels can be determined.

The following points should be kept in mind while designing a silt excluder.

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1. The tunnel discharge through under sluice is kept more then 10% and usually 20% of the canal discharge.

2. The silt excluder should cover only two bays of under sluice as it gives more efficient results.

3. The approach canal needs not to be lined.4. The divide wall should be (1.2 – 1.4) times length of head regulator. 5. The top of the slab of silt excluder should be flushed with the head regulator crest. i.e.

the clear height of the tunnels would be = 13

of depth of the water – slab thickness

6. The roof slab should be designed to carry a full water load in case the tunnels are empty.

7. The first tunnel should completely cover the head regulator length. While other tunnels should be of shorter lengths.

8. The discharge through tunnels will depend upon the head measured above the center line of the tunnel.

Tunnels can be treated as culverts, for which the discharge formula is,Q=CA √2 gH

C = coefficient of dischargeA = tunnel areaH = head of water

The value of “C” is given for concrete box culverts as,

C=[1+0.4 R0.3+ 0.0045 LR1.25 ]

−0.5

Where,R = hydraulic mean radius L = length of the tunnelThe velocity in the tunnels should be 6 ft/sec to 10 ft/sec.

EFFICIENCY OF SILT EXCLUDER:

Haigh defined the efficiency in the following manner,

E=I f −I v

I f

This indicates the reduction of silt intensity in the canal water as compared with that of approach canal.

Where, E = efficiencyI f = silt intensity in the approach canal in ppmI v = silt intensity in the canal.

FACTORS AFFECTING THE EFFICIENCY

1. An increase in escape supply increases the efficiency to a certain point only.

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2. The grade of sediments affects the efficiency i.e. for coarser silt efficiency is more and for smaller grades, it is less.

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irrigation engineering part 2

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