DIVERSION

HEADWORKS/ BARRAGE

DESIGN

Typical Layout of a Canal System

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Plan of Barrage

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Guide Bund

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Terms used in Barrage Design

i) Discharge (Q) = m3/sec

It is the volume metric flow of water during per unit time.

ii) Discharge Intensity (q) = m3/sec

Discharge flowing through per unit width of a structure which is;

q = Q/B and q = 1.70E3/2

iii) Velocity of Approach

The velocity of flowing water approaching to a metering section is

called velocity of approach which is;

Hap = V2/2g

iii) Energy Line (E)

It is equal to depth of water + velocity of approach.

E = D+ Hap

iv) Lose of Head

Head lose is equal to U/S Total Energy Line – D/S Total Energy

Line, HL = TUEL - TDEL

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v) Critical Depth (dc)

It is the depth of water at which Specific Energy is minimum

dc = [q2/g]1/3

vi) Scour Depth (R)

It is the maximum depth measured from the High Flood Level (HFL)

to the lowest bed point which is eroded/ scoured as an outcome of

water current.

R = 1.35 (q2/f)1/3

vii) Wetted Parameter (P)

It is the surface area of any cross section which is wetted by the

flowing water.

P = 4.75 √Q

Where P = B + 2D.

For rivers ‘D’ is negligible comparing to ‘B’ therefore P = B,

hence B = 4.75 √Q

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viii)Conjugate Depth (d1d2)

These are the depth of water it is before and after the formation of

Hydraulic jump.

ix) Discharge over the Weir (Q)

Q = CBH3/2

B = Breadth of the Weir in meter

H = Total Water Depth above the Weir Crest

C = Constant depends upon the Drowning Ration (2.9 – 3.1) in FPS

system and 1.7 in MKS system.

x) Drowning Ratio

It is the ratio between the depth of water above crest at the D/S to

the depth of water above crest on the U/S.

DR = h/D

Where h = depth of water above crest on the D/S side

D = depth of water above crest on the U/S side

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Gibson’s Curves

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Introduction

“Any Hydraulic Structure which supplies water to the off-taking

canal is called a Headwork. Headwork may be divided into two

classes”;

Storage Headwork

Diversion Headwork

i) Storage Headwork

A Storage Headwork comprises of “the construction of a

dam across the river”. It stores water during the period of

excess supplies in the river and releases on demand.

ii) Diversion Headwork

A Diversion Headwork serves to divert the required supply

into the canal from the river. A Diversion Headwork can

further be sub divided into two principal classes;

o Temporary Spurs or Bunds

o Permanent Weirs and Barrages

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Weir

The weir is a solid obstruction put across the river to raise its

water level and divert the water into the canal. If a weir also stores

water for tiding over small periods of short supplies, it is called a

storage weir.

The main difference between a storage weir and a dam is only in

height and the duration for which the supply is stored. A dam

stores the supply for a comparatively longer duration compared to

Diversion Headworks

Further more the Dams are high head structures, which produced

Hydropower besides Irrigation Water whereas the Headworks are

low head structures which only divert river supplies into canal for

Irrigation.

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Types of Weir

Weir may be of different types based on “materials of construction,

design features and types of soil foundation as”;

Vertical drop Weir

o Vertical drop weir without a crest gate is shown in the enclosed

figure. “A crest gate may be provided to store more water during

flood period”. At the upstream and downstream ends of

impervious floor cut off piles are provided. Launching aprons are

provided both at upstream and downstream ends of floor to

safeguard against scouring action.

o A graded filter is provided immediately at the downstream end of

impervious floor to relieve the uplift pressure. This type of weir is

suitable for any type of foundation.

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Vertical Drop Weir

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Sloping Weir of Concrete

o This type is suitable for soft sandy foundation. “It is provided

where difference in weir crest and downstream river bed is not

more than 3.0 m”. Hydraulic jump is formed when water passes

over the sloping glacis. Weir of this type is of recent origin.

Enclosed figure shows a sectional weir of this concrete sloping

weir.

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Parabolic Weir

o “A parabolic weir is almost similar to spillway section of Dam. The

weir or body wall for this weir is designed as low head dam”. A

cistern is provided at downstream as shown in figure.

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Dry Stone Sloping Weir

o It is a dry stone or rock fill weir. “It consists of body wall and

upstream and downstream dry stones are laid in the form of glacis

with some intervening core wall as shown in the figure below”.

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Barrage

“The function of a Barrage is similar to that of weir, but the heading

up of water is controlled by the gates alone. No solid obstruction is

put across the river. “The crest level in the barrage is kept at a low

level”.

During the floods, “the gates are raised to clear off the high flood

level”, enabling the high flood to pass downstream with maximum

afflux.

When the flood recedes, “the gates are lowered and the flow is

obstructed”, thus raising the water level to the upstream of the

barrage.

“Due to this multiple structural components, it is costlier than the

weirs”.

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Plan of Barrage

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Comparison of Barrage Vs Weir

Barrage Weir

Low set crest. High set crest.

Ponding is done by means of

Gates

Ponding is done against the raised

crest or partly against crest and

partly by shutters

Gated over the entire length Shutters in part length

Gates are of greater height Shutters are of low height (2 m)

Gates are raised to pass high

floods

Shutters are dropped to pass

floods

Perfect control on river flow No control of river in high floods

Gates convenient to operate Operation of shutters is slow,

involve labour and time

High floods can be passed with

minimum afflux

Excessive afflux in high floods

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Barrage Weir

Less silting Upstream due to low

set crest.

Raised crest causes silting

Upstream

Longer construction period Shorter construction period

Silt removal is done through under

sluices.

No means for silt disposal.

Road and / or rail bridge can be

constructed at low cost.

Not possible to provide road-rail

bridge.

Costly structure. Relatively cheaper structure

BARRAGES

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Site Selection

The following considerations should be kept in mind when deciding on

the site for a Barrage;

i) The site must have a “good command” over the area to be

irrigated and must also be not too far distant from the

command area to avoid long feeder channels.

ii) “The width of the river at the site should preferably be the

minimum with a well defined and stable river approaches”.

iii) “A good land approach to the site” will reduce the expense of

transportation and, therefore, the ultimate cost of the Barrage.

iv) “A good Catchment Area having minimum infiltration” and

appropriate gradient to generate sufficient discharge with

minimum rainfall.

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v) “Central approach of the river to the Barrage after Diversion”.

This is essential for proper silt control and erosion to avoid

river meandering and minimize the operating expansive.

vi) “The material required for construction should preferably be

available” close to the site to minimize the construction cost.

vii) “If it is intended to convert the existing inundation canals into

perennial canals”, site selection is limited by the position of

the Head Regulator and the alignment of the existing

inundation canals.

viii) “A rock foundation” is the best but in alluvial plains the bed is

invariably sandy.

ix) “Easy diversion of the river after construction”.

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Investigations for Site Selection

i. Topographic Survey

Topographical survey comprises;

o An index plan showing the entire catchment area upstream of

the proposed barrage site with position of gauge and discharge

sites, rain gauge sites, important irrigation works, road and

railway crossing, if any.

o Contour plan of the area around the proposed barrage site

extending upto 5 km on upstream and downstream sides with

contour interval 0.5 m up to an elevation of at least 2.5 m above

HF.

o Cross section of the river at 2 km intervals up to pondage effect

on upstream

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Investigations for Site Selection

o Longitudinal section of the river to indicate observed water

levels along the deep current. In the case of meandering river

the survey is to cover at least two fully developed meanders on

the upstream of the barrage axis and one meander length on

the downstream or as may be required for detailed model

studies.

o The cross levels in the river bed are spaced 10 to 30 m

depending upon the topography of the river. The cross

sections are extended on both banks up to 2.5 m above the

HFL as far as possible, otherwise to an extent such that proper

layout of guide and afflux bunds may be decided.

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Investigations for Site Selection

ii. Collection of Hydrological Data

The Hydrological data are collected to;

o Compute the Design Flood.

o Assess the available weekly or 10 daily and monthly runoff on a

more realistic basis. For these studies it is necessary to obtain

rainfall and runoff data. For the estimation of design flood the

following data are collected.

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Investigations for Site Selection

iii. Surface and Sub Surface Investigations

o Trial pits are excavated to determine the depth of overburden

comprising large size boulders. Where necessary geophysical

method may be employed to locate the rock surface.

o Observations of water table in the area adjacent to the location

of the barrage is also carried out for three-dimensional

electrical analogy studies.

o Log Chute: statistics of logs, such as their numbers, sizes and

periods in which they are handled and other relevant data are

collected.

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Investigations for Site Selection

iv. Construction Materials

o Survey of construction materials, their availability with lead for

determining the type of construction and for preparing

comparative estimates. Availability of hard stone may make

masonry preferable to concrete.

v. Diversion Requirements

o Diversion requirements are worked out in accordance with the

need of the project.

vi. Communication System

o Investigation includes dislocation of existing facilities and their

relocation and additional facilities required during construction

and operation.

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Investigations for Site Selection

vii.Other Miscellaneous Studies

o These include pond survey for the area submerged upto

normal pond level or within the afflux bunds, as acquired, and

all immovable proprieties coming within it are recorded and

valued.

viii.Environmental and Ecological

o The effect of Barrage on ecosystem especially on fish, wild life

and human inhabitants adjacent to the structure is studied.

Site selected should cause minimum environmental

disturbances.

ix. Flood Plain

o Aerial map of the flood plain indicating dominant River Course.

BARRAGES AND WEIRS

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Purpose of Barrage/ Headworks

Headwork serves the following purposes

i) “It raises the water level” in the river so that the commendable

area can be increased.

ii) “It regulates” the intake of water into the canal.

iii) “It controls” the silt entry into the canal.

iv) “It reduces fluctuations” in the level of supply in the river.

v) “It stores water” for tiding over small periods of short

supplies.

vi) “It facilitates the flood management” as well as smooth entry

of river supply into the off-taking canal.

vii) “It provides a road way” over the river crossing for public

facilitations.

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A) Components of Diversion Headworks (Plan)

i. Main Weir

ii. Under Sluice portion

iii. Divide Wall

iv. Fish Ladder

v. Canal Head Regulator

vi. U/S Guide Bund

vii. D/S Guide Bund

viii. Canal Head Regulator

ix. U/S Marginal Bund

x. D/S Marginal Bund

xi. River Training Works

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Plan of Barrage

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B) Components w.r.to X-Section (U/S River Bed)

i) U/S Flexible Protection

ii) U/S Sheet Pile

iii) U/S Concrete Floor

iv) Intermediate Sheet Pile

v) The Main Weir Structure

a) U/S Glacises 1:4

b) Crest

c) D/S Glacises 1:3

vi) D/S Vertical Sheet Piles

vii) Inverted Filter

viii) D/S Flexible Apron

ix) D/S River Bed

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Sectional View of Barrage

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Brief Description of Components of Barrage

The pervious figures show a Typical Barrage Plan and Cross-section.The following are their brief description of a Barrage.

i) Main Barrage Portion;

a) “U/S concrete floor to lengthen the seepage path and to protect

the middle portion” where the piers, gates and bridge are to be

constructed.

b) “A crest at the required height” above the floor on which the gate

rests in its closed position. It also acts as gravity weir during low

supply.

c) “U/S glacis having the necessary slope” to join the U/s floor level

to the highest point, the crest.

d) “D/S glacis of suitable shape and slope”. This joins the crest to

the D/s floor level (which may be at the river bed level or below).

e) “The hydraulic jump forms on the glacis since it is more stable

than on the horizontal floor” and this reduces the length of pucca

work required D/s.

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f) “The D/s floor is made of concrete and is constructed so

as to contain the hydraulic jump”. Thus it takes care of

turbulence which would otherwise cause erosion.

g) “It is also provided with friction blocks” of a suitable

shape and at distances determined by the hydraulic model

experiments in order to increase friction and destroy

residual kinetic energy.

ii) Sheet Piles

a) U/S Sheet Piles

“U/S sheet piles is situated at the U/s end of the U/sconcrete floor”. The piles are driven into the soil beyondthe maximum possible scour that may occur. The functionsare;

To protect the Barrage Structure from the scour;

To reduce the uplift pressure on the Barrage floor;

To hold the sand compacted and densified between two sheetpiles to increase the bearing capacity.

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b) Intermediate Sheet Piles

“Intermediate sheet piles are situated at the end of u/s and

D/s glacis. These serve as the second line of defence”. In

case the U/S or D/S sheet piles collapse due to advancing

scour or undermining. Then these sheet piles give

protection to the main structure of the Barrage.

The intermediate sheet piles also help lengthening the

seepage path and to reduce uplift the pressure.

c) D/S Sheet Piles

“D/S sheet piles are placed at the end of the d/s concrete

floor and their main function is to check the exit gradient”.

Their depth should be greater than the maximum possible

scour.

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iii. Inverted Filter

An inverted filter is provided between the d/s sheet piles

and the flexible protection. “It would typically consist of 6”

fine sand, 9” coarse and 9”gravel”. The filter material may

vary with the size of the particles forming the river bed.

It is protected by placing over it a “concrete block” of

sufficient weight and size (say 4 ft x 2.75 ft x 4 ft as used in

the Kalabagh barrage).

Slits (jhiries) are left between the blocks to allow the water

to escape. The slits are filled with sand.

Its primary function is to check the escape of fine soil

particles in the seepage water. In case of scour, it provides

adequate cover for the d/s sheet piles against the

steepening of the exit gradient.

“The length of the filter should be 2 x D/s depth

of the sheet piles”.

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Inverted Filter

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iv. Flexible Apron

A flexible apron is placed D/S of the filter and consists ofboulders large enough not to be washed away by thehighest likely water velocity.

“The protection provided is such as to cover 1.5 x depth ofscour on the U/s side and 1.5 to 2 x depth of scour on theD/S side at a slope of 3:1”. figure below;

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v. Undersluice

A number of Bays at the extreme ends of the Barrage,

adjacent to the canal regulator will have a Lower Crest Level

than the rest of the Bays.

The main function is “i) to draw water by the formation of a

deep channel in low river flow and, ii) to control the flow of

silt into the canal by reducing the water velocity by the

formation of deep channel in front of the canal.

Accumulated silt can be washed away easily by opening the

undersluice gates to high velocity currents generated by

lower crest levels or a high differential head.

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vi. Divide – Wall

“The divide wall separates the undersluice bays from the

normal bays”. Its length on the U/s side has to be sufficient

to keep the heavy turbulence at the nose of the wall, well

away from the U/s protection of the sluices.

Similarly, on the D/s side it should extend to cover the

Hydraulic jump and the resulting turbulence.

The main functions are;

a) To separate the undersluice from the normal bays to

avoid the heavy turbulence which would otherwise

occur due to a differential head in the two sections.

This helps by creating a still pond in front of the canal

off-take thereby allowing better silt control.

b) To generate a parallel flow and thereby avoid damage

to the flexible protection area of the undersluice

portion.

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vii. Fish Ladder

Fish ladder built along the divide wall, is a device designed

to allow fish to negotiate the artificial barrier in either

direction.

viii. Guide Banks

Guide Banks are earthen embankments with stone

pitching. “The Guide Banks are designed to contain the

floods within the flood plain of the river”. Both height and

length vary according to the back-water effect produced by

the barrage.

The Guide Banks are provided with appropriate apron as

well as stone pitching to defend the water current during

flood.

SURFACE FLOW

CONSIDERATION

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Surface Flow Consideration

Retrogression

Retrogression is a temporary phenomenon which occurs after the

construction of weir or barrage in a river flowing through alluvial

soil.

As a result of back-water effects and the increase in depths, the

velocity of the water decreases resulting in the deposition of the

sediment load.

Therefore, the water overflowing the Barrage having less quantity

of silt, picks up silt from the D/S bed. This results in the lowering

of the D/S river bed for a few miles.

“This phenomenon is temporary” because the river regime, i.e. its

slope, adapts to the new conditions of flow created by the Barrage

within a few years and then the water flowing over the weir has a

normal silt load.

Retrogression value is minimum for a flood discharge and

maximum for a low discharge. “The values vary from 2 feet to 8.5

feet”.

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Accretion

Accretion is the reverse of retrogression and normally occurs u/s

although may also occur d/s after the retrogression cycle is

completed.

Due to construction of a Barrage the water current is obstructed

resulting into lesser velocity on the U/S of Barrage. Due to this

reduction in velocity, the silt load in the flood water settle down

and ultimately deposited at the River Bed. This phenomena results

into Accretion.

There is no accurate method of calculating the values of

“Retrogression and Accretion” but the values that have been

recorded at various barrages may serve as guidelines.

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Designing of Diversion Weir (Surface Flow Consideration)

Step-I

Determined of Designed Discharge (Qm)

The first step is to decide on the Maximum Flood Discharge

likely to be anticipated during the design period. This

discharge is calculated on the basis of 50 or 100 years return

period.

Various Hydrological Methods for calculating the Maximum

Flood Discharge are available such as, rating curve, UH and;

Q = CIA

Where A = Area of Catchment (Km2)

I = The Average Rainfall Intensity (Cm/hr)

C = The Catchment constant depending upon the

catchment and rainfall characteristics.

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Step-II

Width of Weir

The width of the Barrage should be adequate enough to pass

the design discharge amicably for the given pond level.

“Lacey’s Formula can serve as a guide line for fixing the length

of the Barrage”

Pw = 2.67 √Q or P = 4.83 √Q (MKS)

when Pw = Wetted Perimeter

Q = Maximum Flood Discharge

“This is the clear water way required for passing the Design

Discharge. However, using the Lacey’s looseness coefficient

which varies 1 – 1.6”.

The width between the abutment = Wa =Pw x 1.6

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Step-III

Profile of Barrage

“The profile of the Barrage, i.e. the crest level, the D/S floor

level and the shape of the glacis should be fixed in such a way

that Hydraulic Jump for all conditions of flow and for all

conditions of river bed, i.e. normal bed levels, retrogressed and

accreted bed levels is formed on the D/S glacises”.

The Hydraulic Jump is the most economical energy dissipater

and the profile should always be designed to cater for this

requirement.

Friction Blocks are also provided at the toe of the glacis for

efficient energy dissipation and minimizing the water current.

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Step-IV

Fixing of the Crest Level

The crest level is fixed by the requirements of the total head

required to pass the designed flood over the crest.

The pond level is taken as the High Flood Level. Since the width of

the river is known and the maximum depth can be calculated from

Lacey’s scour formula.

R = 0.9 (q2/f)1/3 or R = 1.35 (q2/f)1/3

The velocity of approach will be (q/R) and therefore the velocity

head (V2/2g) can be calculated. This would fix the U/S energy line.

Thus using the Discharge formula.

Q = C.L.H.3/2

Where Q = flood discharge in cusecs

L = length of the barrage crest

H = total energy V2/2g + H

C = 3.1 in FPS and 1.7 in MKS

Hence ‘H’ can be determined. Subtract this ‘H’ from the Total Energy

Line (TEL) which will fix the crest level.

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Example

Calculate the crest level for a gated diversion structure for thefollowing data;

Maximum discharge = 1000 m3/sec, High Flood Level = 100 m

Length of the Barrage = 200 m, f = 0.1

Solution

q = 1000/200 = 5 m3/sec/m

R = 1.35 [q2/f]1/3

R = 1.35 [52/0.1]1/3 = 8.4 m

V = 5/8.4 = 0.59 m/s

V2/2g = 0.192 m

Using Discharge Equation over a broad crested weir

Q = CLH3/2

1000 = 2.03 x 200 x H3/2

H = [1000/200x2.03]2/3 = 1.822 m

Crest Level of Barrage = 100 – 1.82 = 98.18 m

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Step-V

Hydraulic Jump Formation and Fixation of D/S Floor Level

“The Hydraulic Jump should form on the D/S glacis”. It is more stable

on sloping floors than on horizontal floors. Also the total length of the

D/S works will be less if the jump forms on the D/S glacis.

However, when the jump forms on the D/S glacis, there is the risk of

high submergence resulting in a weak jump and reduced energy

dissipation. Therefore the best position for the jump formation is at

toe of the glacis.

The basic equations for the “Hydraulic Jump are used to locate the

position of the jump” on the floor and to calculate the floor levels and

the D/S floor length, “the D/S energy line must be fixed”.

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A suitable value for the loss of head in the jump, HL which is afflux, is

assumed to be as 3 – 4 feet or 15 percent of known ‘H’.

With HL known, D/S Energy Line can be fixed. Using the basic

equation, Ef2, the total D/S energy level can be calculated in order to

fix the D/S floor level.

There are three ready-made methods based on equations which

can be used for Hydraulic Jump Calculations and fixation of D/S

floor level. These are;

a) Blench Curves

b) Crump’s Curves

c) Conjugate Depth method

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Hydraulic Jump Formation and Fixation of D/S Flow Level

Blench curves

This curve is drown between the head loss (HL) v/s Ef2

(Total energy).

Calculate the U/S Discharge Intensity (qb) for various bed

condition i.e. normal flow, accurate and retrogressed.

Find out the U/S and D/S Energy Lines and then the head

loss (HL). = U/S TEL – D/S TEL

For the calculated value of ‘q’ and (HL) the value of

corresponding Ef2 is read from Blench Curve. Then

subtract this value from the D/S Energy Line. This will

fixed the D/S flow level.

The length of floor is taken as 4 – 5 of Ef2

Repeat this procedure for all the three above bed

conditions and take the correct value which will be fixed

the D/S Flow Level.

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Blench Curves

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Blench Curves

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Hydraulic Jump

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Rating Curves D/S of Barrage

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v) Hydraulic jump formation and fixation of d/s floor level

Crump's Curve

This is the set of graphs between (HL)/dc and K+F/dc as

shown the figure.

First calculate discharge intensity (qb) for three bed

conditions.

Find out dc i.e. (q2/g)1/3

Also find out the U/S and D/S Energy Lines for one set of

Flow Condition and Calculate (HL)/dc

For known value of HL/dc read the corresponding value

K+F/dc = 0.5. now K and dc are known then only non-known

‘F’ value be calculated. The ‘F’ is the point of intersection of

Hydraulic Jump with the D/S glacis.

Calculate the value of ‘F’ for critical flow condition and

check weather the Hydraulic Jumps moves on the D/S

glacis.

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Crump's Curve

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Crump’s Curves

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Step-VI

Inverted Filter

An inverted filter is provided between the D/S Sheet Piles and the

flexible protection. “It would typically consist of 6” fine sand, 9”

coarse and 9”gravel”. The filter material may vary with the size of

the particles forming the river bed.

It is protected by placing over it a “concrete block” of sufficient

weight and size (say 4 ft x 2.75 ft x 4 ft as used in the Kalabagh

barrage).

Slits (jhiries) are left between the blocks to allow the water to

escape. The slits are filled with sand.

Its primary function is to check the escape of fine soil particles in

the seepage water. In case of scour, it provides adequate cover for

the d/s sheet piles against the steepening of the exit gradient.

“The length of the filter should be 2 x D/S depth of the

sheet piles”.

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Inverted Filter

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Step-VII

Flexible Apron

“The protection provided is such as to cover 1.5 x depth of scouron the U/s side and 1.5 to 2 x depth of scour (d2 ) on the D/S side ata slope of 3:1”.

The apron in the launched position over the slope of 3:1, the apronmust have a thickness of 90-100 cm. knowing the inclined lengthand the thickness, the total volume of the stone can be calculatedand hence the thickness in the horizontal position in a length of 2.5d2 can be calculated.

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Design of Stone Apron

i) U/S Side

Length according to Lacy = 2.20 √CH

H = Depth of water above the apron level.

C = Lacey's coefficient.

Thickness of Apron is kept 0.3 m over 0.3 – 0.5 concrete block.

ii) D/S Side

Length according to Lacy = 2.20 C√H/13

H = Depth of water above the apron level.

C = Lacey's coefficient.

Thickness of Apron = 4/3 (H – h)/ ρ – 1

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Step-VIII

Divide Wall

A divide wall shown in the enclosed figure is long wall made of stone

masonry or cement concrete placed perpendicular to the weir. It

separates overflow section of weir and under sluices. Divide wall

extends upstream little beyond the canal regulator and D/S upto

launching apron of the weir.

Functions

Divide wall separate the floor level of under sluices or pocket

floor of the weir. Floor level of pocket is normally a bit lower

than main weir floor.

Divide wall helps in forming a pocket of silt to approach the

tunnel of under sluices.

Divide wall serves as a support wall of the fish ladder.

Turbulent action of water and cross currents are prevented by

this long divide wall.

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Divide Wall

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Step-IX

Fish Ladder

Rivers are important source of fishes. Fishes moves upstream to

downstream in winter and downstream to upstream in monsoon.

For easy movement of fishes, fish ladder in irrigation project is

essential.

Enclosed figure is shown the plan and sectional views of fish

ladder. It is made of baffle walls in a zig-zag way so that velocity of

flow within the fish ladder cannot exceed 3 m/sec.

To control the flow, effective gates are fitted at upstream and

downstream ends of fish ladder.

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Fish Ladder

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Step-X

Scouring Sluices or Undersluices, Silt Pocket and Silt

Excluders

The above three components are employed for silt control at the

headworks. Divide wall creates a silt pocket. “Silt excluder

consists of a number under tunnels resting of the floor of the

pocket. Top floor of the tunnels is at the level of sill of the Head

Regulator”.

Various tunnels of different lengths are made as shown in

enclosed figure. “The tunnel near the Head Regulator is of same

length of head regulator and successive tunnels towards the

divide wall are short”. Velocity near the silt pocket is reduced, silts

are deposited at bottom, clear water remains above slab of silt

excluder and is allowed to enter the canal.

“The deposited silt laden water is disposed downstream through

tunnels and Undersluices”. Grade and paned presented a silt

transport concept in tunnel type sediment excluder.

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Scouring Sluices or Undersluices, Silt Pocket and Silt

Excluders

Components of River TrainingThe following are the generally adopted methods fortraining rivers, including bank protection :

i. Marginal Embankments or Levees.

ii. Guide Banks.

iii. Groynes or Spurs.

iv. Artificial cut-offs

v. Pitching of banks and provision of launchingaprons.

vi. Pitched islands.

• Marginal Embankments or Levees.

The marginal embankments retains the floodwaters as a result of back water effect. Thus foreit prevents the river supply from spreading intothe neat by land and Towns, the details shown inthe Fig 8.12

• Guide Banks.

The guide banks provided in pairs, symmetricalin plan and may either be kept parallel or maydiverge slightly up-stream of the structure detailsshown in the Fig

Details of Marginal Bund

Details of Guide Bund

Details of Guide Bund

i. The sloping water side of the entire guide bund as wellas the sloping rear side of the curved portions arepitched with one stone(i.e a stone which can be lifted byone person –weighing 40 to 50 kg) or concrete blocks.

ii. The pitching should extend up to one meter higher thanHFL. The rear side of the shank portion is not pitched,but is generally coated with 0.3 to 0.6 m earth forencouraging vegetation growth, so as to make itresistant against rain, wind, etc.

iii. The thickness of pitching on the river side may becalculated by the formula

iv. The thickness of pitching should be 25% more at theimpregnable head than for the rest of the bund.

Stone Pitching

t = 0.06 Q1/3

Launching apron

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Step-XIII

Canal Head Regulator

Canal Head Regulator is the Hydraulic Structure constructed at the

head of the canal. It consists of a number of spans separated by piers

and operated by gates similar to Barrage. Plan and Sectional Views

shown in the enclosed figure.

Functions

To regulate the required supply by operating the gates between

piers.

To control the silt from entering canal by slightly raising its

floor from floor of under sluices, i.e. a silt.

To prevent flood water from entering the canal by shutting the

gates to the HFL.

A roadway may be provided at the top.

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Canal Head Regulator

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Canal Head Regulator

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Step-XIV

Silt Ejector (or Extractor)

The enclosed figure shows the position of silt ejector. Although silt

excluder at the headworks excludes the silt, yet a portion of silt

enters the canal with water above the sill. The removal of which is

still necessary.

Therefore, the device silt ejector or extractor is provided in the

main canal few metres downstream of head regulator. The device

is a curative measure.

It consists of a horizontal diaphram placed slightly above the canal

bed. Canal bed there is slightly depressed and curved walls as

shown enclosed figure are constructed to have tunnels to dispose

of the extra silt.

Velocity decrease and silt deposited below the diaphram and this

deposited silt is carried to river downstream or to a low

depression.

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Silt Ejector (or Extractor)

SUB SURFACE

FLOW

CONSIDERATION

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Bligh Creep Theory

Bligh in his theory advocated that the design of impervious

floor is directly dependent on the path of percolation. He

assumed that Hydraulic slope or gradient is constant

throughout the impervious length of the apron.

He further assumed that percolating water creeps along the

contact of base profile of the weir and subsoil and thus, head

or energy is lost.

This loss of head is proportional to length of travel of creeping

water. Bligh called this length as creep length.

This creep length is the sum of horizontal as well as vertical

length of creep. He asserted that unless the cutoff walls or

sheet piles extend upto the impervious subsoil strata,

percolation cannot be stopped. The cutoff walls, sheet piles

when provided, can only increase the path of percolation to

reduce the hydraulic gradient.

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Bligh Creep Theory

Considering the enclosed figure-’a’, the creep length ‘L’

according to Bligh is L = l and for the figure-’b’ with two sheet

piles of depth d1 and d2 the creep length is

L = 2d1 + l +2d2

It indicates that vertical cutoff has a weight of two and

horizontal floor has one. If ‘H’ is total loss of head, loss of head

per unit length of the creep (c) is now;

c = H = H

2d1 + l + 2d2 L

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Sub Surface Flow

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Design Criteria

Bligh gave two design Criteria

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(Figure-ii)

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Sub Surface Flow (Bligh’s Creep Theory)

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Sub Surface Flow (Bligh’s Creep Theory)

Figure-ii

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Sub Surface Flow

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Example

The following figure shows the section of a weir on permeable

foundation. Calculate the average Hydraulic gradient. Also calculate

uplift pressures and floor thickness at points A and B. Assume specific

gravity of floor material to be 2.65. Use Bligh Creep Theory.

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Solution:-

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Solution

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Subsoil Flow Considerations

Lane’s Weighted Creep Theory

According to this theory, greater weight should be given to

vertical cut-off than to horizontal floors. The reasons are;

a) In practice the contact between the vertical and steeply

sloping surface is likely to be closer than along horizontal or

slightly sloping surfaces.

b) The soil beneath the structure may settle and leave empty

spaces which will be aggravated by piping. “With vertical

surfaces the void will be filled due to earth pressure”.

c) Vertical cut-off are more effective against horizontal

stratification, and check the free flow through the layers of

low permeability.

d) The results of potential theory described later also indicate

that even in homogenous soils, “resistance against failure by

piping depends to much greater degree on the vertical

elements of the foundation profiles than on the horizontal

flooring”.

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Lane’s Weighted Creep Theory

Lane analyzed more than 200 dams all over the world and from his

analysis, he presented his weighted creep theory in 1932. “He

proposed a weight of three for vertical creep and one for horizontal

creep”. Considering the figure below, the creep length in Lane

Theory, becomes.

L = 3d1 + l +3d2

Although his theory is a modification over Bligh’s Theory, it is still

empirical. There is no rational basis to be used for design.

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Lane’s Weighted Creep Theory

To ensure safety against piping the average Hydraulic gradient H/Lw must not exceed 1/C the values of C are as given below;

Comparison for adopted value of ‘C’ both for Lane and Lacy’stheory is shown as below;-

Material Cj (Lane’s Values) C (Bligh’s Values)

Very fine sand and silt 8.5 18

Fine sand 7.0 15

Coarse sand 5.7 12

Gravel and sand 3.5 to 3 9

Boulders gravel and sand 2.5 to 3 4 to 6

Clayey soils 3.0 to 1.6 --

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Khosla’s Theory

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Khosla’s Theory

108

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Piping

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Khosla’s Theory

The streamlines represent the paths along which the water flows.

Every particle entering the soil at given point upstream traces out

its own path representing a streamline. The first streamline follows

bottom of the floor.

Equipotential lines represent the lines of equal pressure head and

both the lines intersects each other orthogonally and thus, they

form curvilinear square called field. The flow net shown in the

figure below is for a simple weir base profile.

Khosla presented a mathematical solution for the following simple

cases by breaking composite weir profile of given figure into the

following simple profiles shown figures.

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Khosla’s Theory

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Khosla’s Theory

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Subsoil HGL and Piping

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Percolation below Weirs on Sand

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Khosla’s Theory

Figure 6.10

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Khosla’s Theory

For finding pressure at key points E, D, and C, i.e. the points of

contact of the pile with floor and bottom of pile in the given figure

(a), (b), (c) and bottom corner points D1 and D’ of the given figure

(d).

Khosla developed independent curves as shown in the enclosed

figure for calculation of uplift pressures for the following

situations.

i) Figure-I shows a relationship between uplift pressure and 1/α.

ii) The Khosla’s curve is used for calculation of ΦD, ΦE and ΦD’

for the piles and the ends.

iii) Figure-II is used for calculation of Uplift pressure for the

intermediate sheet piles.

iv) Figure-III is used for calculation of Exist Gradient.

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Khosla’s Theory (Figure-I)

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Khosla’s Theory (Figure-II)

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Khosla’s Curve (Figure-II)

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Khosla’s Theory (Figure-III)

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Khosla’s Theory

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α = b/d

b = Total Length

d = Depth of D/S Sheet

pile

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Khosla’s Theory

Methods of Reading Khosla’s Curve

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Khosla’s Theory

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Khosla’s Theory

Figure 6.10

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Khosla’s Method of Independent Variables (Corrections)

Estimations of pressure at key points are made by breaking the

composite profile into four parts [Figures 6.10 (a), (b), (c) and (d)].

In actual practice, weir may have number of piles and its thickness.

Khosla solved this actual problem by an empirical method known

as method of independent variables. He applied the corrections of

floor and mutual interference of piles to the calculated values ΦC,

ΦD and ΦE etc.

The correction due to slope, interference of piles is applied to the

calculated values and net uplift pressures at these control points is

calculated to determine the floor thickness at various points of the

floor length.

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A) Correction for Thickness of the Floor

Let t, t1 and t2 be the thickness of the weir floor at upstream,

intermediate and downstream of the floor respectively and

corresponding depths of piles are d, d1 and d2 as shown in the

enclosed figure.

The figure shows the pressure at key points assuming negligible

floor thickness. Hence percentage pressure determined by the

Khosla’s equations or curves shall pertain to the top level of the

floor while junction of the piles is at the bottom points E1 and C1 of

the floor.

The pressure at E1 and C1 are determined by assuming straight line

or linear variation between the point D and the points E and C.

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Correction for Thickness of the Floor

Figure 24.22

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Correction for Thickness of the Floor

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Correction for Thickness of the Floor

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B) Correction for Mutual Interference of Piles

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Correction for Mutual Interference of Piles (Figure-V)

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C) Correction for Slope

Figure-V

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Correction for Slope

Correction of slope of the floor has also been recommended by

Khosla. The following table gives the recommended slope

correction;

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Correction for Slope►

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Khosla’s Method for Calculation of Depth of D/S Pile

As already discussed, Exit Gradient expression is availablefrom potential theory. It is also shown there that in the case offlush floors, the Exit Gradient value is theoretically infinitywithout a D/S Sheet Pile.According to Khosla it is the D/S sheet pile which controls theExit Gradient value. Hence in Khosla’s method the entire floorand D/S Pile is taken as the elementary profile for thecomputation of the Exit Gradient. For this case an analyticalsolution is available.

Exit Gradient = H 1d π√λ

where λ= 1+ √ 1+ α2

2 and α = b/dwhere b = Total Floor Length (L)

d = Depth of D/S Sheet pile (d)H = Head across

The limiting value of Exit Gradient will fix the D/S Sheet Pile.

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Example-I

Calculate the safe exist gradient with the following data;

Depth of end sheet pile = 7 m

Seepage Head = 4 m

Length of the impervious floor = b = 50 m

α = b/d = 50/7 = 7.14

For α = 7.14

1/π√λ = 0.165

Hydraulic gradient GE = H/d 1/π√λ = 4/7 x 0.165 = 0.094 = 1/10.6

Since the hydraulic gradient is flatter than the permissible value of 1/7the section is safe against piping

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Alternative Solution

α = b/d = 50/7 = 7.14

For α = 7.14

λ = 1 + √1 + α2

2

= 1 + √1 + (7.14)2

2

= 1 + √1 + 50.97 = 51.97

2

= 1 + 7.209 = 8.209 = 4.10

2 2

= 1/π√λ = 0.165

GE = H/d 1/π√λ

= 4/7x 0.156 = 0.089 = 1/11

which is within the safe limit.

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Example-II

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Example-III

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Example-IV

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Design Procedure for Weirs on Permeable Foundation

The Procedure for Designing of Weir on Permeable Foundation is

summarized is as under;

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Causes of Failure of Weirs and their Remedies

i) Piping

“Water seeps under the base of the weirs founded onpermeable soils. When the flow lines emerges out at the D/Send of the impervious floor of the weir, “the HydraulicGradient” or the exit gradient may exceed a certain criticalvalue for the soil. In that case, “the surface soil starts boiling”and is washed away by percolating water.

With the removal of the surface soil, “there is furtherconcentration of flow lines” resulting into the depression andstill more soil is removed.

“This process of erosion thus progressively works backwardtowards the upstream and results in the formation of a channelor a pipe underneath the floor of the weir, causing its failure”.

Remedies; Piping failures can be prevented by;

a) Providing sufficient length of the impervious floor so that“path of percolation is increased and the exit gradient isdecreased”.

b) Providing pile at downstream ends.

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Causes of failure of weirs and their remedies

ii) Rupture of Floor Due to Uplift

If the weight of floor is insufficient to resist the uplift pressure,

“the floor may burst and effective length of impervious floor is

thereby reduced”. The final failure, however, “is due to the

reduction of the effective length” with the consequent increase

in the exit gradient. Example of such failures are Khanki Weir

on Chenab.

Remedies; Failures due to rupture of floor may be prevented by;

a) Providing impervious floor of sufficient length

b) Providing impervious floor of appropriate thickness at

various points and

c) Providing pile at the upstream end so that the uplift

pressure to the d/s is reduced.

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Causes of failure of weirs and their remedies

iii) Rupture of Floor Due to Suction Caused by Standing

Wave/ Hydraulic Jump

The standing wave or Hydraulic Jump formed at the D/S of the

weir causes suction which also acts in the direction of uplift

pressure. If the floor thickness is insufficient, it may fail by

rupture. Examples of such failures are Marala Weir on the

Chenab and Rasul Weir.

Remedies; Failures can be prevented by;

a) Providing additional thickness of floor to counterbalance

the extra pressure due to the standing wave.

b) Constructing the floor thickness in one concrete mass

instead of in masonry layers.

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Causes of failure of weirs and their remedies

iv) Scour on the Upstream and Downstream of the Weir

When the natural waterway of a river is contracted, the water

may scour the bed both at upstream and downstream of the

structure. “The scour holes so formed may progress towards

the structure, causing its failure”. Example of such failures are

Islam Weir and Tounsa.

Remedies; Such failures can be prevented by;

a) Taking the piles at upstream and downstream ends of the

impervious floor, much below the calculated scour level.

b) Providing suitable length and thickness of launching

aprons at u/s and d/s, so that stones of the aprons may

settle in the scour holes.

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B) Subsoil Flow Considerations

There are two considerations for the Design of Barrages founded

on porous soil. They are discussed in detail below;

i) Uplift Pressure

o This is defined as the residual pressure of the seeping

water acting vertically upward with the effect of trying to lift

up the body of The barrage.

o Therefore in the case of gravity floors, the thickness of the

aprons or the glacis must be of greater weight than the

uplift pressure.

o Hence it is very important to determination the exact uplift

pressure at each point under the Barrage profile.

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ii) Undermining

o When the seepage velocity in the microscopic flow

channels in the subsoil under the structure is such that the

seepage force at the exit point becomes greater than the

submerged weight and friction of the soil. Very fine soil

particles become displaced. This can be observed as

muddy water emerging from the soil surface.

o With this continuing process and a subsoil consisting of

fine particles surrounding larger particles, “the removal of

the fine particles causes unequal settlement of the subsoil

and ultimately the collapse of the structure due to piping”.

o The river discharge over the weir further aggravates the

situation “by washing away the loosened soil due to the

excessive exit gradient”.

o The problem consists therefore in “controlling the seepage

force so that it cannot carry away the foundation material”.

Computation of Seepage Discharge

156

Computation of Seepage Discharge

Computation of Rate of Seepage from Flow Net

Computation of Seepage Discharge

Computation of Seepage Discharge