Final Canal Losses

8/2/2019 Final Canal Losses

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BY:

NEELABH GUPTA

GAURAV KAPSE

MELVIN DSOUZA

ADITYA KAPIL

PRIYA GAWADE

ST. JOHNS COLLEGE OF ENGINEERING AND TECHNOLOGY


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1. INTRODUCTION

Not all water taken from a source (river, well) reaches the root zone of theplants.

Part of the water is lost during transport through the canals and in the fields.

The remaining part is stored in the root zone and eventually used by the plants.

In other words, only part of the water is used efficiently, the rest of the water islost for the crops on the fields that were to be irrigated.

The lost of water from canals & distributaries seems to be greater than the lossfrom irrigation into the soil.

This loss is often the cause of unnecessary scarcity in the water supply,especially at the lower end at the canals during the season when water is low.


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2. The irrigation water losses in canals; these are due to:

1. Evaporation from the water surface2. Deep percolation to soil layers underneath the canals3. Seepage through the bunds of the canals4. Overtopping the bunds5. Bund breaks6. Runoff in the drain7. Rat holes in the canal bunds


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SOURCE:


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3. Evaporation Loss :

The evaporation loss is important particularly inwater scarce areas.

It hardly exceeds 1 to 2 percent of total water in unlined channels.

Maximum loss: in summer due to high temp & high wind velocity.

The average evaporation loss per day : 4mm-10mm

Evaporation loss depends on-

1)Climatic Factors such as Temperature, humidity, andwind velocity.

2) Canal Factors such asWater surface area , water depth and velocity of f low.

3) the supply of energy to provide the latent heat of vaporization and

4) the ability to transport the Vapour away from the evaporating surface.


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3.1. Calculation of evaporation loss:

Warnaka and Pochop and Ikebuchi et al. gave the following equation :

The evaporation loss from a canal can be expressed as:

Where

E = Evaporation lost

qe = evaporation discharge per unit length of canal (m2/s);

T = width of free surface (m).

qe = E * T


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4. Seepage Loss:

Seepage loss is the major and the most important part of the total water loss.

By the time the water reaches the field, it has been estimated that theseepage losses are of the order of 45% of the water supplied at the head of thecanal.

The seepage loss results not only in depleted freshwater resources but alsocauses water logging, Stalinization, and ground-water contamination.

Canals in alluvium are lined in general and reduce the seepage in particularSeepage from a lined canal occurs at a reduced rate.


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4.1 SEEPAGE LOSS DEPENDS ON THE FOLLOWING FACTOR

Position of subsoil water table.

Porosity of soil and subsoil.

Extent of absorbing medium.

Design of canal cross section.

a) Depth of water in canal : greater depth , greater loss of water.b)velocity of water : loss decreases with increase in velocity.

Physical properties of canal water:

a)Temperature of water: loss increases with increase in temperature

of water.

b) Amount of silt carried in suspension: losses decreases with

increase in amount of silt carried in suspension.

Condition of canal system:

a) loss decreases with the age of canal.

b)increases with the extent of absorbing medium.


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4.2 Calculation of seepage loss:

The seepage loss from a canal in a homogeneous and isotropic porous

medium, when the water table is at a very large depth, can be expressed as

q s = k*y*F

where qs = seepage discharge per unit length of canal (m2/s);

k = hydraulic conductivity of the porous medium (m/s);

y = depth of water in the canal (m);

F = function of channel geometry


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4.3 Depending upon the position of the water table the seepage lossfrom the canal occurs in two ways :

1. Absorption:

water table considerably below GL

water seeping through pores: unable to join the water table

wets the subsoil locally forming a saturated bulb

Zone between saturated zone and capillary moisture zone-unsaturated

SEEPAGE LOSS BY ABSORPTION SOURCE : B.C. PUNMIA.


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4.4 Percolation:

Water table is close to ground level.

Seepage water establishes a direct & continuous flow between canal and

WT.

Loss depends upon the head difference between the full supply level ofcanal & water -table

SEEPAGE LOSS DUE TO PERCOLATION SOURCE : B. C. PUNMIA


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5. THE LOSSES ARE EXPRESSED IN THEFOLLOWING DIFFERENT WAYS:

a) As cumec per million square meters of the wetted perimeter

b) As a depth of water lost in 24 hours over the area of the

wetted perimeter.

c) As a percentage of the discharge of the channel.

d) As a percentage per km length of the canal.


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6. ESTIMATION OF DESIGN DISCHARGE

Needs:

to design the supply canal and the pump (if any)to check that enough water is available from the source.

The amount of water required by a crop depends on :the local environment, the climate, the crop and its stage of growth,

and the degree to which the crop may be stressed.

This requirement may be expressed as a uniform depth of water overthe area in millimetres per day(mm/d).

Irrigation requirements:

Reference evapotranspiration (ETo) is the water use of grass (in mm/d)under standard conditions. Local estimates may be available frommeteorological offices.

For most crops, the reference evapotranspiration at mid-season can be

taken as a reasonable estimate of the peak water requirement.


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It is reasonable to assume that 70 per cent of average rainfall is available

to the crop; the net irrigation requirement (In mm/d ) can be estimated as:In = ETo - (0.70 x P)

where P (mm/d) is the average rainfall.

Additional water has to be supplied to take account of field applicationlosses which, with surface irrigation, are typically about 40 per cent, givingan application efficiency of0.60.

The field irrigation requirement (If) can be estimated as:If = In = ETo - (0.70 x P)

0.60*0.60

The field irrigation requirement represents the rate (in mm/d) at whichwater must be delivered to the field to prevent the crop suffering a shortageof water.


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The required canal discharge depends on the field area to be irrigated

(known as the 'command area'), and the water losses from the canal.

For a design command area A (m sq), the design discharge required Q(l/s) for irrigation hours (H) every day, is given by the field-irrigationrequirement multiplied by the area, divided by the time (in seconds):

When the water-surface width in the canal is W metres, a drop of S

millimetres per hour corresponds to an average canal loss of:

canal loss = W x S l/s per metre length60 x 60

Q = If x A plus canal lossesH x 60 x 60


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Example: What design discharge is required for a canal to irrigate anarea of 10 hectares in the semi-arid subtropics, when the mean dailytemperature is 30oC, and the mean rainfall is 0.2 mm/d during the

peak period (midseason)? The canal is 800m long and is to operate for12 hours per day. Losses from a similar canal are measured as 48mm perhour with a water-surface width of 1.5m.

1) ETo = 7.5 mm/d

2) Hence the net irrigation requirement is: In = ETo - (0.70 x P)= 7.5 - (0.7 x 0.2) = 7.36 mm/d;

3) the field irrigation requirement is: If = In = ETo - (0.70 x P)0.60*0.60

= 7.36/0.60 = 12.3 mm/d

4) Canal losses = W x S60 x 60

= 48 x 1.5 = 0.02 l/s per metre length60 x 60


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5) A = 10 ha = 10 x 10 000 m2

6) Q = If x A canal lossesH x 60 x 60

= 12.3 x (10 x 10 000) + 800 x 0.0212*60*60

= 28 + 16 = 44 l/s

This design discharge of 44 l/s should be compared with the wateravailable from the source. If less is available, the area may need to bereduced, or the irrigation time increased.


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The loss from canals is known to be large, and often produces aserious problem in their management.

This loss is often the cause of unnecessary scarcity in the watersupply, especially at the lower end of the canal when the water is low.

The loss in clay soils is less than in sandy soils or gravely soils.

From standpoint of economy , it is important to know the losses andthereby estimating the design discharge to meet the irrigation demand

of a system.

As a remedial measures to losses, impervious linings or coatings offine material as silt or clay can be applied thus making canal lined.


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[1] Irrigation and Water power engineering, Dr. B. C. Punmia, Dr. Pande B.B, Laxmipublication

[2] Ian Smout and Rod Shaw WEDC Loughborough University Leicestershire LE11 3TU

UK www.lboro.ac.uk/departments/cv/wedc/

[3] Harr, M. E. (1962). Groundwater and seepage. McGraw-Hill, New York.Measurement of seepage losses from canals. (1980). IS: 9452 (part I
http://www.lboro.ac.uk/departments/cv/wedc/http://www.lboro.ac.uk/departments/cv/wedc/


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Final Canal Losses

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