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Design of open irrigation canals.

By: Eng. Y. levy 1- introduction:

In order to explain the engineering aspects and the factors influences on the flow character, stability and the design methods of open canals generally and grassy canals in particular , following engineering summary is prepared . The method of grassy – canals initially developed by the hydraulic laboratory of still water – Oklahoma, U. S.A, recommendations related canals - stability and the design tablets of the canals capacities based on the experiences of this laboratory. The method developed basically to the western gulf zone, but we can study out of it, the values of “Manning” coefficient “n” and the design tablets &relevant solutions in all zones. The irrigation - engineers need to combine the engineering experiences in system design of the irrigation methods (open surface &pressurized systems) with the water transportation and delivery canals, specifically to the irrigation areas named “low –land”. For the “up – land”, energy need to be developed, in this case modern pressurized systems will adapt more economically. Delivery of the irrigation water to the agricultural areas in most of the developing countries is by channels and the irrigation done by open system irrigation methods, basically because of two reasons: 1- Availability of Know –how in design and applications of

modern's systems. 2- Needs for initial cash flow and high investments in the

preliminary project stages.

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2 - Open canals flow:

2.1 – general: Canal determined as water - conduit in which the up-water surfs is free. The hydraulics of pipes in which the flow is not in full section is similar to the canal hydraulics. Below are the determinations of the flow characterizing: 2.1.1 – Stability flow: If the discharge at the canal sections remand constant, the flow named: stability flow. 2.1.2 –Non-stability flow: If the discharge at the canal sections is not constant, the flow named: non - stability flow. 2.1.3 – Uniform flow: If the flow is stability flow and the flow - velocity is equal in all sections of the canal, this determined as uniform flow. 2.1.4 – Non –uniform flow: If the flow- velocity is not equal in all the sections of the canal and changed from section to section it is determined as non – uniform flow.

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2.2 – Parameters determinations: The engineering determinations of flow and the canal parameters concentrate at the tablet below:

Parameter: Symbol: Units:

Canal discharge Q m^3/s , or: m^3/hr.

Flow section A m^2

Average flow velocity V m/s

“Manning” coefficient n -

Hydraulic radius R m

Canal slope S m/m , or: %

Canal flow depth h m

Canal depth y m

Canal bottom wide B m

Water surfs wide b (0,1,2,,,,) m

Canal up - wide B up m

Canal wall, side-slope Z m : m

Flow perimeter P m

3 - Flow characteristics in canals & Delivery Canal

Design The flow at the canal is basically turbulent – flow, while the water flow direction in each point is variable and changed with the time. In a canal design problem, the independent variables are known either explicitly or implicitly, or as inequalities, mostly in terms of empirical relationship. The velocity V (m/s) is the average velocity of all velocity components which are parallel to the canal bottoms. V=Q/A

In grassy- canal, big velocity deference’s at the flow section depending on the section geometry.

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Normally a trapezoidal section is adopted. Rectangular cross –section is also in use in special situations, such as in rock cuts, steep chutes and in cross –drainage works. The side slope, expressed as “m” horizontal: “1” vertical, depends on the type of canals Lined or unlined and type of soil. The slopes are designed to withstand seepage forces under critical conditions: 1- A canal running full with banks saturated due to rainfall. 2- The sudden drawdown of canal supply.

Side slopes for: unlined - canals:

No: Type of soil “m”

1 Sandy soil 1.5 – 2

2 Sandy loam 1 – 1.5

3 Sandy – gravely 1 – 2

4 Murom, hard soil 0.75 – 1.5

5 Rock 0.25 – 0.5

Delivery irrigation Canal Design:

The canals design for irrigation based on the average flow velocity, for a uniform - flow in a canal.

To calculate all the dimensions of the canal we use the “manning” formula:

3

2

2

11

RSn

V

0r:

5

3

2

2

11

RASn

Q

P

AR

Where “A” and “R” in general, functions of the geometric elements of the canal. If the canal is of trapezoidal cross –section, Q=f (n, y, s, B, m), This equation has six variables out of which one is a dependent variable and the rest five are independent ones.

Canal Section Design:

To determine the dimensions of the cross-section of the delivery canal from point to point, first of we have to consider if the canal is earth lined canal or: anther type like for example concrete lined canal. Then by estimating the maximum flow velocity, based on the permissible velocity available or the total critical expected discharge in the canal. Typical trapezoidal Cross-sectional of Canal:

Permissible Data:

Y

Y

B

1

m

2

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Permissible maximum velocities, in lined canals: The permissibly maximum velocities normally adopted for a few soil types and lining materials as indicated in the table below:

No: Nature of boundary Permissible maximum velocity (m/s)

1 Sandy soil 0.3-0.6

2 Black soil 0.6-0.9

3 Murom, hard soil 0.9-1.1

4 Firm clay &loam 0.9-1.15

5 Gravel 1.2

6 Disintegrated rock 1.5

7 Hard rock 4

8 Brick masonry with cement pointing

2.5

8 Brick masonry with cement plaster

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9 Concrete 6

10 Steel lining 10

Permissible maximum velocities, in lined grassy- canals:

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Grass caver Canal range of Slopes (%)

Stable soil Erosions soil

Bermuda grass 0-5 2.5 2

“ 5-10 2.3 1.6

“ >10 2 1.4

Buffalo grass &similar 0-5 2.3 1.6

“ 5-10 2 1.4

“ >10 1.6 1

Mixed grasses 0-5 1.6 1.4

“ 5-10 1.4 1

Lespedeza 0-5 1.15 0.8

Alfalfa & Sudan grass 0-5 1.15 0.8

Commends:

• Velocities above 1.6 m/s accepted only for intensive crowded grass and good guarding.

• It is recommended not to use higher slopes than 5%during the design only if grass caver canal bank combined.

• In general, better not to design slopes above 5%.

• Manning - formula is used for uniform flow, in regular conditions grassy canals are suitably for this. but, in canals with sharp slopes and canals with artificial banks (like concerts or still) the below considerations need to be considered:

• The design of the inlet to the canal and the outlet from the canal and the design influence on the flow.

• Canals slop Variability’s.

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• Cross –section Variability’s.

“Manning” coefficient values: The manning - coefficient is according to the flow characteristic and the canal situation, the coefficient values are:

flow characteristic

canal situation n

Not -Constantly flow

Uncovered, straight uniform

0.017-0.025

“ Not uniform, rough 0.025 -0.035

Constantly flow Uncovered bottom & multiplication (M=B*y)

0.030

“ M<1 0.030-0.040

“ 2<M<1 0.020-0.030

“ M>2 0.017-0.025

“ Covered said, rough bottom

0.040

“ M<1 0.045-0.050

“ 2<M<10 0.040-0.045

“ M>2 0.030-0.040

“ Grass flow disturbed canal

0.015

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EXAMPELS & engineering calculations: Example -1: Delivery canal from the main canal to pumping point for pressurized systems applications:

CANAL TYPE Earth lined canal

Concrete lined canal

Permissible Velocity(m/s)

0.3-0.6 0.6-2

Manning’s coefficient

0.03 0.018

Derivation of the above formula:

Example, m = 2

Section area =A= ½(Y*2Y) + B*Y + ½(Y*2Y) = 2Y^2 +B*Y

2

1 Y

B

b

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A = Y (2Y + B) Perimeter = P =2*(Y^2+ (2Y) ^2) ^0.5 + 2B +4Y =2*(5Y^2) ^0.5 +2B+4Y P=2*5^0.5*Y+4Y+2B R=A/P R= Y (2Y + B)/ (2*5^0.5*Y+4Y+2B) Economical design --------- B/Y = 2 Performance of B=2Y in the above equation R=0.32Y Table with the deferent’s depth data need to be prepared & taking with the other data from survey.

B(m) Y(m) S n R(m) V(m/s) Q(m3/hr)

Estimations: Base (B) = 1m up to 2.2m Slope(S) = from field survey data. Depth (y) = 0.2m up to 1

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Example, given data:

Q = 6000m3/hr = 1.66m3/s

Table only with the iteration of the depth and taking the other data from survey.

Need to be calculated by XL software, like below example table:

b(m) Y(m) S n R(m) V(m/s) Q(m3/hr)

1 0.2

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

* The shade values are approximately equal to the available discharge and the permissible velocity.

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* Now the engineer can select the appropriate dimensions of the canal’s cross-section with minimum deviation to the design values Q.

* The data shade need to be approximate to the design discharge and it is within the allowable velocity range.

*The data shaded more approximate to the available discharge and the velocity is within range for point-1 to point-2 according to the flow direction.

Inlet and Outlet canal Design:

The inlet and the outlet portion of the delivery canal must be based on the method necessary to avoid the contaminations which are involved to the water from the inlet point. If the design engineer need to combine (from the main supply canal!), pressurized systems and to develop pumping point for pressurized irrigation application methods. Good to take this as a criterion and shape the end points of the channel. The engineering solutions need to be conceder three parts of engineering analysis, as shown below:

P

Inlet from

main canal

Inlet

to

pump

station

Part-1 Part-2 Part-3

Gate

Sediment

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Side view of the delivery canal (section A-A)

Top view of the delivery canal

Section B-B

A A

B

B

Y

B

1

2

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Different design alternatives of sectional parts:

By taking different depth to with ratio we will check the effect on the available discharge and velocity. Beside see the economical advantage of these different orientations:

1. Shallow section: -Assumption Y/B = 0.25

The calculation will be Calculate by XL as explained above and fill up the below tablet:

B Y S n R V Q

Y

B

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- From this table the design engineer can conclude that having very shallow section with keeping slop leads very small amount of water supply from main canal to the pump station in this regard the available discharge will not be gained. 2. Deep section: -Assumption Y/B = 0.5 or 0.8

The calculation will be Calculate by XL and fill up the below tablet:

B Y S n R V Q

Y

B

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- From this table the designer can also conclude that having dipper section and keeping the slope the same to the above calculation the water supplied from main canal to the pump station will be very large with high velocities.

Example – 2:

Design a reservoir fed canal to carry a discharge of: 5 m^3/s:

Data: n =0.025 Vmax=0.65 m/s Q=5 m^3/s Solution: Vmax=Q/Amin 0.65=5/A ---------A=5/0.65 ------------A~=7.7 m^2

Y

B

1

1

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The chart will generally not be supplied as a data, so the engineers using the relation between” B” and “Y” If: Q<15 m^3/s ------Y=0.5B^0.5 ---------or: B=4Y^2 Using a trapezoidal canal section with 1:1 side slop, (m=1). A=B*Y+Y^2=Y (B+Y) =Y (4Y^2+Y) =4Y^3+Y^2=7.7 Solve by hit &trial

Y(m) L.H. S A(m^2)

1 4+1 5

1.2 6.91+1.44 8.35

1.165 ~7 7 o.k.

The above calculation gives only approximate value; the designer can choose depth “Y” as anything like 1.165 m, for example let choose: Y=1.2m. From the above --------- A=Y (B+Y) 7=1.2(B+1.2) ------------B=7/1.2-1.2=5.2 m Use: B=5.2 m Y=1.2 m The bed canal slope can be determined according “maning” formula:

3

2

2

11

RASn

Q

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Where: A=7.7 m^2 P=5.2+2*2^0.5*1.2 = 8.6 m. R=A/P =7.7/ 8.6=0.895 m. 5=1/0.025*7.7*0.895^2/3*S^0.5 --------------S^0.5 = 0.0175 S=~ 0.13

Side slopes for unlined canals in cutting:

S no: Type of soil m

1 Sandy soil 1.5-2.0

2 Sandy loam 1.0-1.5

3 Sandy to gravely soil 1.0-2.0

4 Murom, hard soil 0.75-1.5

5 Rock 0.25-0.50

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Typical section of a lined irrigation canal:

The free –board: The free - board for lined canals is the vertical distance between the full supply levels (F.S.L) to the top of the lining.

The free – board depends on: - Canal size. - Location. - Flow velocity. - Depth of flow. The relevant engineering experiences and current practice of providing free-board recommended being as at the tablets below:

Q = Canal discharge (m^3/s) Free board (m)

Q<0.15 0.30

0.15<Q<0.75 0.45

0.75<Q<1.5 0.60

1.5<Q<9 0.75

Q>9 0.09

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The width to depth ratio:

The relationship between width and depth depending upon the engineering design practice. The hydraulically most – efficient canal section as above drawing is: m = 1/3^0.5 B = 2Y/3 = 1.155Y B/Y = 1.155 If any other “m” value is used the value of “B/Y” for efficient section would be: B/Y = 2 * ((1+m^2) ^0.5 – m) Practically it is usual to adopt a shallower section. Value of “B/Y” larger than the above recommendation for “B/Y” as a function of discharge is at the below table:

Q = Canal discharge (m^3/s) “B/Y”

0.3 2

3 4

14 6

28 7.5

140 14

285 18

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Example – 3:

Trapezoidal canal. Q= canal discharge = 50 m^3/s S =Maximum slope that can be used= 0.0004 Need to design this canal:

For example, lined canal with concrete lining. The Lined concrete canal design example format: Side slop of 1:1--------- m=1 “n” for concrete -----------n=0.013 Recommended “B/Y” for Q=50 m^3/s is about 8. For B/Y=8---------------Y/B=0.125 Generally (empirical equation) -- @=Q*n/S^0.5*B^8/3 =0.0314 Substituting: Q=50, n=0.013, s=0.0004 in the above -> B=13.56m Adopt B=13.5 and recalculated the @ value @=50*0.013/0.0004^0.5*31.5^8/3=0.0314 Corresponding Y/B=0.125 Giving: Y=1.7 m A= (13.5+1.7) *1.7 = 25.84

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V= 1.935 m/s. This value is greater than V min. Of 0.3 m/s and its approximately v= 2m/s, further is less than the maximum permissibly velocity of 6 m/s for concrete canal. So, the selection of: “B” & “Y” are all right. The recommended geometric parameters of the canal are therefore: B=13.5 m, m=1, S=0.0004 Adopt the free board of 0.75m. The normal depth for n=0.013 will be 1.7 m.

Canal standards The size of canal standards unit at the below recommended table is in “m”. The velocity value is in “m/s”.

Item V<0.3 0.3<V<1 1<V<5 5<V<10 10<V<30 30<V<150

MWBC 1 1.5 2 2.25 2.5 3

WRW - 3.5 3.5 5 5 6

FB 0.3 0.4 0.5 0.6 0.75 0.9

DEC 0.5 0.5 0.5 0.5 0.8 1

Distributors M d M d MD MD MBC MBC

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Acronyms: M d Minor distributaries canals. MD -> Major distributaries canals.

MBC -> Main Branch canal. MWBC ---- Maximum width of the bank crests. WRW ------ Width of roadway. FB ----------- free board. DEC --------- Depth cover over saturation gradient. Width of berms:

Maximum berm width =0.6 m +1/4th of the width of combined side lops of cutting and embankment.

And/or:

Maximum berm width =0.6 m +1/2th the width of combined slopes.

Width of land to be acquired when canal cutting is deeper than

the balancing depth.

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For M d & MD canals: ----- Half the height of the bank above ground, subject to minimum of 1.5 m. For MBC canals: --- Full height of bank +5m.

Width of land to be acquired when canal cutting is lesser than

the balancing depth. For M d & MD canals: ----- Full height of bank above ground level plus 1.5 m. For MBC canals: --- Full height of bank +5m. By: Eng. Y. Levy. Irrigation Engineer, Team leader. Pal- Yal Engineering Ltd.