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DEFINITION OF IRRIGATION:

The science of artificial application of water to the land, in accordance with the „crop requirements‟

throughout the „crop period‟ for full – fledged nourishment of the crops.

TYPES OF IRRIGATION

Factors affecting the choice of the method of irrigation

Certain methods of irrigation are of general applicability and are used in almost all irrigation regions, but

there are other methods of limited applicability which can be used only when special conditions prevail.

As such a proper selection of the method of irrigation is very essential and is based on the following

factors.

(i) Soil characteristics of the land to be irrigated.

(ii) Topography of the country – slope of land surface, roughness of the surface, etc.

(iii) Size of the stream supplying irrigation water to the land to be irrigated.

(iv) Available water supplies and the rate of advance of irrigating water.

(v) Length of run and time required for wetting the total area of the land to be irrigated.

(vi) The water requirements of the crops grown and the growth habits of the plants.

(vii) Rate of infiltration of the soil.

(viii)Depth of the root zone of the plants.

(ix) Depth of the water table.

(x) Possible erosion hazard.

(xi) Amount of water to be applied during each irrigation.

Irrigation may broadly be classified into:

i) Surface irrigation ii) Sub-surface irrigation

Techniques of water Distribution in the Farms:

There are various ways in which the irrigation water can be applied to the fields. Their main

classification is as follows:

(i) Free flooding (ii) Border flooding

Chapter

1

IRRIGATION METHOD &

WATER REQUIREMENTS Syllabus: Irrigation, Types Of Irrigation , Factors, Water Requirements

Of Crops, Soil Moisture , Duty And Delta , Factors Affecting Duty Of

Water , Potential Evapotranspiration (Pet) And Actual

Evapotranspiration (Aet) , Irrigation Efficiencies , Irrigation

Requirements Of Crops. wieghtage= 25%


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(iii) Check flooding (iv) Basin flooding

(v) Furrow irrigation method (vi) Sprinkler irrigation method

(vii) Drip irrigation method

These methods are briefly discussed below:

1. Free flooding or ordinary flooding: In this method, ditches are excavated in the field, and they

may be either on the contour or up and down the slope. Water from these ditches, flows across the

field. After the water leaves the ditches, no attempt is made to control the flow by means of levees,

etc. Since the movement of water is not restricted, it is sometimes called wild flooding. This

method may be used on rolling land (topography irregular) where borders, checks, basins and

furrows are not feasible.

MAIN SUPPLYDITCH

MAINSUPPLYDITCH

OUTLETS

SUBSIDIARY DITCH

OUTLET

Free flooding (plan view)

2. Border flooding: In this method, the land is divided into a number of strips, separated by low

levees called borders.

3. Check flooding: Check flooding is similar to ordinary flooding except that the water is controlled

by surrounding the check area with low and flat levees.

DITCH GATE OPERATED OPENINGS

10 TO 20MLOW LEVEES

BORDERS

100 TO400 m

4. Basin flooding: this method is a special type of check flooding and is adopted specially for orchard

trees.

5. Furrow irrigation method: In furrow irrigation method only one-fifth to one-half of the land

surface is wetted by water. Furrows are narrow field ditches, excavated between rows of plans and

carry irrigation water through them.

6. Sprinkler irrigation method. In this water is applied to the soil in the form of a spray through a

network of pipes and pumps.


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The conditions favouring the adoption of this method are:

i) When the land topography is irregular, and hence unsuitable for surface irrigation

ii) When the land gradient is steeper and soil is easily erodible.

iii) When the land soil is excessively permeable.

iv) When the water table is high.

Disadvantages of Sprinkler Irrigation

Limitations of Sprinkler Irrigation

1. Wind may distort the sprinkler pattern, thus resulting in non-uniform application of irrigation

water.

2. The first investment involved in the sprinkler irrigation method is high.

3. A constant water supply is needed for the most economical use of the equipment of the sprinkler

irrigation system.

4. Water must be clean and free sand etc., to prevent the clogging of the sprinklers.

5. The power requirement is high since continuous pumping of water is required in this method of

irrigation.

6. Heavy soils with poor intake cannot be irrigated efficiently by the sprinkler irrigation method.

7. Drip Irrigation Method. Drip irrigation, also called trickle irrigation, is the latest field irrigation

technique, and is mean for adoption at places where here exists acute scarcity of irrigation water and

other salt problems. In this method, water is slowly and directly applied to the root zone of the

plants, thereby minimizing the losses by evaporation and percolation.

WATER REQUIREMENTS OF CROPS:

SAR:-

The proportion of sodium ions present in the soils, is generally measured by a factor called Sodium-

Absorption Ratio (SAR) and represents the sodium hazards of water. SAR is defined as:

SAR = Na+

Ca +++ Mg ++

2

S.No Type of water

1. Low sodium water (SI). SAR value lying between 0 to 10.

2. Medium sodium water (S2) SAR value Lying 10 o 18.


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3. High sodium water (S3). SAR value lying between 18 to 26.

4. Very high sodium water (S4). SAR value Above 26

SOIL MOISTURE:-

Classes and Availability of Soil Water:-

Water present in the soil may be classified under three heads:

1. Hydroscopic water 2. Capillary water 3.Gravitational water.

Super FluousWater

Non AvailableWater

Oven Dried Soil

Hydroscopicwater

GravitationalWater

Capillary Water

AvailableWater

WitingCoeff.

Available moisture for the plant = Fc –

(i)

Mois

ture

Co

nte

nt

(m.c

)

Optimum moisturecontent

Field capacity

Field capacity m.c

Available orCapillary water

Wilting Point m.c.

Non available orm.c

Hygroscopic water

Time

Readily Available Moisture

Availablemoisture

Mo

istu

re c

on

ten

t o

f o

f so

il

Time

Wilting P.T.m.c

Optimumm.c

Fold Capacity

Impervious strata

Soil Moisture

W.T.

Ground water

Soil zone or root zone

Intermediate zoneCapillary zone

(ii) Readily available moisture for the plant = Fc - Mo

Here, Fc = Field capacity

= Willing point or wiling coefficients below which Plant can‟t survive.


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Mo = Readily available moisture content.

(iii) Frequency of irrigation = Available / Readilyavailablemoisturedepth

consumptiveuserate

(iv) C

Weight of waterstoredinsoilof unit areaF

Weight of samesoilof unit area

where. Weigh of water stored in soil of unit area = 𝛾w .dw.1.

Weigh of same soil of unit area = 𝛾.d.1

dw = depth of water stored in root zone.

(v) Available moisture depth to plant.

'

w c

.dd (F )

w

(vi) Readily available moisture depth to plant

'

w c 0

w

.dd (F m )

(vii) Where, G = specify gravity and n = Porosity

Fc = n/G

CROPS

Two main crop seasons of India viz., Rabi and Kharif. The crops grown during these crop seasons are

designated as Rabi crops and Kharif crops. Rabi crops are also known as winter crops and Kharif crops

are known as monsoon crops. Normally Rabi crops are sown in the month of October and are harvested

by the end of March, while Kharif crops are sown in the month of April and are harvested by the end of

September.

The crops may also be classified on the basis of their irrigation requirements as Dry crops and Wet crops.

Dry crops are those crops which are ordinarily grown without irrigation, but they irrigation facilities are

normally not available. However, sometimes dry crops are those areas where the irrigation facilities are

normally not available. However, sometimes dry crops are also irrigated, especially in the years of

deficient rainfall, in which case these are known as irrigated dry crops. On the other hand wet crops are

those crops which cannot normally be grown without irrigation.

DUTY AND DELTA:-


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Duty: The duty of water is the relationship between the volume of water and the area of the crop it

matures. It is defined as the area irrigated per cumec of discharge running for base period B. The duty is

generally represented by D.

Delta: It is the total depth of water required by crop during the entire base period and is represented by

the symbol∆.

Crop Average delta (cm)_

Rice 120

Wheat 37.5

Cotton 45

Tobacco 60

Sugarcane 90

Relation between Duty and Delta:-

8.64B

D

where , = Delta in meter

D = Duty in Ha/cumec

B = Base period in days

Also

2B

D

where, ∆ = Delta in feet

B = Base period in days

D = Duty in acre /cusec.

FACTORS AFFECTING DUTY OF WATER

The duty of water mainly depends on the following factors.

(1) Type of crop.

(2) Climate condition of the area.

(3) System of irrigation.

(4) Method of irrigation.

(5) Quality of irrigation water.

(6) Method of cultivation.

(7) Time of irrigation and frequency of cultivation.

(8) Type of soil and sub-soil of the irrigated field.


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(9) Type of soil and sub-soil of the area through which canal passes.

(10) Canal conditions.

(11) Method of assessment of irrigation water rate.

(12) Skill of the cultivator.

(13) Topography of land.

(14) Base period of crop.

(1) Type of crop. Different crops require varying quantities of water and therefore duty of water varies

from crop to crop. The crops which require large quantity of water have lower duty of water than for

the crops which requires less quantity of water.

(2) Climatic condition of the area. The water requirement of a crop varies with the climatic condition

of the area and hence it also affects the duty of water. The climatic conditions which affect the duty

of water are (i) temperature, (ii) Wind velocity, (iii) humidity and (iv) rainfall. If the temperature is

high, the loss of water due to evapotranspiration will be more and hence the duty of water will be

less. Similarly higher wind velocity will result in greater loss of water due to evapotranspiration and

hence lower duty of water. On the other hand higher is the humidity higher will be the duty of water

since the loss of water due to evapotranspiration will be less. Further if during the base period of a

crop there is rainfall then since less quantity of irrigation water will be require to be supplied the

duty of water will increase.

(3) System of irrigation. In the perennial irrigation system the soil of the irrigated area remains

continuously wet and hence less quantity of water is required for initial saturation of the soil. Further

in the areas where perennial irrigation is practiced the water table is generally high and hence the

loss of water due to deep percolation is less. Also in the perennial system of irrigation a better

control on the use of irrigation water may be enforced. On the other hand in the inundation irrigation

system there is a wasteful use of water. As such for the perennial irrigation system the duty of water

is higher than for the inundation irrigation system. Similarly for the flow irrigation system the duty

of water is low due to the transmission losses in the network of the canals. However, for tank

irrigation system the duty of water may be high if the land to be irrigated is in the close vicinity of

the tank and through proper control efficient use of water is made. On the other hand for the lift

irrigation system the duty of water is generally high because the land to be irrigated is mostly near

the well and hence the transmission losses are considerably reduced.

(4) Method of irrigation. The method of irrigation or the mode of applying water to the fields affects

the duty of water because the water application efficiency varies with the method of irrigation. Thus

among the various surface irrigation methods the duty of water is higher for furrow method than for


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any of the flooding methods. This is so because in furrow method since water is not applied to the

entire land surface the water losses are considerably reduced. Further as compared to the surface

irrigation methods the duty of water is high for the sprinkler irrigation method as well as for the sub-

surface irrigation methods, and generally for the drip irrigation method the duty of water is the

highest.

(5) Quality of irrigation water. If the irrigation water contains an appreciable amount of harmful salts

and alkalies dissolved in it then it is required to be applied in large quantity so that the salts are

leached off. This however results in a lower due to the wastage of considerable amount of water. On

the other hand if the water is of better quality and contains useful salts dissolved in it then the

wasteful use of water is avoided and it results in a higher duty of water.

(6) Method of cultivation. If the land is properly ploughed upto the required depth and made quite

loose before irrigating, the soil will have high water retaining capacity in the root zone of the plants.

This will reduce the number of watering and hence result in a higher duty of water. Further the use of

modern methods of cultivation gives higher duty of water than the old conventional methods of

cultivation.

(7) Time of irrigation and frequency of cultivation. In the initial stages the land to be cultivated may

not be properly leveled and hence more than the required quantity of water may be applied, which

will result in a lower duty of water. The land slope may gradually improve with time then only the

required quantity of water will be applied and hence the duty of water will be high. Further gradual

rise of water table with time due to continuous irrigation will make water available in the root zone

of the plants, thus relatively less quantity of water will be required to be applied which will result in

a higher duty of water.

Frequent cultivation of land reduces the loss of moisture through weeds and evaporation from soil

and hence result in a higher duty of water. Moreover, frequent cultivation helps to maintain the soil

structure in good condition so that the soil is in good tilth higher water retaining capacity and hence

results in a higher duty of water.

(8) Type of soil and sub-soil of the irrigated field. If the and sub-soil of the field to be irrigated is

coarse grained then due to high percolation loss the duty of water will be low. However, the presence

of an impervious layer below the root zone of the plants i.i., at a depth of 1 to 2 metres below the

surface will reduce percolation of water and hence the duty of water will be high.

(9) Type of soil and sub-soil of the area through which canal passes. If the canal is unlined and it

passes through coarse grained soil then since there will be greater percolation los the duty of water


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will be low. On the other hand if an unlined canal is passing through the grained soil then the

percolation loss will be less and hence the duty of water will be high.

(10) Canal conditions. In an earthen canal, the percolation loss will be high which will result in a low

duty of water. However. The canals with good maintenance will have a higher duty of water than

those having poor maintenance. Further if a canal is so aligned that the land to be irrigated is

concentrated along it then since the transmission losses will be less the duty of water will be high.

However, the dispersion of the fields to be irrigated with respect to the canal will increase the

transmission losses and hence the duty of water will be low.

(11) Method of assessment of irrigation. The assessment of irrigation water on volumetric basis

prevents wastage of water and over irrigation by the farmers, thus leading to economy of water and a

higher duty of water. However, if the assessment is made on flat rate basis or on the basis of area

under cultivation then the farmers are tempted to use more water which leads to wastage of water

and a lower duty of water.

(12) Skill of cultivator. The judicious use of water by the cultivators can save large quantity of water,

which can be used to irrigate more area of the land thus resulting in a higher duty of water. However,

the economical use of water will mainly depend on the skill of the cultivators and therefore if the

cultivators are not skilled they should be trained to use irrigation water with utmost care and do

avoid its wastage.

(13) Topography of land. If the land to be irrigated is properly leveled then uniform application of water

will be possible which will result in economical use of water and hence a higher duty of water. On

the other hand if the land is not leveled then the lower portion will receive more water than the

higher portion, resulting in a wasteful use of water and hence the duty of water will be low.

(14) Base period of crop. In general when the base period of a crop is long, more water may be required

thus resulting in a lower duty of water. However, the requirement of water is not directly

proportional to the base period of the crop.

Paleo Irrigation (or Paleo)

It is defined as the watering done prior to the sowing of a crop. This is done to prepare the land for

sowing and to add sufficient moisture to the soil which would be required for the initial growth of the

crop.

Kor Watering, Kor Depth and Kor Period

The total quantity of water required by a crop is applied through a number of watering at certain interval

during the base period of the crop. However, the quantity of water required to be applied during each of


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these watering is not same. In general for all the crops during the first watering after the plants have

grown few centimeters high, the quantity of water required is more than that during the subsequent

watering. The first watering after the plants have grown a few centimeters high is known as kor watering

and the depth of water applied during this watering is known as kor depth. The kor watering must be done

in a limited period which is known as kor period.

Crop Ratio

Crop ratio is defined as the ratio of the areas of the land irrigated (or anticipated to be irrigated) during

the two main crop season viz., Rabi and Kharif.

Factors affecting consumptive use of water

The various factors affecting the consumptive use of water are as follows.

(1) Evaporation from the soil (2) Temperature

(3) Wind velocity (4) Relative humidity of air

(5) Precipitation (6) Day time hours

(7) Intensity of sunlight (8) Soil type and topography

(9) Type of crop (10) Cropping pattern

(11) Length of growing of the plant (12) Stage of the growth of the plant

(13) Amount of foliage of plants (14) Nature of leaves of plants

(15) Method of irrigation (16) Quantity of irrigation water applied

(17) Quantity of readily available soil moisture.

Potential Evapotranspiration (PET) and Actual Evapotranspiration (AET)

For a given set of climatic conditions, evapotranspiration obviously depends on the availability of water.

If sufficient moisture is always available to completely meet the needs of the plants the resulting

evapotranspiration is called Potential evapotranspiration (PET). Potential evapotranspiration no longer

critically depends on soil and plant factors but depends essentially on climatic factors. The real

evapotranspiration occurring in a specific situation is called actual evapotranspiration (AET). At the field

capacity, since the water supply to the plants is adequate AET will be eaual to PET, i.e., the ratio

(AET/PET) is equal to 1.

Consumptive use determination by use of Equations:-

The following are some of the commonly used methods.

1. Penman- Method

2. Blaney-Criddle method


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3. Hargreaves class A pan evaporation Method

4. Evaporation method

Irrigation Efficiencies:-

i) Water Conveyance -Efficiency,(𝑛c)

fC

r

wn 100

w

where ,wf = water delivered to the field.

wr = water delivered from the reservoir.

ii) Water application efficiency (𝑛a)

sa

f

wn 100

w

Where, ws = Water stored in the root zone.

wf = Water delivered to the field.

iii) Water use Efficiency (𝒏𝒖)

Where, uu

f

Wn 100

W wu= Water use consumptively

wf = Water delivered to the field.

iv) Water Storage Efficiency(𝒏𝐬)

S'S

n

Wn 100

w

Where, ws‟ = Actual water stored in the root zone.

wn = Water needed to store to bring the water content up to field capacity.

v) Water Distribution Efficiency, (𝒏d)

d

yn 1 100

d

Where, y = Average numerical deviation in the depth of water stored from the average depth of

irrigation stored.

d = Average depth during irrigation.

vi) Consumptive use efficiency (𝒏cu)


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cucu

d

wn 100

w

where, wcu or Cu = water used by plant consumptively.

wd = Net amount of water depleted from root zone.

Irrigation Requirements of Crops:

(1) Consumptive Irrigation Requirement, (CIR)

u effCIR C P

It is defined as the amount of irrigation water that is required to meet the evapotranspiration needs of

a crop during its full growth. However, if during the growth period of a crop rain occurs then since a

part of it will be retained by the soil in the root zone and the same will be available to meet a part of

the evapotranspiration requirements of the crop, the quantity of irrigation water required to be

applied will be correspondingly reduced.

Where, Cu = Total consumptive use requirement.

Peff = Effective rainfall.

(2) Net Irrigation Requirement (NIR)

NIR=CIR + Leaching requirement

It is defined as the amount of irrigation water required to be delivered at the field to meet the

evapotranspiration needs of a crop as well as the other needs such as leaching, presowing

requirement.

(3) Field Irrigation Requirement (FIR). It is defined as the amount of water to meet the „net irrigation

requirements‟ plus the amount of water lost as surface runoff and through deep percolation. As

indicated in the previous section the water application efficiency ηa accounts for the loss of irrigation

water by surface runoff and through deep percolation and hence

a

NIRFIR

(4) Gross Irrigation Requirement (GIR). It is defined as the amount of water required to meet the

field irrigation requirements plus the amount of irrigation water lost in conveyance through the canal

system by evaporation and by seepage. Again as indicted in the previous section the water

conveyance efficiency ηc accounts for the conveyance losses and hence

c

FIRGIR

Leaching Requirement (LR)


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Leaching requirement (LR) is the minimum amount of irrigation water supplied that must be drained

through the root zone to control soil salinity at the given specific level. For sandy lam to clay loam soils

with good drainage and where rainfall is low the leaching requirement can be obtained from the following

expression.

For surface irrigation methods including sprinkler

5

w

e w

ECLR

EC EC

…(4.44)

For drip irrigation and high frequency sprinkler irrigation (nearly daily)

2 Max.

w

e

ECLR

EC …(4.45)

in which

LR = leaching requirement expressed as a fraction of total irrigation water required for plant consumption

and for leaching;

ECw= electrical conductivity of the irrigation water, m mhos/cm;

ECe= electrical conductivity of the soil saturation extract for a given crop apporopriate to the tolerable

degree of yield reduction.

Max.ECe = maximum tolerable electrical conductivity of the soil saturation extract for a given crop.

DEGRADING TYPE

River training implies various measures adopted on a river channel along a certain alignment with a

certain cross-section. These measures are required to be adopted because rivers in alluvial plains

frequently alter their courses and cause damage to land and property adjacent to their banks.

OBJECTIVES OF RIVER TRAINING

Chapter

2 RIVER TRAINING

AND CANALS Syllabus: River training,objectives of river training, classification of

river training works , marginal embankments or levees , guide banks ,

spurs or groynes , functions of groynes , types of alignment , canal

irrigation, layout of a diversion head works and its components

Wieghtage= 15%


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(i) To prevent the river from changing its course and to avoid outflanking the structures like bridges,

weirs, aqueducts, etc.

(ii) To prevent flooding.

(iii) To protect the river banks by deflecting the river away from the attacked banks.

(iv) To ensure effective disposal of sediment load.

(v) To provide minimum water depth required for navigation.

CLASSIFICATION OF RIVER TRAINING WORKS

On the basis of the purpose required to be served the river training works may be classified into the

following types:

(1) High water training

(2) Low water training

(3) Mean water training

(1) High water training. High water training is undertaken with the purpose of providing expeditious of

maximum floods and thus provide protection against damage due to floods. It is mainly concerned

with the most suitable alignment and height of marginal embankments for disposal of floods and

may also include other measures of channel improvement for the same purpose. Thus high water

training can also be called Training for discharge.

(2) Low water training. Low water training is undertaken with the purpose of providing sufficient

depth for navigation during the low water season. This is achieved by contracting the width of the

channel at low water and is usually carried out with the help of groynes. Thus low water training can

also be called Training for depth.

(3) Mean water training. Mean water training is undertaken to provide efficient disposal of bed and

suspended sediment load and thus to preserve the river channel in good shape. Thus mean water

training can also be called Training for sediment.

The following are the generally adopted methods for raining rivers:-

(1) Marginal embankments or Levees.

(2) Guide banks.

(3) Groynes or Spurs.

MARGINAL EMBANKMENTS OR LEVEES.


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Marginal embankments are generally earthen embankments, running parallel to the river. They may be

constructed on both sides of the river or only one one side, for some suitable river length, where the river

is passing through towns or cities or any other places of importance.

Effect of Marginal Embankments of River Flow during Floods

The effect of confining the flood waters of a river between marginal embankments or levees is :

(1) to increase the rate at which the flood waver travels down the stream,

(2) to increase the water surface elevation at flood,

(3) to increase the maximum discharge at all points downstream,

(4) to reduce the water surface slope of the stream on the upstream of the leveed portion, and

(5) to increase the velocity and the scouring action through the leveed sections.

Merits and Demerits of River Training by Marginal Embankments

The various merits of marginal embankments as a method as a method of river training for flood

protection are as follows.

(i) Embankments are the only means to prevent the spreading of flood waters over the large aeras of

flood plains. The spreading of flood waters would cause considerable hardship to the inhabitants of

the villages on the flood plains and would result in damage to their houses and other belongings.

Such damages would, however, be avoided by providing the embankments.

(ii) The initial cost of embankments is low, although when raised subsequently, they become expensive.

(iii) The construction of embankments is easy and presents no difficulty, as it can be done by utilizing

local materials and unskilled labour. Also the maintenance of embankments is simple and cheap.

(iv) The embankments may be constructed in parts, provided that the ends are properly protected.

The various demerits of marginal embankments as a method of river training are as follows.

(i) Embankments cause raising of high flood levels.

(ii) Embankments may fail by piping due to boring by small animals like crabs and worms. As such they

need to be supervised closely during flood and protected as soon as they are in danger.

(iii) In the event of a breach there is a sudden and considerable inflow of water which may cause damage

to the neighbourhood and may result in the deposition of considerable quantities of san rendering

vast areas unproductive. Moreover, embankment breaches may result in flooding almost the entire

area, protected by the embankment system.

(iv) In a flood plain unprotected by embankments the flood waters spread over the plain during every

flood season and leave a deposit of fine silt behind them as they recede. Thus the land gets benefitted

by way of inundation irrigation as well as adding new fertile soil during every flood season. When


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embankments are provided for flood protection the land in the flood plains would be deprived of

these benefits.

GUIDE BANKS

The guide banks are generally provided in pairs, symmetrical in plan and may either be kept parallel or

may diverge slightly up-stream of the works. The guide banks, usually, consists of two heavily built

embankments in the river in the shape of a bell mouth (named after the name of its inventor – Mr. Bell.)

SPURS OR GROYNES.

Groynes are the embankment type structures, constructed transverse to the river flow, extending from the

bank into the river. That is why; they may also be called “Transverse Dykes”. They are constructed, in

order to protect the bank from which they are extended, by deflecting the current away from the bank.

FUNCTIONS OF GROYNES

Gyrones serve one or more of the following functions.

(i) Training the river along a desired course by attracting, deflecting or repelling the flow in the river.

(ii) Creating a slack flow with the object of silting up the area in the vicinity.

(iii) Protecting the river bank by keeping the flow away from it.

(iv) Contracting a wide river channel, usually for the improvement of depth for navigation.

TYPES OF ALIGNMENT

A groyne aligned perpendicular to the bank line is known as ordinary groyne or a normal groyne.

Left bank

U/SD/S

Right bank Normal groyne

U/S

Scourhole

Angle 10 – 30° withthe normal to bank

Right bankof river

Still pocketRepelling groyne

D/S

A groyne pointing upstream has the property of repelling the flow away from it. Such groynes are called

repelling groynes.


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Scourhole

60° – 80°

U/S

Right bank of river Attracting groyne

Left bank

On the other hand, a groyne pointing downstream has the property of attracting the flow towards it, and is

called an attracting groyne.

T-SHAPED GROYNES.

Denehey‟s T-shaped groyne is a special type of groyne developed in India. It is an ordinary provided

with an extra cross groyne at the head giving it a T-shape. The cross groyne protects the main groyne on

the same principles as the main groyne saves the bank. The longer arm (ab) of the T is provided on the

upstream, and he shorter one (bc), on the downstream.

HOCKEY- SHAPED GROYNES:-

These groynes are shaped like a hockey stick at their lower end these groynes exert an „attracting‟ type of

influence on the flow and hence are not useful for bank protection for repelling the current away from it.

CANAL IRRIGATION

The works which are constructed at the head of the canal, in order to divert the river water towards the

canal, so as to ensure a regulated continuous supply of slit-free water with a certain minimum head into

the canal, are known as Diversion Head Works.

Layout of a diversion Head Works and its components:-

A typical layout of a canal head –works is shown in below Such a head-works consists of :

(a) Weir proper.

Weir Barrage:- If the major part or the entire ponding of water is achieved by a raised crest and a

smaller part or nil part of it is achieved by the shutters, then this barrier is known as a weir. On the

other hand, if most of the ponding is done by gated and a smaller or nil part of it is done by the raised

crest, then the barrier is known as a Barrage or a River Regulator.


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POND LEVEL

PONDING BY(REST SHUTTER

P2

P1

PONDING BYRAISED CREST TOTAL

PONDING(P)

(WEIR)P > > > P1 2

CREST SHUTTER

CREST LEVEL

Weir with shutters

POND LEVEL

TOTALPONDINGBY GATE

P = 01

P = P2

(BARRAGE)

(b) Under-sluices.

The undersluices are the openings provided in the weir wall with their crest at a low level. These

openings are fully controlled by gates. They are located on the same side as the offtaking canal.

However, if two canals take off, one on either side of the river, then it would be necessary to provide

undersluices on either side.

Functions of undersluices

The functions of undersluices are as follows.

(i) They preserve a clear and well defined river channel towards the canal head regulator.

(ii) They scour the silt deposited on the river bed in the pocket upstream of the canal head regulator.

(iii) They pass low floods without the necessity of dropping the weir crest shutters.

(iv) They help to lower the high flood level by supplementing the discharge over the weir during

high floods.

(c) Divide wall.

A divide wall is a long masonry or concrete wall or groyne (an embankment protected on all sides by

stone or concrete blocks) which is constructed at right angles to the axis of the weir to separate the

Undersluices from the rest of the weir. If two canals take off, one on either side of the river, then two

divide walls are required, one on each side. The top width of divide wall is about 1.5 to 2.5 m. the

divide wall extends on the upstream side upto a distance little beyond the beginning of the canal head

regulator and on the downstream side upto the end of the loose protection or „talus‟ of the

undersluices.

Functions of divide wall

The functions of a divide wall are as follows.

(i) Since the floor level of the undersluices is generally lower than the floor level of the weir, the

two have to be separated from each other and this is done by the divide wall.


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(ii) It provides a comparatively quiet pocket in front of the canal head regulator resulting in

deposition of silt in the pocket and entry of clear water into the canal.

(iii) It provides a straight approach through the pocket and thus helps to concentrate scouring action

of the undersluices for washing out the silt deposited in the pocket.

(iv) It keeps the cross currents, it any, away from the weir. A cross current will develop when the

main current in the river tends to approach the bank opposite the canal head regulator and the

weir forces the water to flow towards the regulator. The cross current cause formation of

vortices and result in deep scour. As such sometimes additional divide walls are provided at

equal interval along the weir which will keep this current away from the weir and provide safety

against any possible damage.

(d) River training works such as marginal bunds, guide banks, groynes, etc.

At the canal head works river training works are required for providing a smooth non – tortuous

approach to the work and prevent the river from outflanking the work. This purpose is usually

accomplished by providing guide banks on either side. In addition, marginal bunds and required

upstream of the work to prevent additional area from getting submerged due to raised high flood

level caused by the afflux created by the weir. The marginal bunds have to be continued till they join

high contours above the high flood level. Further „spurs‟ or embankments projecting into the stream

from the side banks may be required to protect the marginal bunds, or to deflect the current to the

opposite bank.

(e) Fish ladder.

The anadromous fish have been found o be moving from upstream (hills) to downstream (plains) in

the beginning of the winter season in search of warmer waters and return to their spawning grounds

upstream, slightly before monsoons, in the month of May and June.

A structure which enables the fish to pass upstream is called a ‘fish ladder’

(f) Canal Head Regulator.

A canal head regulator is a structure constructed at the head of a canal taking off from the upstream

of a weir or a barrage. It consists of a number of spans separated by piers which support the gates

provided for regulation of flow into the canal. The spans of 6 to 8 m are commonly used with

counterbalanced steel gates which are operated manually by winches. However, larger spans may

also be used if necessary and economical.


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Functions of canal head regulator

A canal head regulator serves the following functions.

(i) It regulates the supply of water into the canal.

(ii) It controls the entry of silt into the canal.

(iii) It completely excludes the high flood from entering into the canal.

(g) Weir’s ancillary works, such as shutters, gates, etc.

(h) Slit Regulation Works.

Although some control on the entry of silt into the canal taking off from a head works is provided by

the two methods of regulation as described in the previous section, but in both the methods of

regulation certain amount of silt may enter the canal. As such special devices are required to be

provided to control the entry of silt into a canal. These devices are of two types as indicated below.

(1) Silt excluders

(2) Silt extractions or silt ejectors.

The fundamental principle on which both the types of silt control devices operate is that in a

stream of water carrying silt in suspension, the concentration of silt in the lower layers is greater than

in the upper ones. Hence the device is so designed that the top and bottom layers are separated

without any disturbance. The to water which is relatively clear is allowed to flow in the canal while

the bottom wate which is heavily silt laden is allowed to go as a waste.

1. The Canal Head Regulator or Head Sluices:-

A canal head regulator (C.H.R) is provided at the head of the off-taking canal, and serves the

following functions:

a) It regulated the supply of water entering the canal.

b) It controls the entry of slit in the canal.

c) It prevents the river floods from entering the canal.


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MARGINALBUND

IVS GUIDEHANK

PIERS

OFF TAKING

CAUAL

AXIS OFREGULATOR

90° to 110°

FISHLADDER

GUIDE BANK UNDERSLUICEPOCKET

DIVIDE WALL

WEIRPROPER

Alignment of Canal Head Regulator 2. Slit Controls Devices:-

The entry of slit into a canal, which takes off from a Head-Works, can be reduced by constructing

certain special works, called slit controls works.

a) Silt Excluders:-Silt excluders are those works which are constructed on the bed ofthe river,

upstream of the head regulator. The clearer water enters the head regulator and the silted water

enters the silt excluder. In this type of works the silt is, therefore, removed from the water

before it enters the canal.

River bank

1–3Ways of under sluicepacket being coveredby excluder funnels

C

C

Under sluiceportion of weir

Weir proper

Headerregulator

Tunnel

Tunnel

Tunnel

Pier

Divide wall

(PLAN)Weir

Divide wallside

Tunnel slab

Other bays ofunder sluice pocket

Pier

Crest or ?oor level of under sluices

Tunnel

1 to 3 way of water sluices

Tunnel Tunnel

SECTION AT C.C.Silt Excluder

Crest level ofHead regulator

Regulator side

b) Silt Ejectors:- Silt ejectors, also called silt extractors, are those device which extract the silt

from the canal-water after the silted water has travelled a certain distance in the off-take canal.


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These works are, therefore, constructed on the bad of the canal, and a little distance downstream

from the head regulator.


Whenever the available natural ground slope is steeper than he designed bed slope of the channel, the

difference is adjusted by constructing vertical „falls‟ or „drops‟ in the canal bed at suitable intervals.

Such a drop in a natural canal bed will not be stable and, therefore, in order to retain this drop, a masonry

structure is constructed. Such a pucca structure is called a canal fall or acanal drop.

NECESSITY AND LOCATION OF FALLS

When the natural slope of the ground over which channel is to be constructed is greater than the designed

bed slope of the channel, the difference in the slopes is adjusted by providing vertical falls or drops in the

bed slope of the channel at suitable intervals. The location of a fall is decided according to various

considerations as indicated below.

(1) A fall may be provided at a location where the F.S.L. of the channel outstrips the ground level

channel the bed of the channel comes into filling.

(2) A fall should be so located that as far as possible there is no loss of the commanded area of the

channel.

(3) Al fall should be such that below the fall the F.S.L. of the channel remains below the ground level

for 1

2 to

3

4 kilometer but not much more. This will, however, not result in any loss of commanded

area as upto this extent the area can be irrigated by a watercourse from an outlet at high level

upstream of the fall.

(4) The location of a fall may also be affected by the possibility of combining it with a regulator or a

bridge or some other structure. Such combination often result in economy.

(5) For the location of falls relative economy of providing a large number of small falls or small number

of large falls should also be considered subject to the condition that the commanded area is not

reduced. For a small length of the channel both upstream and downstream of a fall, there is

unbalanced earthwork which should be kept at a minimum and almost equal on either side so that

extra earth from below may be used to meet the requirements above. For a larger fall the quantity of

Chapter

3 CANAL FALLS

Canal falls, necessity and location of falls , types of falls , canal

escapes, types of canal escapes, cross drainage works , types of cross-

drainage works, aqueduct and syphon aqueduct , super-passage and

syphon, level crossing, wieghtage = 20 %


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unbalanced earthwork below may be used to meet the requirements above. For a larger fall the

quantity of unbalanced earthwork will be larger. As such the provision of even small number of large

falls will result in large quantity of unbalanced earthwork, but the reduction in the number of falls

will result in saving in the cost of the falls. This saving has to be balanced against the extra cost of

earthwork to determine the relative economy.

1. Types of Falls:-

a) Ogee Falls: The „Ogee type fall‟ was constructed in olden days on projects like Ganga canal. The

water was gradually led down by providing convex and concave curves.

DRAW DOWN

WATER SURFACE

C : C.1 : 2 : 4D/S BED

RUBBLE MASONARY

U/S BED

b) Trapezoidal Notch Falls: It consists of a number of trapezoidal notches constructed in a high

crested wall across the channel with a smooth entrance and a flat circular lip projecting downstream

from each notch to spread out the failing jet.

SILL

TOP OF CANAL BANK U/S FSL NOTCHPIER

SIDE WALL

FRONT ELEVATION

U/S CANALBED

FOUNDATIONWALL

SILL

U/S FSL NOTCHPIER

c) Well Type Falls or cylinder Falls or siphon Well Drops:- This type of a fall consists of an inlet

well with a pipe at its bottom, carrying water from the inlet well to a down streams well or a cistern.

d) Simple Vertical Drop Type and Sarda Type Falls:- A raised crest fall with a vertical impact was

first of all introduced on Sarda Canal system in U.P, owing to its economy and simplicity.


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In this type of a high crested fall, the nappe impinges into the water cushion below. There is no

clear hydraulic jump and the energy dissipation is brought about by the turbulent diffusion, as the

high velocity jet enters the deep pool of water downstream.

U/S CANAL BED

U/S HFL

U/S BEDPITCH HING

DROP WALL

FLOOR OF WATER CUSHION

Simple Vertical Drop fall

D/S BED PITCHING

D/S BED

D/S HFL

e) Straight Glacis Falls:-In this type of a modern fall, a „straight glacis‟ (generally sloping 2:1) is

provided after a „raised crest‟. The hydraulic jump is made to occur on the glacis, causing sufficient

energy dissipation. This type of falls gives very good performance if not flumed.

U/S WING WALL

U/S HFL

U/S CANALBED

U/S CURTAINWALL

D/S WING WALL RETURN WALL ORRETURN WING

D/S HFL

2

11

5

TOP OF PITCHING

SLOPE PITCHING

D/SHFL

PROFILEWALLD/S BED

BED PITCHING

TOE WALL

Straight Glacis fall DEFLECTOR WALL ORD/S CURTAIN WALL

f) Montague Type Falls. The energy dissipation on a straight glacis remains incomplete due o vertical

component of velocity remaining unaffected. An improvement in energy dissipation may be brought

about in this type of fall by replacing the straight glacis by a parabolic glacis, commonly known as

“Montague Profile”.

D/SBED

U/S WING WALL

U/S FSL

U/S BED

D/S WING WALL

D/S HFL

RETURNWING

SLOPEPITCHING

PROFILEWALL

TOE WALL

Montague Type fall


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g) Inglis Falls or Baffle Falls:- A straight glacis type fall when added with a baffle platform and a

baffle wall was developed by Englis, and us called „Englis Fall or „Baffle Fall‟. They are quiet

suitable for all discharges and for drops of more than 15 m.

urs BED

u/s HFL

BRICKPITCHING U/S FLOOR

CREST

U/S RETURN WING ORRETURN WALL

PROFILEWALL

BAFFLEWALL

D/SRETURN

WING

U/S WING WALL

D/S WINGWALL

TOP OFPITCHING

SLOPEPITCHING

D/S HFL

PROFILEWALL

D/S BED

DEFLECTORWALL

„Baf?e fall‟ or „English - fall‟

TOE WALL

CANAL ESCAPES:-

An escape is a side channel constructed to remove surplus water from an irrigation channel (main canal,

branch canal, or distributary, etc.) into a natural drain.

1. TYPES OF CANAL ESCAPES:-

I. Weir type:-In this type, the crest of the weir wall is kept at R.L. equal o canal FSL. When

water level rises above FSL, It gets escaped.

NATURALDRAIN

PITCHING

Weir type escape or ‘Tail escape’

STEPS

CREST

TOP OF CANAL BANK

CANAL FSL

CANAL BEDLEVEL (C.B.L)

WEIR WALL

II. Regulator type (Sluice type). In this type, the still of the escape is kept at canal bed level and

he flow is controlled by gates. This type of escapes are preferred these days, as they give better

control and can be used for completely emptying the canal.

2. METERING FLUMES:-

A meter is a structure constructed in a canal for measuring its discharge accurately. A metering

flume is an artificially flumed (narrowed) section of the channel, which can be utilized for

calculating the discharge in the channel.


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GATE

PIERS

BRIDGE ROAD

ESCAPECHANNEL

CANAL FSL

PIERS

BED LEVELOF CANAL

FLOW DIRECTIONFOR CANAL

PLATFORM FORGATE OPERATION

Regulator type escape or ‘Surplus water escape’

3. TYPES OF METERING FLUMES:-

(a) Non-modular venturi flume or drowned venturi-flume, generally called venture flume.

(b) Standing wave flume; or modular venture flume; or free flow venture flume.

Cross Drainage Works

A cross drainage work is a structure which is constructed at the crossing of a canal and a natural drain, so

as dispose of drainage water without interrupting the continuous canal supplies.

TYPES OF CROSS-DRAINAGE WORKS

Aqueduct and Syphon Aqueduct:- The canal is taken over the natural drain, such that the drainage

water run below the canal either freely or under syphoning pressure. When the HFL of the drain is

sufficiently below the bottom of the canal, so that the drainage water flows freely under gravity, he

structure is known as an Aqueduct. However, if the HFL of the drain is higher than the canal bed and

the water passes through the aqueduct barrels under symphonic action, the structure is known as Syphon

Aqueduct.

DRAIN BED

SUPPORTING PIERSAT SUITABALE

SPACING

HFL OFDRAIN

PEIR NOSE

INSPECTIONROAD

CANAL

FSL


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Super-passage and syphon:- In these works, the drain is taken over the canal such that the canal water

runs below the drain either freely or under siphoning pressure. When the FSL of the canal is sufficiently

below the bottom of the drain trough, so that the canal water flows freely under gravity, the structure is

known as Super passage. However, if the FSL of the canal is sufficiently above the bed level of the

drainage rough, so that the canal flows under symphonic action under the rough, the structure is known as

a canal syphon or a Syphon.

DRAIN

CANAL FSL

CANAL BED

FSL OFCANAL

HFL

DRAIN

CANAL

CANAL

D/S FSL OFCANAL

U/S FSL OFCANAL

Level Crossing:- In this type of cross-drainage work, the canal water and drain water are allowed to

intermingle with each other. A level crossing if generally provided when a large canal and a huge

drainage (such as stream or a river) approach each other practically at the same level.

FACTORS AFFECTING SUITABILITY OF AQUEDUCT AND SIPHON AQUEDUCT

In addition to the above noted factors the choice between aqueduct and siphon aqueduct is also affected

by the following factors.

(i) Suitable canal alignment.

Note:- A super passage is thus the reverse of an aqueduct, and similarly, a siphon is a reverse

of an aqueduct siphon.


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(ii) Nature of foundation.

(iii) Ground water likely to be met with at foundation level and available dewatering equipment.

(iv) Suitability of soil for making embankment.

FEATURES OF DESIGN OF CROSS DRAINAGE WORKS

The important features of design of cross drainage works are as follows.

(A) Hydraulic design

(1) Determination of maximum flood discharge and high flood level (H.F.L.).

(2) Determination of waterway of the drain.

(3) Head loss through siphon barrels.

(4) Contraction of canal waterway or fluming.

(5) Determination of uplift pressure on the underside of the through (or the barrel roof).

(6) Determination of uplift pressure on the floor of the barrel.

(7) Design of bank connections.

(B) Structural design

The structural design deals with the following aspects.

(1) Design of side walls of trough and rood and floor of barrels.

(2) Design of piers and abutments.

(3) Design of foundation.

The various features of design of cross drainage works are discussed in the following section.


A dam may be defined as an obstruction or a barrier built across a stream or a river. At the back of this

barrier, water gets collected, forming a pool of water. The side on which water gets collected is called the

upstream side, and the other side of the barrier is called the down streams side. The lake of water which

is formed upstream is often called a reservoir, or dam reservoir, or a river reservoir, or a storage

reservoir.

TYPES OF DAMS:-

1. Earth Dams:- Earth dams are made of soil that is pounded down solidly. They are built in areas

where the foundation is not strong enough to bear the weight of a concrete dam, and where earth is

more easily available as a building material compared to concrete or stone or rock.

2. Rock fill Dams:-Rock fill are formed of loose rocks and boulders piled in the river bed. A slab of

reinforced concrete is often laid across the upstream face of a rock fill dam to make it water-tight.

3. Solid-masonry Gravity dams:-These big dams are expensive to be built are more durable and solid

than earth rock dams. They can be constructed on any dam site, where there is a natural foundation

strong enough to bear the great weight of the dam. eg: BahakraNangal Dam.

4. The hollow masonry gravity dams.

5. Steel Dams.

6. Timber dams:- These are short lived, since in a few years time, rotting sets in. Their life is no more

than 30 to 40 years and must have regular maintenance during that time.

7. Arch dams: Arch dams are very complex and complicated. They make use of the horizontal arch

action in place weight to hold back the water. They are best suited at sites where the dam must be

extremely high and narrow.

SPILLWAY

A spillway is a structure constructed at a dam site, for effectively disposing of the surplus water from

upstream to downstream.

REQUIREMENTS OF A SPILLWAY

Chapter

4 DAMS AND

SPILLWAYS Syllabus: Types of dams , spillway, requirements , types of spillways,

forces acting on gravity dam , modes of failure & criteria for structural

stability of gravity dams. Weightage= 15%


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The essential requirements of a spillway are as follows.

(i) The spillway must have sufficient capacity.

(ii) It must be hydraulically and structurally adequate.

(iii) It must be so located that it provides safe disposal of water i.e., spillway discharges will not erode or

undermine the downstream toe of the dam.

(iv) The bounding surfaces of the spillway must be erosion resistant to withstand the high scouring

velocities created by the drop from the reservoir surface to tail water.

(v) Usually some device will be required for dissipation of energy on the downstream side of the

spillway.

VARIOUS TYPES OF SPILLWAYS

Depending upon the type of the structure constructed for disposing of the surplus water, the spillways can

be of the following major types:

(1) Straight Drop Spillway.

This is the simplest type of spillway and may be constructed on small bunds or on thin arch dams,

etc. It is a low weir and simple vertical fall type structure. The downstream face of the structure may

be kept vertical or slightly in clined. The crest is some times extended in the form of an overhanging

lip, which keeps small discharges away from the face of the overfall section. The water falls freely

from the crest under the action of gravity. Since vacuum gets created in the underside portion of the

falling jet, sufficient ventilation of the nappe is required in order to avoid pulsating and fluctuating

effects of the jet.

(2) Overflow Spillway generally generally called Ogee Spillway.

The side channel spillway differs from the chute spillway in the sense that while in a chute spillway,

the water flows at right angles to the weir crest after spilling over it, whereas in a side channel

spillway the flow of water after spilling over the crest, is turned by 90° such that it flows parallel to

the weir crest.

(3) Chute Spillway often called Trough Spillway or Open channel Spillway.

An ogee spillway is mostly suitable for concrete gravity dams especially when the spillway is located

within the dam body in the same valley. But for earthen and rockfill dams, a separate spillway is

generally constructed in a flank or a saddle, away from the main valley, as explained earlier.

Sometimes, even for gravity dams, a separate spillway is required because of the narrowness of the

main valley. In all such circumstances, a separate spillway may have to be provided. The Trough

Spillway or Chute Spillway is the simplest type of a spillway which can be easily provided

independently and at low costs. It is lighter and adaptable to any type of foundations; and hence

provided easily on earth and rockfill dams. A chute spillway is sometimes known as a waste weir.

(4) Side Channel Spillway


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The side channel spillway differs from the chute spillway in the sense that while in a chute spillway,

the water flows at right angles to the weir crest after spilling over it, whereas in a side channel

spillway the flow of water after spilling over the crest , is turned by 90° such that it flows parallel to

the wire crest.

(5) Shaft Spillway

In a shaft spillway the water from the reservoir enters into a vertical shaft which conveys this water

into a horizontal tunnel which finally discharges the water into the river downstream. Sometimes, the

vertical shaft may be excavated through some natural rocky island or rocky spur existing on the u/s

of the river near the dam. Sometimes, artificial shafts may be constructed. For small heights, the

shafts may be constructed entirely of metal or concrete, or clay tiles. But for larger heights,

reinforced cement concrete may be used.

(6) Syphon Spillway.

A siphon spillway essentially consists of a siphon pipe, one end of which is kept on the upstream

side and is in contact with the reservoir, while the other end discharges water on the downstream

side. Two typical installations of siphon pipes are shown in fig. Both these types of siphon spillway

are the variation of a saddle siphon spillway or simply called a siphon spillway.

TILTED OUTLET TYPE OF A SYPHON SPILLWAY

The siphon pipe in fig has been installed within the body of the dam. When the valley is very narrow

and no space is available for constructing a separate spillway, the siphon pipes can be installed

within the dam body, as shown in fig. An air vent may be connected with the siphon pipe. The level

of the air vent may be kept at normal pool level, while the entry point of the siphon pipe may be kept

still lower, so as to prevent the entry of debris, etc. in the siphon. The outlet of the siphon may be

submerged so as to prevent the entry of the air in the siphon from its d/s end.


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Tail water level

Submerged Outlet

Normal pool level

DAMP

Crown of Siphon

Siphon pipe

H.G. lineDischarging or

operating

Reservoir level duringdischarge/f

Air vent

Siphon pipe installed within the gravity dam.

lood

Minimumtail water level

DLip.

Normal pool level

Lower limbor Leg

H2

Op

erat

ing

hea

d

H1

TailW.L.

Lip.

Inletor Mouth

A

B Crest

(B)

C

Reservoir levelduring ? ood

Throat

Deprimer hood

Air ventCrown (C)

Syphon Hoodor Cowl

Reservoirlevel

Siphon installed over the over?ow spillway to increase

its effectiveness and discharging capacity.

When the water in the reservoir is upto or below the normal pool level, air enters the siphon through

the vent and siphonic action cannot take place. When once the water level in the reservoir goes

above the normal pool level, and if once the siphon is filled with water (i.e., it is primed); the water

will start flowing through the siphon by siphonic action. The outflow will continue till the water

level in the reservoir falls back to normal pool level. As soon as it happens the air will enter the

siphon through the new exposed air vent, and the flow will stop.

FORCES ACTING ON GRAVITY DAM:-

i) Water Pressure

P=1

2𝛾𝑤 H2


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Acing at H

3 from base

Where 𝛾w = Unit weight of water.

H

W.M.L.

P = 12

Where g = unit weight of waterw

9.81 kN/m = 1000 kgf/m3 3

ii) Uplift Pressure.

a) When drainage Gallery is not Provided

b) When Drainage Gallery is Provided.

iii) Earthquake Force

𝛼H = 0.1g to 0.2g

Where,𝛼H = Horizontal acceleration

𝛼𝑣 = vertical acceleration, 𝛼 = Seismic coefficient

αv = 0.75αH

β = Soil foundation system factor, I = Important factor

αo = Basic seismic coefficient which depends upon

seismic zone of country.

Where, Fg = Body force

g = acceleration due to gravity, +ve

for upward & -ve for downward.

MODES OF FAILURE & CRITERIA FOR STRUCTURAL STABILITY OF GRAVITY DAMS.

(1) Failure by Overturning About Toe.

RS

0

MF 1.5

M Where Fs = Factor of safety

MR = Restoring moment about toe (due to Fv )

𝛼 = βlα0

F𝑔 = w

g (g +αv)


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M0 = Overturning moment about toe (due to FH )

2 2

R H VF F F

Where FR = Resultant force

e = Eccentricity

B

e x2

=×− Distance of from toe.FR

→

(2) Failure due to Sliding:

VS

H

FF

F

H

V

FSlidingfactor

F

External factor

v = Developed friction

H

V

FS = Factor of safety due o sliding.

SF 1slidingfactor

Case: If shear strength is also accounted then factor of safety is called shear frictional factor (S.F.F)

S.F.F = 𝛍 𝐅𝐯+𝐪 (𝐁 𝐱 𝟏)

𝐅𝐇

S.F.F >3

Where, B = Width in meter.

(3) Failure due o Compression or Crushing:-

when toe failure occurs.

𝛔𝐦𝐚𝐱 = 𝐅𝐯

(𝐁 𝐱𝟏) 𝟏 +

𝟔𝐞

𝐁

when heel failure occurs.

σmin = 𝐅𝐯

(𝐁 𝐱𝟏) 𝟏 −

𝟔𝐞

𝐁


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σmax ≤Fc

for no failure

where Fc = Crushing strength.

(4) Failure due to Tension:-

𝝈𝒎𝒊𝒏 ≥𝟎

e≤𝐁

𝛔


(1) REGIME CHANNELS:-

A Channel is said to be in a state of „Regime‟ if the flow is such that „silting and scoring‟ need no

special attention. Such a state is no easily possible in rivers, but in artificial channels, such a state

can be obtained by properly designing the channel.

(2) KENNEDY’S THEORY (1895):-

R.G Kennedy, an Executive Engineer of Punjab P.W.D, carried out extensive investigations on some

of the canal reaches in the upper Bari Doab Canal System.

Eddies are generated due to the friction of the flowing water with the channel surface. The vertical

component of these eddies try to move he sediment up, while the weight of the sediment tries to

bring it down, thus keeping the sediment in suspension. So if the velocity is sufficient to generate

these eddies, so as to keep the sediment just in suspension, silting will be avoided. Based upon this

concept, he defined the critical velocity (Vo) in a channel as the mean velocity (across the section)

which will just keep the channel free from silting or scouring, and related it to the depth of flow by

the equation.

Vo = 0.55 y0.64

Kennedy later introduced a factor (m) in this equation, to account for the type of soil through which

the canal was to pass. This factor, which was dependent upon the silt grade, was named as critical

velocity ratio (C.V.R) and denoted by m.

The equation for critical velocity was, thus, modifies as:

Vo = 0.55 my0.64

where Vo = Critical velocity in the channel in m/s.

Chapter

5 DESIGN THEORY

Syllabus: Regime Channels, Kennedy’s Theory ,Drawbacks ,

Design Procedure, Kutter’s Formula, Manning’s Formula,

Chezy’s Formula , Lacey’s Theory , Design Procedure ,

Drawbacks Weightage= 15%


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y = water depth in channel in m

𝑚= 𝐶. 𝑉. 𝑅

For sands coarser than the standard, the value of m were given from 1.0 to 1.2, and for sands finer

than the standard, m was valued between 1.0 to 0.7.

Recommended Values of C.V. R. (m)

S.No Type of silt Value of m

1. Silt of River Indus (Pakisan) 0.7

2. Light sandy silt in North Indian Rivers 1.0

3. Light sandy silt, a little coarser 1.1

4. Sandy loamy silt 1.2

5. Debris of hard soil 1.3

DRAWBACKS IN KENNEDY’S THEORY

Limitations of Kutter‟s Equation are incorporated in Kennedy‟s theory.

No equation for bed slope (S) by Kennedy

Complex phenomenon of silt transportation is incorporated in a single factor called „m‟

Involves trial and error.

DESIGN PROCEDURE:-

The critical velocity Vo is done by assuming a trial depth, and then determine area by dividing discharge

by velocity. Then determine channel dimensions. Finally, compute the actual mean velocity (V) that will

prevail in the channel of this cross-section, by using Kutter‟s formula, Meaning‟s formula, etc. If the

two velocities 𝑉0 and V work out o be the same, then the assumed depth is all right, otherwise change it

and repeat the procedure, till 𝑉and 𝑉o become equal.

For unlined alluvial channels the value of rugosity coefficient N depends on the nature of the material in

the bed and sides of the channel as well as on the condition of the channel.

Channel condition Value of N

1. Very good 0.025

2. Good 0.0250


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3. Indifferent 0.0275

4. Poor 0.0300

KUTTER’S FORMULA:-

1 0.0015523 RS

n SV

0.00155 n1 23

S R

MANNING’S FORMULA:-

2/3 1/21V R .S

n

Where V = Velocity of flow in metres/ sec.

R = Hydraulic mean depth in metres.

S = Bed slope of the channel.

n = Rugosity coefficient.

The values of n in both these equations depend upon channel condition and also upon discharge. The

values of n may be taken as given in Table.

Table. Recommended Values of Manning’s Coefficient n for Unlined Channels

Condition of channel Value of n

Very good

Good

Indifferent

Poor

0.0225

0.025

0.0275

0.030

CHEZY’S FORMULA:-

V = C 𝑅𝑆where C = a constant depending upon the shape and surface of the channel. R and S have the

same meaning .

The actual mean velocity (V) generated in the channel can be computed by any of these three

resistance equations, but generally Kutter‟s equation is used with Kennedy‟s theory.


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Use of Garret’s Diagrams for Applying Kennedy’s Theory:- To save mathematical calculations,

graphical solution of Kennedy‟s equations was evolved by Garret.

LACEY’S THEORY

Lacey’s Theory (1939):- Lacey, an eminent civil engineer of U.P. Irrigation Department, carried out

extensive investigations on the design of stable channels in alluviums. On the basis of his research work,

he found many draw backs in Kennedy‟s Theory (1895) and he put forward his new theory.

Lacey’s regime channels:- It was stated by Kennedy that a channel is said to be in a state of „regime‟ if

there is neither silting nor scouring in the channel. Bu Lacey came out with the statement that even a

channel showing no silting no scouring may actually not be in regime. He, therefore, differentiated

between there regime conditions: (i) True regime; (ii) Initial regime; and (iii) Final regime.

According o him, a channel which is under „initial‟ regime, is not a channel in regime (though outwardly

it appears to be in regime, as here is no silting or scouring) and hence, regime theory is not applicable to

such channels. His theory is therefore applicable only to those channels, which are either in true regime

or in final regime.

True regime:-A channel shall be in regime, if there is neither silting nor scouring . For his condition to

be satisfied, the silt load entering he channel must be carried through, by the channel section

Initial regime and final regime:- When only the bed slope of a channel varies d due o dropping of slit,

and its cross-section or wetted perimeter remains unaffected, even then the channel can exhibit „no silting

no scouring‟ properties , called Initial regime.

If there is no resistance from the sides, and the entire variable such as perimeter, depth, slope etc. are

equally free to vary and finally get adjusted according to discharge and silt grade, then the channel is said

to have achieved permanent stability, called Final Regime.

DESIGN PROCEDURE FOR LACEY’S THEORY:-

1. Calculate the velocity from equation.


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1/62Qf

v m / sec140

where𝑄 is in cumec:

V is in m/s; and 𝑓 is the silt factor, given by

𝑓 = 1.76 𝑑𝑚𝑚

Where dmm = Average particle size in mm.

2. Work out the hydraulic mean depth (R) from the equation.

25VR

2f

Where 𝑉 is in m/sec ;𝑅 is in m.

3. Compute area of channel section𝐴 = 𝑄

𝑉

4. Compute wetted perimeter,𝑃P = 4.75 𝑄

Where 𝑃 is in m; 𝑸 is in m3/sec.

5. Knowing these values, the channel section is known; and finally the bed slope S is determined by the

equation.

5/3

1/6

fS

3340Q

where𝑓𝑖s the slit factor

Q is the discharge in cumec.

Lacey’s Normal Scour Depth* (R’)

1/3

2R' 1.35q / f

Where q is the discharge intensity per unit width of stream = Q/L

where LIs the actual river width at the given site.

COMPARISON BETWEEN KENNEDY’S AND LACEY’S THEORIES

(1) Kennedy indicated that silt carried by water flowing in a channel is kept in suspension by the vertical

component of eddies generated from the bed of the channel only and hence he gave a relation

between V and D. Lacey indicated that the silt carried by water flowing in a channel is kept in

suspension by the vertical component of eddies generated from the entire perimeter of the channel

and hence he gave a relation between V and R.


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(2) Kennedy introduced critical velocity ratio m to make his equation applicable to channels flowing in

soils of different grades, but he did not give any method to determine the value of m. Lacey

introduced silt factor f to make his equation applicable to channels flowing in soils of different grades

and also provided a relation between f and mean particle size mr for determining the value of f.

(3) Kennedy recommended the use of Kutter‟s equation for determining the mean velocity of flow.

Lacey gave his own flow equation for determining the mean velocity of flow.

(4) Kennedy did not give any equation for the bed slope of the channel, which is decided on the basis of

the available slope of the ground of the ground or it is determined from Wood‟s table. Lacey gave an

equation for the bed slope of the channel. (5) Lacey indicated that a true regime channel has a

semi-elliptical section. Kennedy did not mention anything about the shape of the section of a stable

channel.

(6) The design of channel by Kennedy‟s method involves trial and error procedure. The design of a

channel by Lacey‟s method does not involve any trial and error procedure.

(7) Lacey made distinction between two types of resistances offered to the flow in unlined alluvial

channel viz., frictional resistance which depends on the nature of the material in the boundary

surface; and form resistance or shock which depends on irregularities of the channel. Kennedy did

not make any such distinction.

DRAWBACKS IN LACEY’S THEORY

The various drawbacks in Lacey‟s theory are not precisely defined.

(1) The characteristics of a regime channel are not precisely defined.

(2) The true regime conditions defined by Lacey are only theoretical and may not be achieved in actual

practice.

(3) The derivation of various equations by considering a single factor called silt factor f is not

satisfactory. Further the value of f may be different for the bed and the sides and no account of this

fact has been made regarding the concentration of silt as an independent variable.

(4) A considerable increase in the concentration of silt will result due to the loss in absorption which

may be 12 to 15% of the total discharge of a channel. In Lacey‟s theory, however, no consideration

has been made regarding the concentration of silt as an independent variable.

(5) According to Lacey a regime channel is inherently free from external restrain and shock and has

therefore constant Na for a given size of the material in the boundary surface. However, a regime

channel being a sediment transporting channel and will normally have a changing pattern of bed

ripple formation, this statement is unlikely to be correct.

(6) Silt charge and silt grade have not been properly defined by Lacey.


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(7) „Regime silt charge‟ was defined by Lacey‟s equations do not include silt charge.

(8) Lacey indicated that a true regime channel has a semi-elliptical section but the same is not supported

by any of his equations and also it may not be so in actual practice.

(9) The actual dimensions of stable channels are often found to be markedly different from those given

by Lacey‟s equations with f based on the size of the bed material only. Moreover, the values of f

obtained from various equations of Lacey are often quite divergent.

(10) Lacey‟s equations being empirical and based on the data obtained from channels flowing in a

particular type of material, before these equations can be applied in general it would be necessary to

determine the values of the constants by making observations on exiting stable channels flowing in

other types of material.


Rivers take off from mountains, flow from the mountainous plain terrains, and finally join the oceans.

They are formed along more or less defined channels, drain away the land water obtained by precipitation

and snow melting in high altitudes, and discharge the unutilized waters back into the sea, thus completing

the hydrological cycle.

Types of Rivers and their characteristics

Classification of Rivers on the Basis of the Topography of the River Basin. Depending upon the

topography of the basin, the river reaches can be classified into two main classes i.e,

(i) River in hills (Upper reaches)

(ii) Rivers in alluvial plains, known as rivers in flood plains (Lower reaches); and

(iii) Tidal rivers

All these three types of river reaches are described below :

(i) Rivers in Hills (Upper Reaches). The rivers generally take off from the mountains and flow

through the hilly regions before traversing the plains. These upper reaches of the rivers may be

termed as Rivers in Hills. They can be further sub-divided into :

(a) Incised or Rocky River stage; and

(b) Boulder River stage.

(a) Rocky Stage or incised River Stage. In this type, the flow channel is generally formed by the

process of degradation (erosion). The sediment transported in this reach is often different from

the river bed material, since most of it comes from the catchment due to denundation and soil

erosion. These river-reaches are highly steep with swift flow, and forming rapids along their

courses. The beds and banks of such rivers are less susceptible to erosion. The bed-load carried

by such river-reaches cannot be determined on the basis of usual bed load transportation

formulas, derived on the basis of bed characteristics.

Chapter

6 MEANDERING

Syllabus: Rivers , types of rivers and their characteristics,

classification of rivers in flood plains, meanders, aggrading, degrading

type, merits and demerits of river training wieghtage= 10%


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(b) Boulder River Stage. The river bed in these reaches consists of a mixture of boulders, gravels,

shingles and alluvial sand-deposits created by itself. Still these river reaches differ considerably

from those carrying sand and silt and flowing through plains. In the latter stage, the river flows

through deep well defined beds and wider flood plains and develops zig-zag courses. On the

other hand, in the boulder stage, the river flows through wide shallow beds and interlaced

channels, and develops a straighter course. During a flood, the boulders, shingles and gravels

are transported downstream, but as the flood subsides, the material gets deposited in heaps. The

water, then unable to shift these heaps, go round them, and the channel often wanders in new

directions, often attacking the banks and consequently widening the bed.

(ii) Rivers in Alluvial Flood Plains {Lower Reaches). The chief characteristics of these river reaches is

the zig-zag fashion in which they flow, called meandering. They meander freely from one bank to

another and carry sediment which is similar to bed material. Material gets eroded constantly from the

concave bank (outer edge) of the bend and gets deposited either on the convex side (inner edge] of

the successive bends or between two successive bends to form a bar, as shown in diagram.

x

x

Concave bank(outer edge)

Sediment

Flow

Convex bank(inner edge)

Outer bank

Scouring

Silt deposit

Innerbank

SECTION - X X

Silting and scouring in a meandering river

When once a straight moving river, just slightly deviates from its axis, the unbalance created goes on

multiplying with constant erosion from the concave side and deposition on the convex side. If

unchecked, the process continues, resulting in the formation of large meanders.

Rivers in flood plains can be further classified as :

(a) Aggrading (b) Degrading ; (c) Stable ; (d) Braided ; and (e) Deltaic.

If the river is collecting sediment and is building up its bed, it is called an aggrading or of an accreting

type. If the bed is getting scoured year to year, it is called a degrading type. If there is no silting or

scouring, it is called a stable river.

It is not necessary that a river reach should be of one type in its entire alluvial length, rather it is generally

of more than one type of reach, in its length. In other words, the same river reach may behave as of

aggrading or of degrading or of stable type. How and under what circumstances, the river may change its

type, will become very clear, if we study these types in details, as given below :


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(a) Aggrading or Accreting Type. An aggrading river is a silting river, as shown in diagram. Such a

river increases its beds slope, which is called building up of slope. The silting may be due to various

reasons, such as heavy sediment load; construction of an obstruction across the river, such as a dam

or a weir; sudden intrusion of sediment from a tributary; etc. This type of river, usually, has straight

and wide reaches with shoals in the middle, which shift with floods, dividing the flow into a number

of braided channels.

Final bedof river

Silting

Original inadequatebed slope

Aggrading River (b) Degrading Type. If the river bed is constantly getting scoured (eroded) to reduce and dissipate

available excess land slope, as shown in diagram, then the river is known as a degrading river. It may

be found either above a cutoff or below a dam or a weir or a barrage, etc. For example, the Colorado

River [U.S.A.] became a degrading type on the downstream, after the construction of the Boulder

dam.

Original excess bedslope of river

Scouring

Final bed slope of river

Degrading River (c) Stable Type. A river which does not change its alignment, slope and its regime significantly, is

called a stable river. Changes such as silting or scouring or advancement of delta into the sea may

take place, but they are negligible and may fail to produce any change in the regime of the channel,

except, perhaps, that the river may shift within its Khadirs. Most of the sediment load carried by

them is brought to the sea.

The behaviour of a particular reach (whether to be aggrading, degrading or Stable depends mainly

upon the variations of silt charge [size as well as quantity) and flow discharge with time.

(d) Braided Rivers. When a river flowe in two or more channels around alluvial islands as shown in

diagram, it is known as a braided river.

A typical braided reach of river


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The braided pattern in a river develops after local deposition of coarser material, which cannot be

transported under prevailing conditions of flow; and which subsequently grows into an island

consisting of coarse as well as fine material.

(e) Deltaic Rivers. A river before it joins the sea, gets divided into branches, thus forming a shaped

delta, as shown in diagram.

As the river approaches the sea, its velocity is reduced, and consequently the channel gets silted and

water level rises resulting in spills and eventual formations of new channels. These branches

multiply in their number as the river approaches the sea. Causes of delta formation and factors

responsible for its shape and growth etc., are beyond the scope of this book. The delta river indicates

a stage, rather than a type of a river.

Deltaformation

SEA

Main river

Branches

Delta formation

(e) Tidal Rivers. The tail reaches of the rivers adjoining the oceans are affected by the tides in the

ocean. The ocean water enters the river during the flood tide and goes out into the ocean during the

ebb tide The river, therefore, undergoes periodical rise and fall in its water level, depending upon the

nature of the tide. The distance upto which the tidal effect is experienced, depends upon various

factors, such as the shape and configuration of the river, the tidal range, freshet discharge, etc. The

detailed description of tidal reaches is beyond the scope of this book.

Classification of Rivers on the Basis of Flood Hydrographs. The rivers may be classified on the basis

of stage and nature of their flood hydrograph, into the following two types :

(i) Flashy rivers and (ii) Virgin rivers.

(i) Flashy Rivers. If the flood rise and flood fall in a river is sudden, then it is called . In flashy rivers,

the flood flows, therefore, occur suddenly, and rise and fall of water level is very quick. The flood

hydrographs are very steep, indicating floods, all of a sudden.

(ii) Virgin Rivers. In arid zones (deserts), a„ river water may completely dry before it joins another river

or the ocean. Such a river is called a virgin river. After flowing for a certain distance from its source,

the water of such a river disappears due to high percolation or due to excessive evaporation. There

are several virgin rivers in the States of Kutch and Rajasthan in India.


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CLASSIFICATION OF RIVERS:-

A rivers pass through the following four stage:-

(1) Rocky stage

(2) Boulder stage

(3) Trough stage

(4) Deltaic sage

In the rocky or hilly stage the river cross-section is made up of rock or very large boulders and hence in

this stage river training works are generally not required. In the other three stages the river through the

deposits created by the river itself. In the boulder stage the bed and banks of the river consists of a

mixture of boulders, gravels, shingles and alluvial sand deposits. In the trough and the deltaic stage the

river flows through alluvial sand deposits. It is in these lower stages of the river, the river training works

are required.

The rivers on alluvial plains may be broadly classified into the following types:-

(1) Meandering type.

(2) Aggrading type

(3) Degrading type.

MEANDERS:-

If a river deviates from is axial path and a curvature of reverse order is developed with short straight

reaches, the river is stated to be a meandering river.

Clockwise bend

Anticlockwisebend

Meanderingriver

AGGRADING TYPE

An aggrading type of river or an aggrading reach of a river is in the process of building up it bed to a

certain slope. This may be due to the following causes;

(i) Excessive sediment entering a river with a sudden reduction of slope on the plain.

(ii) Excessive sediment entering a river with water surface slope flattened due to construction foa weir or

a barrage or a dam across a river downstream of a reach.

(iii) The sudden influx of sediment from a tributary.

(iv) Extension of delta at the river mouth.


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DEGRADING TYPE

A degrading type of river or a degrading reach of a river is in the process of losing its bed gradually in the

form of sediment load of the river. A degrading reach may develop on a river downstream of a weir or a

barrage or a dam due to holding up of the sediment load above and the same being replenished by the

water flowing on the downstream side by scouring the river bed. It may also result due to the sudden

increase in slope which takes place when a loop a meandering river is out off by a straight river channel.

River training implies various measures adopted on a river channel along a certain alignment with a

certain cross-section. These measures are required to be adopted because rivers in alluvial plains

frequently alter their courses and cause damage to land and property adjacent to their banks.

CLASSIFICATION OF RIVER TRAINING WORKS

On the basis of the purpose required to be served the river training works may be classified into the

following types:

(1) High water training (2) Low water training (3) Mean water training

(1) High water training. High water training is undertaken with the purpose of providing expeditious of

maximum floods and thus provide protection against damage due to floods. It is mainly concerned

with the most suitable alignment and height of marginal embankments for disposal of floods and

may also include other measures of channel improvement for the same purpose. Thus high water

training can also be called Training for discharge.

(2) Low water training. Low water training is undertaken with the purpose of providing sufficient depth

for navigation during the low water season. This is achieved by contracting the width of the channel

at low water and is usually carried out with the help of groynes. Thus low water training can also be

called Training for depth.

(3) Mean water training. Mean water training is undertaken to provide efficient disposal of bed and

suspended sediment load and thus to preserve the river channel in good shape. Thus mean water

training can also be called Training for sediment.

Effect of Marginal Embankments of River Flow during Floods

The effect of confining the flood waters of a river between marginal embankments or levees is :

(1) to increase the rate at which the flood waver travels down the stream,

(2) to increase the water surface elevation at flood,

(3) to increase the maximum discharge at all points downstream,

(4) to reduce the water surface slope of the stream on the upstream of the leveed portion, and

(5) to increase the velocity and the scouring action through the leveed sections.


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MERITS AND DEMERITS OF RIVER TRAINING BY MARGINAL EMBANKMENTS

The various merits of marginal embankments as a method as a method of river training for flood

protection are as follows.

(i) Embankments are the only means to prevent the spreading of flood waters over the large aeras of

flood plains. The spreading of flood waters would cause considerable hardship to the inhabitants of

the villages on the flood plains and would result in damage to their houses and other belongings.

Such damages would, however, be avoided by providing the embankments.

(ii) The initial cost of embankments is low, although when raised subsequently, they become expensive.

(iii) The construction of embankments is easy and presents no difficulty, as it can be done by utilizing

local materials and unskilled labour. Also the maintenance of embankments is simple and cheap.

(iv) The embankments may be constructed in parts, provided that the ends are properly protected.

The various demerits of marginal embankments as a method of river training are as follows.

(i) Embankments cause raising of high flood levels.

(ii) Embankments may fail by piping due to boring by small animals like crabs and worms. As such they

need to be supervised closely during flood and protected as soon as they are in danger.

(iii) In the event of a breach there is a sudden and considerable inflow of water which may cause damage

to the neighbourhood and may result in the deposition of considerable quantities of san rendering

vast areas unproductive. Moreover, embankment breaches may result in flooding almost the entire

area, protected by the embankment system.

(iv) In a flood plain unprotected by embankments the flood waters spread over the plain during every

flood season and leave a deposit of fine silt behind them as they recede. Thus the land gets benefitted

by way of inundation irrigation as well as adding new fertile soil during every flood season. When

embankments are provided for flood protection the land in the flood plains would be deprived of

these benefits.

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