Lecture on Channels04.02.10

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DESIGN OF STABLE OPEN DESIGN OF STABLE OPEN

CHANNELSCHANNELS

Wapda Engineering Academy Wapda Engineering Academy FaisalabadFaisalabad

MUHAMMAD KHALID PERVAIZMUHAMMAD KHALID PERVAIZ

CHIEF ENGINEER CDO (W) WAPDACHIEF ENGINEER CDO (W) WAPDA

WAPDA HOUSE, LAHOREWAPDA HOUSE, LAHORE

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1. Introduction to Irrigation Channels

2. Fundamental Equations and Concepts

3. Design of Lined and Unlined Channels

4. Maximum Permissible Velocity Method

5. Tractive Force Methods

6. Miscellaneous Considerations

7. Design Examples

SEQUENCE OF LECTURE

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1. INTRODUCTION TO IRRIGATION CHANNELS

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INTRODUCTION TO IRRIGATION CHANNELS

Irrigation

Artificial application of water to land for raising crops for food and fiber is called irrigation.

Drainage

Artificial removal of excess water from land to improve soil moisture conditions and for healthy plant growth is called drainage.

Channel

Channel is a natural or artificial passage in the ground for flow of water. Various types of artificial channels are:

• Open Channel

• Alluvial Channel

• Stable Channel

• Irrigation Channel

• Power Channel

• Lined Channel

• Unlined Channel

• Drainage Channel

• Sewerage Channel

• Flood Carrier Channel

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Open Channel

Open Channel is a channel which has open top and free water surface subjected only to atmospheric pressure. Flow in open channel is caused by gravity component along bed slope of channel. Various types of open channel are:

• Rivers

• Canals

• Sewers

• Tunnels

• Pipelines

Alluvial Channel

Alluvial channel is a channel in which flow transports sediment having same characteristics as that of material in channel bottom.

Stable Channel

Stable alluvial channel is a channel in which sediment inflow into channel reach is equal to sediment out flow. Therefore, channel cross section and bed slope do not change due to erosion or deposition.

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Irrigation Channel

Channel conveying water for irrigation is called irrigation channel. Various types of irrigation channel are:

• Canals

• Chutes

• Flumes

• Tunnels

Canal

Canal is an artificial earthen channel having mild slope usually trapezoidal in section, constructed on ground to carry water over long distance from source for:

• Irrigation

• Hydropower

• Water Supply

• Drainage

• Flood Control

Flume

Channel supported above ground and built at wood, metal or concrete is called flume.

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Power Channel

Channel conveying water for power generation is called power channel.

Lined Channel

Channel whose prism has been protected with impervious material mainly to stop seepage through it, is called lined channel.

Unlined Channel

Earthen channel whose prism has not been protected with impervious material is called lined channel.

Classification of Channels

Channels can be classified as:

• Non-erodible Channel

• Erodible Channel

• Regime Channel

Non-erodible channels are also called rigid boundary channels and erodible channels are also called loose boundary channels.

Power Channel

Channel conveying water for power generation is called power channel.

Lined Channel

Channel whose prism has been protected with impervious material mainly to stop seepage through it, is called lined channel.

Unlined Channel

Earthen channel whose prism has not been protected with impervious material is called lined channel.

Classification of Channels

Channels can be classified as:

• Non-erodible Channel

• Erodible Channel

• Regime Channel

Non-erodible channels are also called rigid boundary channels and erodible channels are also called loose boundary channels.

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Canal System

Canal or conveyance system is a network of canals constructed to convey water from source to field for irrigation purposes.

Canals may be classified into different types such as:

Based on Flow Conditions

As per flow conditions:

• Gravity Canal• Lift Canal

Based on Canal System

As per status in canal system:

• Main Canal• Branch Canal• Distributary Canal• Minor Canal• Water Course

Based on Lining

As per status of bed and sides:

• Lined Canal• Unlined Canal

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Based on Silt

As per silt in canal water :

• Silt Carrying Canal• Silt Free Canal

Based on Purpose

As per purpose of canal:

• Link Canal• Feeder Canal

Based on Flow

As per flow of canal:

• Perennial Canal• Non-perennial Canal

Design Approach

Two approaches are used for design of stable alluvial channels:

Empirical Approach

• Regime Theory

Rational Approach

• Tractive Force Method

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Regime theory is empirical in nature and was based on observation on number of canals in sub continent. Tractive force approach is rational in nature, since it utilizes laws governing sediment transport and resistance to flow and was developed in Europe.

Regime

A natural channel which is neither silting nor scouring is said to be in regime. Types of regime are:

• Initial Regime

• True Regime

• Final Regime

Regime Conditions

Regime conditions are:

• Discharge is constant.

• Silt grade and charge are constant.

• Alluvium is incoherent, unlimited and of same characteristics as sediment charge carried by water.

Incoherent Alluvium

Soil composed of loose granular material, which can be scoured away with same ease with which it is deposited, is called incoherent alluvium.

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Discharge Capacity of Irrigation Channels

Methods for fixing discharge capacity of irrigation channels are:

Irrigation Department Method

In this method capacity of each outlet is determined from its water allowance and area under its command. Discharge of outlets is added and then considering conveyance losses, full supply discharge for canal reaches is calculated.

Consumptive Use Method

In this method capacity of outlets is determined from actual water requirement of various crops in a given period of time and area under command of each outlet. The rest of procedure remains the same as above.

Recommended Velocity in Canals

Recommended velocity in canals is:

• Lined Canal = 8 fps

• Unlined Canal = 3 fps

However, flow velocity should not be less than regime velocity for silt carrying channels.

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Recently Constructed Canals

Some recently constructed canals are:

• Chashma Right Bank Canal (CRBC)

• Pehur High Level Canal

• Ghazi Brotha Power Channel

Under Construction Canals

Some under construction canals are:

• Kachhi Canal

• Rainee Canal

• Greater Thal Canal

Proposed Canals

Some proposed canals are:

• Chashma Right Bank Canal (Lift and Gravity)

• Thar Coal Power Project Canal

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2. FUNDAMENTAL CONCEPTS AND EQUATIONS

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FUNDAMENTAL CONCEPTS AND EQUATIONS

Flow

Direction of movement of water in a channel is called flow. Classifications of flow are:

Classification - I

Steady Flow

If flow velocity at given point does not change with respect to time, then flow is called steady flow.

Unsteady Flow

If flow velocity at given point changes with respect to time, then flow is called unsteady flow.

Uniform Flow

If flow velocity at given instant of time does not change within given reach of channel, then flow is called uniform flow.

Non - Uniform Flow

If flow velocity at given instant of time varies with within given reach of channel, then flow is called non-uniform or varied flow.

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Gradually Varied Flow

If flow depth varies gradually with respect to distance, then flow is called gradually varied flow e.g. reservoir behind dam.

Rapidly Varied Flow

If flow depth varies rapidly in short distances then flow is called rapidly varied flow e.g. hydraulic jump.

Classification - II

Subcritical Flow

If flow velocity is less then critical velocity, then flow is called subcritical flow, for which Fr < 1.

Critical Flow

If flow velocity is equal to critical velocity, then flow is called critical flow, for which Fr = 1.

Supercritical Flow

If flow velocity is greater than critical velocity, then flow is called supercritical flow for which Fr > 1.

Froude Number ( Fr )

Froude number is ratio of inertia and gravity forces:

Fr = V / √ g y

Where V = Velocity of flow y = Depth of flow

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Critical Velocity ( Vc )

Flow velocity at critical depth is called critical velocity.

Vc = √ g yc

where yc = Critical depth

Critical Depth ( yc )

Flow depth producing maximum discharge for a given specific energy is called critical depth.

yc = ( q2 / g )1/3

where q = Discharge per unit width

Maximum Discharge for Given Specific Energy ( qm )

Maximum rate of discharge for given specific energy is given by:

qm = √ g yc3

Specific Energy ( E )

Total energy of a section with reference to channel bed is called specific energy.

E = y + V2 / 2g

where y = Depth of flow

V = Velocity of flow

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Total Head ( H )

Total head is expressed as:

H = z + p / γ + V2 / 2g

where H = Total head

z = Elevation head

p / γ = Pressure head

V2/2g = Velocity head

Continuity Equation

Continuity equation for uniform flow is:

Q1 = Q2 or A1 V1 = A2 V2

Bernoulli Equation Bernoulli equation of uniform flow is::

z + p / γ + V2 / 2g = Constant

Hydraulic Jump

Hydraulic jump is formed whenever supercritical flow changes to subcritical flow.

y2 / y1 = 1/2 ( -1 + √ 1+ 8 Fr2 )

y1 = Depth before jump

y2 = Depth after jump

Fr = Froude number

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3. DESIGN OF OPEN CHANNELS

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DESIGN OF OPEN CHANNELS

Design of Open Channels

Design of open channels consists in finding depth, bed width, side slope and longitudinal slope of channel, so as to produce non-silting and non-scouring velocity for given discharge and sediment load.

Type of Channels from Design Consideration

Types of channels from design consideration are:

• Lined Channels

• Silt Carrying Channels

• Silt Free Channels

• Unlined Channels

• Silt Carrying Channels

• Silt Free Channels

Design of each type of channel is as under:

LINED CHANNELS

Silt Free Lined Channels

Lined channels carrying silt free water can be designed

by Chezy or Manning formula.

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Chezy Formula

French Engineer Antoine Chezy in 1775 developed

following formula for velocity of flow in open channels.

V = C √ R S

Where V = Mean velocity

R = Hydraulic mean depth (A / P)

S = Bed slope of channel

C = Chezy coefficient

Kutter, Brazin and Powell developed formulae for determination of Chezy coefficient (C). This formula has been widely used for design of open channels. Many earlier canals were designed according to this formula.

Manning Formula

Irish Engineer Robert Manning in 1889 presented following formula for velocity of flow in open channels.

V = ( 1.49 / n ) R2/3 S1/2

Where V = Mean velocity in fps

R = Hydraulic mean depth in ft. (A / P)


n = Manning coefficient

And Q = Discharge in cfs ( A V )

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Due to its simplicity of form and satisfactory results, this formula was widely used for design of open channels.

Design Procedure

Procedure for design of channel by Manning formula is as follows:

Manning equation for discharge is:

Q = V A = 1.49 / n ( A R2/3 S1/2 )

From above equation:

A R2/3 = n Q / 1.49 S1/2

Left hand side of the above equation is known as section factor. For given values of n, Q and S, the above equation is solved to determine normal depth of flow. This may be done by using design charts or by trial and error method.

Silt Carrying Lined Channels

Slit carrying lined channels are also designed by Manning formula. However, mean velocity should be more than regime velocity and Froude number ( Fr ) should be less than one to ensure sub critical flow.

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UNLINED CHANNELS

Silt Carrying Unlined Channels

Unlined channels carrying silt laden water can be designed by empirical method or by rational method.

Empirical Method

Lacey Regime Theory

Gerald Lacey, Executive Engineer in United Province, studied and analyzed data of different canal systems and developed his regime theory in 1929.

He obtained the following regime equations.

V = 1.1547 √ f RAf2 = 3.8 V5

V = (Q f2 / 3.8)1/6

He further developed the following equations:

V = 16 R2/3 S1/3

V = ( 1.346 / Na) R3/4 S1/2

Na = 0.0225 f1/4

f = 1.76 √d

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He also derived the following equations from the above equation.

P = 2.667√ Q

A = 1.26 ( Q5/6 / f1/3 )

S = 1/1844 (f5/3 / Q1/6 )

fRS = 192 R1/3 S2/3

fVR = 0.75 ( V2 / R )

Rs = 0.474 ( Q / f )1/3

Rs = 0.90 ( q2 / f )1/3

Lacey assumed channel side slope of 1 : 1/2

Where V = Stable velocity in fps

f = Lacey silt factor

R = Hydraulic mean radius in ft( A/P)

Q = Discharge in cfs


Na = Coefficient of roughness

d = Diameter of silt particles in mm

P = Wetted parameters in ft

A = Area of cross section in ft2

Rs = Depth of scour in ft

q = Discharge per unit width in cfs / ft

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In 1934 Central Board of Irrigation adopted Lacey equations for design of stable alluvial channels and many canals were designed according to Lacey regime theory and many other were successfully remodeled. The same equations are still used to design regime channels.

Design Procedure

Procedure for design of channel by Lacey equations is as follows:

• Area is determined from A = 1.26 ( Q5/6 / f1/6 )• Perimeter is determined from P = 2.67 √ Q

• Computed values are equated to expression for P and A and resulting equations are solved to determine bed width and flow depth.

• Bed slope is determined from Lacey equation.

Further Development in Regime Theory

Further development in regime theory are:

• Lacey Shock Theory• Claude Inglis Modification• Blench and King Modification• Simons and Albertson Method• S S Kirmani Modification

However, silt carrying unlined channels are still designed according to Lacey regime theory.

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Rational Method

Design of unlined channels by rational method method involves problem of sediment transport. Canal sections will be stable if velocity, slope and cross section are such that all sediment entering in canal is swept away from the section.

Sediment load is divided into:

• Bed Load

• Suspended Load

Separate functions have been derived by various authors for both. These functions are empirical in nature, being based on laboratory experiments and filed data.

Bed load transport formulae are :

• Duboy Formula

• Meyer - Peter Formula

• Einstein - Brown Formula

However, non of the formula has gained general acceptance. Suspended load concentration is calculated from suspended load function:

Above formula in combination with Manning and other formula are used to design channel by rational method.

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Design of Unlined Channels as per Rational Method

These does not exist a generalized, comprehensive and well defined procedure for designed based on the above theoretical approach. Due to lack of explicit relationship between various parameters such as slope, area, sediment transport capacity, shape etc. it is essential to follow field experience. However, problem can be tackled in many ways and what follows can be considered as one possible method.

Mayer-Peter Formula Mayer-Peter Formula for bed load transport is

gs = 4700 [τo (N’/N)3/2 - τc ]3/2

gs = Rate of bed load transport in kg/m /hr

τo = Unit tacctive force in kg/m2

τc = Critical tractive force kg/m2

N’ = Coefficient of rugosity of unrippled bed= d1/6 /24, d is diameter of particles in mm

N = Coefficient of rugosity on rippled bed

= 0.02 to for discharge >15 cumecs and

0.0225 for smaller discharge

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Other Design Methods

Other methods for design of unlined channels are:

• Permissible Velocity Method

• Tractive Force Method

• Hydraulic Design Criteria Method

Silt Free Unlined Channels

Slit free unlined channels flowing through alluvium can also be designed by Lacey equations as water can pick up silt and deposit it at lower reaches. However, lower value of silt factor ‘f’ should be used due to fine silt in such channels. Silt free unlined channels can also be designed by above methods.

Longitudinal Section of Canal

Calculated bed width, full supply depth, bed slope, mean velocity etc and other relevant information like full supply discharge, free board, roughness coefficient or silt factor, full supply level, bed level and natural surface level etc. are plotted, starting from head to tail, against reduced distances (RDs) in form of longitudinal section of canal. Locations of fall and other structures are also marked on longitudinal section of canal.

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4. PERMISSIBLE VELOCITY METHOD

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PERMISSIBLE VELOCITY METHOD

Permissible Velocity Method

In permissible velocity method, channel size is selected such that mean flow velocity for design discharge under uniform flow conditions is less than permissible velocity.

Permissible Velocity

Permissible velocity is defined as the mean velocity at or below which bottom and sides of channels are not eroded. Permissible velocity depends upon:

• Type of soil• Size of particles• Depth of flow• Curvature of channel

Maximum permissible velocities for different materials are given in the table. The values listed in the table are for straight channels having flow depth of about 3.50 ft. These values should be reduced for sinuous channels as below:

• Slightly sinuous channels = 5%• Moderately sinuous channels = 13% • Highly sinuous channels = 22%

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For other flow depths, these velocities should be multiplied by correction factor to determine permissible flow velocity. Correction factor ‘k’ for wide channels is:

k = y1/6

where k = Correction factor

y = Depth of flow

Design Procedure

Procedure for design of channel by permissible velocity method is as follows:

• Permissible velocity is found from the table.

• Area is found from continuity equation A = Q / V and hydraulic radius ‘R’ from Manning equation.

• Wetted perimeter is determined from P = R / A.

• Computed values are equated to expressions for P and A and resulting equations are solved to determine channel bed width and depth of flow.

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5. TRACTIVE FORCE METHOD

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TRACTIVE FORCE METHOD

Tractive Force Method

Scour and erosion process can be viewed in rational way by considering forces acting on particles lying on channel bottom or sides. The channel is eroded if resultant of forces tending to move particles is greater than resultant of forces resisting motion. This concept is referred as tractive force approach.

Tractive Force

The force exerted by flowing water on bottom and sides of channel is called tractive force. In uniform flow, this force is equal to component of weight acting in direction of flow and is given by:

τo = γ R So = γ y So

Where τo = Tractive force γ = Unit weight of water R = Hydraulic mean radius y = Depth of flow So = Bed slope of channel

Critical Tractive Force

The force at which channel material begins to move from stationery condition is called critical tractive force ( τc ).

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Distribution of Tractive Force

Distribution of tractive force or shear stress over channel perimeter is not uniform. For trapezoidal channels, tractive force at channel bottom may be assumed equal to γ y So and at channel sides equal to 0.76 γ y So .

τs = 0.76 γ y So

Reduction Factor for Channel Sides

Reduction factor for critical tractive force on channel sides is:

К = √ 1- (Sin2 θ /Sin2 Φ)

Where К = Reduction factorθ = Slope of sidesΦ = Angle of repose

Effect of angle of repose should be considered only for coarse non cohesive materials and can be neglected for fine cohesive materials. Critical shear stress for cohesive and non cohesive materials is given in the figures. These values are for straight channels and should be reduced for sinuous channels as below:

Slightly sinuous channels = 10%Moderately sinuous channels = 25%Highly sinuous channels = 40%

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Design Procedure

Procedure for design of channel by tractive force method is as follows:

• Permissible shear stress is found from the figure.

• Reduction factor for channel sides is determined.

• Unit tractive force on the side ( 0.76 γ y So ) is equated to permissible shear stress and depth of flow is determined.

• Bed width is determined from Manning equation.

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6. MISCELLANEOUS CONSIDERATIONS

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Miscellaneous Considerations

Miscellaneous consideration in design of irrigation channels are:

Alignment of Irrigation Channels

• Distributaries should be aligned along secondary ridges.

• Main and branch canals should be aligned along

main ridges.

• Irrigation channels should not cross drainage

system of area.

• Obstacles such as roads, towns, railway lines,

canals etc. should be avoided.

• Direct irrigation should not be done from main

and branch canals.

• Main canal should be split into branch canals

• Irrigation channels should be straight as far as

possible

• In case of curvature, suitable radius of curves

should be provided. Radius of curve depends on

discharge of canal but should be not less than 10

to 15 times bed width of canal.

MISCELLANEOUS CONSIDERATIONS

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Longitudinal Slope

Longitudinal slope is fixed as per Lacey equation. If slope of canal is flatter than grade of land, falls are provided at suitable intervals and if slope of canal is steeper than grade of land the later is adopted.

Side Slope

Side slope of canal should be so selected that they remain stable under all operating conditions. Side slope ranges from vertical to 1:3 for lined canals to 1:1/2 to 1:3 for unlined canals, depending on site conditions.

Free Board

Free board is vertical distance between full supply level and top of canal banks. It depends on full supply depth and discharge of canal and generally ranges from 1 ft. to 4 ft. for small distributaries and main canals carrying 3000 cfs discharge. For canals carrying 10000 cfs or more discharge, it is 5.5 ft. Following equation provides estimate for free board.

F = √ C y

Where F = Free board in ft.

y = Design depth in ft.

C = Coefficient which varies from 1.5 ft. for

20 cfs to 2.5 for 3000 cfs or more.

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Drainage behind Lining

In case of hydrostatic pressure behind lining, drainage of soil behind lining should be provided. Drainage may consists of filter blanket or transverse and longitudinal drains under the lining.

Super Elevation

Bed of canal is elevated on outer side as compared with inner side on curves to overcome effects of curvature, which is called super elevation. The effect of curvature is negligible if ratio of radius of curvature to distance to center of canal is greater than 3 times bed width of canal. Super elevation can be calculated from the following equation.

h = V2 b / g R

Where h = Super elevation

V = Average sub critical velocity

b = Width of canal

R = Distance from center of curve to center line of canal

Berm Width

Berm is distance between edge of canal section and inner toe of canal bank. Berm width is usually kept between 2D to 4D, where D is full supply depth.

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7. DESIGN EXAMPLES

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References

1. Design of Small Canal Structures (USBR)

2. Irrigation and Hydraulic Structures (Dr.Iqbal Ali)

3. Irrigation and Drainage Engineering (Iqtidar Saddiqui)

4. Open Channel Hydraulics (Ven Te Chow)

5. Open Channel Flow (M. Hanif Chaudhry)

6. Open Channel Flow (F.M. Henderson)

7. Irrigation Canals (Iqtidar Saddiqui)

8. Irrigation Channels (WAPDA)

9. Hydraulic Engineering (J Roberson & Hanif Ch)

10. Design of Channels in Alluvium Soil (PEC)

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Canal Lining

Lining means protection of canal prism with impervious material. Canal lining may be rigid, semi rigid or flexible.

Need of Lining

Canal is lined in case of:

• Track of land through which canal is passing is highly pervious.

• Less of water through seepage is very high.• Danger of water logging.• Very high velocity is desired in canal.• Head and tailrace of hydel power station.

Advantages of Lining

Advantages of lining area:

• Water losses are reduced.• Water logging is controlled• Discharge capacity of channel is increased• Mainframe cost is reduced.• Risk of pilferage of water is reduced.• Weed growth is controlled.• Danger of erosion and breach is eliminated• Salt absorption is checked

CANAL LINING

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Lined Canal

Canal whose prism has been protected with impervious material mainly to stop seepage through it, is called lined canal.

Types of Canal Lining

Various types of lining are.

• Shotcrete Lining

• Concrete Lining

• Asphalt Lining

• Brick Lining

• Earth Lining

• Clay Lining

• Bentonite Lining

• Membrane Lining

• Asphalt Membrane

• Plastic Film

• Gootextile Lining

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Laminar Flow

It liquid particles appear to move in definite smooth paths and flow appears to becis movement of this layers on tap each other, then flow is called laminar flow

Turbulent Flow

In turbutent flow, liquid particial as more in irregular paths, which are not fined with respect to either time or space.Transition from laminar to turbulent flow in free surface flows accures for re of

Lecture on Channels04.02.10

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