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DESIGN OF ERODIBLE

CHANNELS

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Introduction

Under the gravitational Irrigation system the water is conveyed

from the source to the field level through irrigation canal.

The source of water for irrigation is available from River, Reservoir,

Lakes and Tanks. Flow Irrigation may be Diversion type, Storage,

Perennial and Inundation etc.

Irrigation canals are generally Trapezoidal in shape, constructed at

the highest elevation for batter command. The channel could be

cutting or filling.

IRRIGATION CANALS

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Kennedy’s Silt Theory

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Kennedy’s Silt Theory►

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Kennedy’s Silt Theory

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Steps Required to Design Canal by Kennedy’s Theory

i) Assume a trial value of Depth ‘y’

ii) Find the velocity by the equation V = 0.55 mr y0.64

iii) Find Flow Area A =Q/V

iv) Find ‘B’ from ‘A’ = (B + zy) y

Where ‘A’ is known from step (3), ‘z’ is assumed or given, ‘y’ is

assumed in step (1)

v) Find P = B +2y √1 + z2

vi) Find R =A/P

vii) Now find velocity ‘V’ by Kutter’s equation

viii)If the velocity obtained in step (ii) and in step (vii) are not almost

equal, assume second trail values of Depth ‘y’ and report steps (i)

to (viii) above.

DESIGN OF ERODIBLE CHANNELS

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Steps Required to Design Canal by Kennedy’s Theory

ix) Repeat this process, until velocity in step (ii) = velocity in step

(vii). The value of ‘y’ at which the two velocities are almost same,

is the required depth of flow.

x) Find ‘B’ when ‘y’ is known.

xi) Assume a reasonable free board (FB). Usually 0.6 when Q < 10

m3/sec and (FB) > 0.75 when Q > 100 m3/sec

This trial and error method is very tedious and assumption of first

trial values of ‘y’ is very difficult. The design of canal by Kennedy’s

theory can be designed from Garret’s diagram which provide a

graphical solution of Kennedy’s equation.

DESIGN OF ERODIBLE CHANNELS

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Lindley’s Regime Theory

DESIGN OF ERODIBLE CHANNELS

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Example-I

Design a trapezoidal channel by Kennedy’s theory with side slope 1:1

in alluvial soil to carry a discharge of 30m3/sec in bed slope of 1/5000.

rugosity coefficient of Kutter is 0.0225, CVR = 1

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DESIGN OF ERODIBLE CHANNELS

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Lacey’s Regime Theory

Better and modified method was developed by G. Lacey. His

regime theory postulates that dimensions of bed width, depth and

shape attain a state of equilibrium called Regime State.

Lacey has defined a Regime Channel as a stable Channel

transporting a minimum bed load consistent with fully active bed.

According to his theory a channel will be in regime if it carries a

constant Discharge and it flows uniformly in unlimited incoherent

alluvium of the same character.

Lacey also differentiated between initial and final regime condition

of channel. The initial condition is attained shortly after it is put

into operation after construction and the canal begins to adjust its

bed slope either by silting or scouring although bed width is not

changed.

DESIGN OF ERODIBLE CHANNELS

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Lacey’s Regime Theory

The canal then appears to have attained stability, but it is not

actually the final state of stability and hence, it still represents the

initial regime condition.

Eventually continuous action of water overcomes the resistance of

the banks and sets up a condition such that the canal adjusts its

complete section. At this stage the final or true regime condition is

attained.

DESIGN OF ERODIBLE CHANNELS

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Regime Equations of Lacey

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Regime Equations of Lacey

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Regime Equations of Lacey

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Regime Equations of Lacey

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Regime Equations of Lacey

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Regime Equations of Lacey

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Summary of Lacy’s Formula

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Steps Required in Design by Lacey’s Theory

Known Data is Q, sediment size, side slope zH: IV (if not given assume

1/2H : 1 or 1H : IV)

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Steps Required in Design by Lacey’s Theory

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Example-I

Using Lacey’s silt theory, design an irrigation canal in alluvial soil with

a Discharge of 14m3/sec flows through the channel in soil of 0.33 mm

diameters. Recommend FB.

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Example-II

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Example-II

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Sediment Transportation

in Irrigation Channels

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Sediment and its Transport

Sediment in transport in open canal plays an important role and

exerts as great influence on the attainment of regime in earthen

canals. Excessive silt in the canal get deposited in the bed, thereby

reducing the discharge capacity of the canal.

On the other hand, silt-free water flowing in earthen canals in

regime would tend to scour the bed until a new regime is

established.

Therefore, a regime channel should be capable of transporting the

total sediment load and there should be no silting nor scouring.

However, practically it is very difficult to attain this situation

particularly when the channel is newly constructed.

SEDIMENT TRANSPORT

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Development of Canal Bed

After water starts flowing in a canal, its bed adopts various shapes

depending upon the velocity of flow.

At a very low velocity, there is no movement in the canal bed. With

the increase in velocity, the sediments start moving. The stage at

which the movement starts, is called ‘threshold of motion’.

If the bed is made of fine sand, saw tooth ripples start to appear in

the bed. With the increase in velocity, the saw tooth get rounded off

and become dunes with ripples. With further increase in velocity,

they take the shape of rounded dunes.

With increase in velocity the dunes disappear and flat surface

appears. Further increase in velocity will now form sand waves in

association with surface waves. Still further increase in velocity

will result in movement of wave systems and formation of sand

waves, called antinodes. At this stage Froude’s number

approaches unity.

SEDIMENT TRANSPORT

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Development of Canal Bed

SEDIMENT TRANSPORT

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Sediment Load

The sediment moving in a canal can be broadly divided into two

parts namely bed load and suspended load. Bed load is that parts

of the sediment load which due to its size and weight does not go

in suspension in the moving water but moves along with the bed of

the canal. This load moves forward by rolling, sliding or salutation,

i.e. non-moving bed.

Suspended load is that part of the sediment load whose particles

being small in size and weight, are thrown in suspension with the

increase in the velocity of moving water in the channel. A particle is

thrown in suspension by the upward component of the turbulent

eddies.

This load does not move in contact with the bottom thought there

may be a continuous interchange of particles from the bed. In this

process some grains are dropped while others go in suspension.

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Tractive Force Theory

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Tractive Force Theory

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Tractive Force Theory

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Tractive Force Theory

SEDIMENT TRANSPORT

Φ

Φ

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Estimation of Suspended Load

SEDIMENT TRANSPORT

Estimation of Bed Load

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Estimation of Bed Load

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Estimation of Bed Load

SEDIMENT TRANSPORT

Meyer-Peter Formula

SEDIMENT TRANSPORT

Einstein – Brown Formula

SEDIMENT TRANSPORT

Einstein – Bed Load Function

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Estimation of Suspended Load

SEDIMENT TRANSPORT

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Example

SEDIMENT TRANSPORT

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SEDIMENT TRANSPORT

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SEDIMENT TRANSPORT