Diversion Headworks

Module-III

• Diversion Headworks:• Types- selection of the suitable site for the

diversion headwork- components of diversionheadwork- Causes of failure of structure onpervious foundation- Khosla’s theory- Designof concrete sloping glacis weir

Diversion Head-Works• The works which are constructed at the head

of the canal in order to divert the river watertoward the canal, so as to ensure a regulatedcontinuous supply mostly silt free water withcertain minimum head into the canal, areknown as diversion headworks.

Diversion Head-Works

Objectives of Diversion Head Works• The Following are the objective of Diversion

Head works• To Raise the water level at the head of canal.• To form a storage by construction of dykes on

both side of banks of the river so that water isavailable throughout the year.

• To control the entry of silt into the canal and tocontrol the deposition of silt at the head of canal.

• To control the fluctuation of water level in theriver during different seasons.

Objective of Diversion Head works

Objective of Diversion Head works

Objective of Diversion Head works

Objective of Diversion Head works

Selection of Site for Diversion Head Works

• The following points should be considered toselect a site for this diversion headworks.

• The river should be straight and narrow at thesite

• The elevation of site should be higher than thearea to be irrigated for gravity flow.

• River banks at site should be well defined andstable.

• Valuable land upstream of the barrier likeweir or barrage should not be submerged.

Selection of Site for Diversion Head Works

• Material of construction should be locallyavailable.

• Roads or railway communication to the site isessential to carry the material of construction.

• Site should be close to the cropland tominimize loss of water due to seepage andevaporation of canal.

• The site should provide a good foundation forconstruction of weir or barrage.

Selection of Site for Diversion Head Works

Components of Diversion Headworks

Components of Diversion Headworks

Components of Diversion Headworks• The components of diversion headworks are:• Weir or barrage• Canal head regulator• Divide Wall• Fish Ladder• Scouring Sluices Under sluices• Silt excluder• Silt ejector.• Marginal embankment or dikes• Guide bank• Silt pocket or trap.

Weir or Barrage• Weir is a solid obstruction placed across the

river. Its main function is to raise the waterlevel so that water can be diverted by canal tocrop field due to difference of head.

• Barrage is practically a low weir with anadjustable gate over this low weir. Heading upof water is affected by gate.

Weir

Weir

Weir

Barrage

Barrage

Barrage

Comparison Between Weir & a Barrage

Types of Weir

Types of Weir• Weir may be of different types based on material of

construction, design features and types of soilfoundation as:

• Vertical Drop Weir• A crest gate may be provided to store more water

during flood period. At the upstream and downstreamends of impervious floor cut off piles are provided.Launching apron are provided both at upstream anddownstream ends of floor to safeguard againstscouring action. A graded filter is providedimmediately at the downstream end of imperviousfloor to relieve the uplift pressure. This type of weir issuitable for any type of foundation.

Vertical Drop Weir

Vertical Drop Weir

Vertical Drop Weir

Types of Weir• Sloping Weir of Concrete:• This type is suitable for soft sandy foundation.

It is used where difference in weir crest anddownstream riverbed is not more than 3 m.Hydraulic jump is formed when water passesover the sloping glacis. Weir of this type is ofrecent origin.

Sloping Weir of Concrete

Sloping Weir of Concrete

Types of Weir• Parabolic Weir:• A parabolic weir is almost similar to spillway

section of dam. The weir body wall for thisweir is designed as low dam. A cistern isprovided at downstream.

Types of Weir

Types of Weir• Dry Stone Slopping Weir:• It is dry stone or rock fill weir. It consists of

body wall and upstream and downstream drystones are laid in the form of glacis with someintervening core wall.

Dry Stone Slopping Weir

Dry Stone Slopping Weir

Barrage• When the water level on the upstream side of

the weir is required to be raised to differentlevels at different time, then the barrage isconstructed. Practically a barrage is anarrangement of adjustable gates or shutters atdifferent tiers over the weir. The water levelcan be adjusted by the opening of gates.

Barrage

Divide Wall• The Divide Wall is a long wall constructed at right angle to the

weir or barrage, it may be constructed with stone masonry orcement concrete. On the upstream side, the wall is extendedjust to cover the canal regulator and on the down stream side,it is extended up to the launching apron. The functions of thedivide wall are as follows,

• (a) To form a still water pocket in front of the canal head sothat the suspended silt can be settled down which then latercan be cleared through the scouring sluices from time to time.

• (b) It controls the eddy current or cross current in front of thecanal head.

• (c) It provides a straight approach in front of the canal head.• (d) It resists the overturning effect on the weir or barrage

caused by the pressure of the impounding water.

Divide Wall

Scouring Sluices or Under Sluices• The Scouring sluices are the openings provided

at the base of the weir or barrage. Theseopenings are provided with adjustable gates.Normally, the gates are kept closed. Thesuspended silt goes on the depositing in front ofthe canal head regulator. When the siltdeposition becomes appreciable the gates areopened and the deposited silt is loosened with anagitator mounting on a boat. The muddy waterflows towards the downstream side through thescouring sluices. The gates are closed. But, at theperiod of flood, the gates are kept opened.

Under Sluices

Scouring Sluices or Under Sluices

Fish Ladder• The Fish Ladder is provided just by the side of the

divide wall for the movement of fishes. Rivers areimportant source of fishes. There are various types offish in the river. The nature of fish varies from typeto type. But in general, the tendency of fish is tomove from upstream to downstream in winters andfrom downstream to upstream in monsoons. Thismovement is essential for their survival.

Fish Ladder• Due to construction of weir or barrage, this

movement gets obstructed, and is detrimental tothe fishes. For the movement of the fishes alongthe course of the river, the fish ladder is essential.In the fish ladder, the baffle walls areconstructed in the zigzag manner so that thevelocities of flow within the ladder does notexceed 3 m/s. The width, length, and height ofthe fish ladder depends on the nature of the riverand the type of the weir or barrage.

Fish Ladder

Fish Ladder

Canal Head Regulator• A structure which is constructed at the head of

the canal regulator to regulate the flow of wateris known as canal head regulator. It consists of anumber of piers which divide the total width ofthe canal into a number of spans which areknown as bays. The pier consists of a number oftiers on which the adjustable gates are placed.The gates are operated from the top by suitablemechanical device. A platform is produced onthe top of the piers for the facility of operatingthe gates. Again some piers are constructed onthe downstream side of the canal head to supportthe roadway.

Canal Head Regulator

Canal Head Regulator

Silt Excluder• When still pocket is formed in front of the canal

head by constructing the divide wall, then it is foundthat the lower layer of water contains heavy silt andthe upper layer contains very fine silt. The fine silt isvery fertile and it may be allowed to enter the canal.But the heavy silt causes sedimentation in thepocket.. To eliminate the suspended heavy silt, thesilt excluder is provided. It consists of a series oftunnels starting from the side of the head regulatorup to the divide wall.

Silt Excluder• The tunnel nearest to the head regulator is

longest, and the successive tunnels decrease inlength, the tunnel nearest to the divide wall isshortest. The tunnels are covered by R.C.C. Slab.The top level of the slab is kept below the silllevel of the head regulator. So, the completelyclear water is allowed to flow in the canalthrough the head regulator. The suspendedheavy silt carried by the water enters the siltexcluder tunnels and passes out through thescouring sluices.

Silt Excluder• Silt excluders are those works which are

constructed on the bed of the river, upstreamof the head regulator. The clearer waterenters the head regulator and silted waterenters the silt excluder. In this type ofworks, the silt is, therefore, removed from thewater before in enters the canal.

Silt Excluder

Silt Ejectors• Silt ejectors, also called silt extractors, are

those devices which extract the silt fromthe canal water after the silted water has

• traveled a certain distance in the off-takecanal. These works are, therefore, constructedon the bed of the canal, and little distancedownstream from the head regulator.

Silt Ejectors

Marginal Embankments or dykes• The marginal embankments or dykes are earthen

embankments which are constructed parallel to theriver bank on one or both the banks according to thecondition. The top width is generally 3 to 4 m and sideslope is generally 1 ½ : 1 to 2: 1. The height of theembankment depends on the highest flood level. Asuitable margin is provided between the toe of theembankment and the bank of the river. To resist theeffect of erosion on the embankment, wooden piles aredriven along the river banks throughout the length ofdyke. The length of the dyke is protected by boulderspitching with cement grouting and the downstreamside is protected by turfing.

Marginal Embankments or dykes• The Marginal Bunds are constructed for the

following purposes.• (a) It prevents the flood water or storage

water from entering the surrounding area.• (b) It retains the flood water or storage water

within a specified section.• (c) It Protects the towns and village from

devastation during the heavy flood.• (d) It protects valuable agricultural lands.

Marginal Embankments

Guide Bank• When a barrage is constructed across a river which

flows through the alluvial soil, the guide banks mustbe constructed on both the approaches to protect thestructure from erosion. It is an earthen embankmentwith curved head on both the ends.

• The Guide Bank serves the following purposes.• It protects the barrage from the effect of scouring and

erosion.• It controls the tendency of changing the course of the

river.• It controls the velocity of the flow near the structure.

Guide Bank

Guide Bank

Guide Bank• Components of Guide Banks are• Upstream curved head• Downstream curved head• Shank portion which joins upstream and

downstream curved end• Sloping apron• Launching apron• Pile protection

Guide Bank

Causes of Failure of weir or barrage on permeable foundation

• The combined effect of surface flow and surface flow maycause the failure of the weir or barrage.

• (i) Failure due to subsurface flow• (a) By piping or undermining: The water from the

upstream side continuously percolates through the bottomof the foundation and emerges at the downstream end ofthe weir or barrage floor. The force of percolating waterremoves the soil particles by scouring at the point ofemergence. As the process of removal of soil particles goeson continuously, a depression is formed which extendsbackwards towards the upstream through the bottom ofthe foundation. A hollow pipe like formation thus developsunder the foundation due to which the weir or barragemay fail by subsiding. This phenomenon is known asfailure by piping or undermining.

By Piping or Undermining

By Piping or Undermining

Causes of Failure of weir or barrage on permeable foundation

• (b) By uplift Pressure: The percolating water exerts an upward pressureon the foundation of the weir or barrage. If this uplift is notcounterbalanced by the self weight of the structure, it may fail byrapture.

• 2. Failure by Surface Flow:• (a) By Hydraulic Jump: When the water flows with a very high velocity

over the crust of the weir or over the gates of the barrage, thenhydraulic jump develops. This hydraulic jump causes a suctionpressure or negative pressure on the downstream side which acts inthe direction of uplift pressure. If the thickness of the impervious flooris not sufficient, then the structure fails by rapture.

• (b) By Scouring During floods: The gates of the barrage are kept openand the water flows with high velocity. The water may also flow withvery high velocity over the crest of the weir. Both the cases can resultin scouring effect on the downstream and on the upstream side of thestructure. Due to scouring effect on the downstream and on theupstream side of the structure, its stability gets endangered by shearing.

By Uplift Pressure

By Hydraulic Jump

By Scouring During floods

Causes of Failure of Weir, and Remedies

• If a weir is constructed on permeable soil, weir may failby piping, uplift force, suction caused by standing waveand scouring on both upstream and downstream of theweir.

• When hydraulic gradient or exit gradient exceeds thecritical value of soil, surface soil at down end startsboiling first and is washed away by percolating water.This process of removal or washing out of soil continuousand eventually a channel in the form of pipe is formed byseepage water. This is called piping which may cause thefailure of foundation. Similarly uplift force of percolatingwater is acting on ther floor from bottom and if theweight of floor is not enough to resist this uplift force,floor may fail by cracking or bursting.

Causes of Failure of Weir, and Remedies

• The main remedies against failure are:• Path of percolation or creep length of seepage

water should be increased by providing sheetpiles at upstream, downstream or atintermediate point to reduce the hydraulicgradient.

• Floor thickness should be increased toincrease its self weight to balance the upliftforce.

Precautions Against Failure• The following precautions can be taken to prevent

failure.• (a) The length of the impervious layer should be

carefully designed so that the path of the percolatingwater is increased consequently reducing the exitgradient.

• (b) Sheet piles should be provided on the upstreamside and the downstream side of the imperviousfloor to increase to the length of percolating waterso that the uplift pressure is considerable reduced.

• (c) The thickness of the impervious floor should besuch that the weight of the floor is a sufficient tocounterbalance the uplift pressure.

Precautions Against Failure• (d) Energy dissipater blocks like friction

blocks, impact blocks, should be provided.• (e) Inverted filter should be provided with

concrete blocks on the top so that thepercolating water does not wash out the soilparticles.

• Deep foundation like well foundation shouldbe provided for the barrage piers

Precautions Against Failure

Precautions Against Failure

Energy Dissipater Blocks

Increasing Floor Thickness

Flow Net• A flownet is a graphical representation of two-

dimensional steady-state groundwater flowthrough aquifers.

• The method consists of filling the flow area withstream lines and equipotential lines, which areeverywhere perpendicular to each other, makinga curvilinear grid.

• Stream Lines: The streamlines represent the pathsalong which the water flows through the sub-soil.

• Equipotential lines: Equipotential lines are lines of• equal hydraulic head.

Flow Net

Khosla’s Theory• Khosla’s Theory and Concept of Flow Nets• Many of the important hydraulic structures,

such as weirs and barrage, were designed onthe basis of Bligh’s theory between the periods1910 to 1925. In 1926 – 27, the upper Chenabcanal siphons, designed on Bligh’s theory,started posing undermining troubles.Investigations started, which ultimately lead to

• Khosla’s theory. The main principles of this theoryare summarized below:

• (a) The seepage water does not creep along thebottom contour of pucca flood as started by Bligh,but on the other hand, this water moves along a setof stream-lines. This steady seepage in a verticalplane for a homogeneous soil can be expressed byLaplacian equation:

Khosla’s Theory• The equation represents two sets of curves

intersecting each other orthogonally. The resultantflow diagram showing both of the curves is called aFlow Net.

• Stream Lines: The streamlines represent the pathsalong which the water flows through the sub-soil.

• Every particle entering the soil at a given pointupstream of the work, will trace out its own path andwill represent a streamline. The first streamlinefollows the bottom contour of the works and is thesame as Bligh’s path of creep. The remainingstreamlines follows smooth curves transiting slowlyfrom the outline of the foundation to a semi-ellipse, asshown below.

Khosla’s Theory

• Equipotential Lines: (1) Treating the downstream bedas datum and assuming no water on the downstreamside, it can be easily started that every streamlinepossesses a head equal to h1 while entering the soil;and when it emerges at the down-stream end into theatmosphere, its head is zero. Thus, the head h1 isentirely lost during the passage of water along thestreamlines.

Khosla’s Theory• Further, at every intermediate point in its path,

there is certain residual head (h) still to be dissipatedin the remaining length to be traversed to thedownstream end. This fact is applicable to everystreamline, and hence, there will be points ondifferent streamlines having the same value ofresidual head h. If such points are joined together,the curve obtained is called an equipotential line.

Khosla’s Theory

Khosla’s TheoryEvery water particle on line AB is having aresidual head h = h1, and on CD is having aresidual head h = 0, and hence, AB and CDare equipotential lines.Since an equipotential line represent thejoining of points of equal residual head, henceif piezometers were installed on anequipotential line, the water will rise in all ofthem up to the same level as shown in figurebelow

Khosla’s Theory

Khosla’s Theory• The seepage water exerts a force at each point in the

direction of flow and tangential to the streamlines asshown in figure above. This force (F) has an upwardcomponent from the point where the streamlines turnsupward. For soil grains to remain stable, the upwardcomponent of this force should be counterbalanced bythe submerged weight of the soil grain. This force hasthe maximum disturbing tendency at the exit end,because the direction of this force at the exit point isvertically upward, and hence full force acts as itsupward component.

Khosla’s Theory• For the soil grain to remain stable, the

submerged weight of soil grain should be morethan this upward disturbing force. Thedisturbing force at any point is proportional tothe gradient of pressure of water at that point(i.e. dp/dt). This gradient of pressure of water atthe exit end is called the exit gradient. In orderthat the soil particles at exit remain stable, theupward pressure at exit should be safe. In otherwords, the exit gradient should be safe.

Critical Exit Gradient

• This exit gradient is said to be critical, whenthe upward disturbing force on the grain isjust equal to the submerged weight of thegrain at the exit. When a factor of safety equalto 4 to 5 is used, the exit gradient can then betaken as safe. In other words, an exit gradientequal to ¼ to 1/5 of the critical exit gradientensured, so as to keep the structure safeagainst piping.

• The submerged weight (Ws) of a unit volumeof soil is given as:

Critical Exit Gradient

Khosla’s Method of independent variables for determination of pressures and exit gradient for

seepage below a weir or a barrage• In order to know as to how the seepage below the

foundation of a hydraulic structure is taking place, it isnecessary to plot the flow net. In other words, we mustsolve the Laplacian equations. This can beaccomplished either by mathematical solution of theLaplacian equations, or by Electrical analogy method,or by graphical sketching by adjusting the streamlinesand equipotential lines with respect to the boundaryconditions. These are complicated methods and aretime consuming. Therefore, for designing hydraulicstructures such as weirs or barrage or perviousfoundations, Khosla has evolved a simple, quick and an

• accurate approach, called Method of IndependentVariables.

Khosla’s Method of independent variables fordetermination of pressures and exit gradient forseepage below a weir or a barrage

• In this method, a complex profile like that of a weir isbroken into a number of simple profiles; each ofwhich can be solved mathematically. profiles whichare most useful are:

• (i) A straight horizontal floor of negligible thicknesswith a sheet pile line on the u/s end and d/s end.

• (ii) A straight horizontal floor depressed below the bedbut without any vertical cut-offs.

• (iii) A straight horizontal floor of negligible thicknesswith a sheet pile line at some intermediate point.

Khosla’s Method of independent variables fordetermination of pressures and exit gradient forseepage below a weir or a barrage• The key points are the junctions of the floor and the

pole lines on either side, and the bottom point of• the pile line, and the bottom corners in the case of a

depressed floor. The percentage pressures at these key• points for the simple forms into which the complex

profile has been broken is valid for the complexprofile

• itself, if corrected for• (a) Correction for the Mutual interference of Piles• (b) Correction for the thickness of floor• (c) Correction for the slope of the floor

(a) Correction for the Mutual interference of Piles

The correction C to be applied as percentage of head due to this effect, is givenby

Where,b′ = The distance between two pile lines.D = The depth of the pile line, the influence of which has to be determined on

the neighboring pile of depth d. D is to be measured below the level atwhich interference is desired.

d = The depth of the pile on which the effect is consideredb = Total floor length

The correction is positive for the points in the rear of back water, andsubtractive for the points forward in the direction of flow. This equationdoes not apply to the effect of an outer pile on an intermediate pile, if theintermediate pile is equal to or smaller than the outer pile and is at adistance less than twice the length of the outer pile.

(a) Correction for the Mutual interference of Piles

(a) Correction for the Mutual interference of Piles

• Suppose in the above figure, we are considering the influence ofthe pile no (2) on pile no (1) for correcting the pressure at C1.Since the point C1 is in the rear, this correction shall be positive.While the correction to be applied to E2 due to pile no (1)shall be negative, since the point E2 is in the forward directionof flow. Similarly, the correction at C2 due to pile no (3) ispositive and the correction at E2 due to pile no (2) is negative.

(b) Correction for the Thickness of Floor

• In the standard form profiles, the floor isassumed to have negligible thickness. Hence,the percentage pressures calculated byKhosla’s equations or graphs shall pertain tothe top levels of the floor. While the actualjunction points E and C are at the bottom ofthe floor. Hence, the pressures at the actualpoints are calculated by assuming a straightline pressure variation.

(b) Correction for the thickness of floor

(b) Correction for the thickness of floor• Correction for the slope of the floor a correction is applied for

a slopping floor, and is taken as positive for the downwardslopes, an negative for the upward slopes following thedirection of flow. Values of correction of standard slop suchas 1 : 1, 2 : 1, 3 : 1, etc. are tabulated in Table below

(b) Correction for the thickness of floor

• The correction factor given above is to be multipliedby the horizontal length of the slope and divided bythe distance between the two pile lines betweenwhich the sloping floor is located. This correction isapplicable only to the key points of the pile line fixedat the start or the end of the slope.

Exit gradient (GE)

• It has been determined that for a standard formconsisting of a floor length (b) with a vertical cutoffof depth (d), the exit gradient at its downstream endis given by

Important Questions• Explain Khosla’s method of independent variables.• Explain the term ”Diversion Head Work” and

clearly mention its different functions.• Explain Bligh’s Creep Theory in details.• What is weir? How does it differ from a barrage

structure?• What are the functions of a canal head regulator?

Reference

• Irrigation Engineering : N.N. Basak

• Irrigation Engineering & Hydraulic

Structures: S.K.Garg

• Internet Websites

ThanksGHT