diversion headworks/ barrage...
TRANSCRIPT
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
BARRAGES
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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
BARRAGES AND WEIRS
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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.
BARRAGES AND WEIRS
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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.
BARRAGES AND WEIRS
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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.
BARRAGES AND WEIRS
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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.
BARRAGES AND WEIRS
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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
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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