diversion headworks

Diversion Headworks

Module-III

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

for the diversion headwork- components of diversion headwork- Causes of failure of structure on pervious foundation- Khosla’s theory- Design of concrete sloping glacis weir

Diversion Head-Works• The works which are constructed at

the head of the canal in order to divert the river water toward the canal, so as to ensure a regulated continuous supply mostly silt free water with certain minimum head into the canal, are known 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 is available throughout the year.

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

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

Objective of Diversion Head works

Selection of Site for Diversion Head Works

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

• The river should be straight and narrow at the site

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

• River banks at site should be well defined and stable.

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


• Material of construction should be locally available.

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

• Site should be close to the cropland to minimize loss of water due to seepage and evaporation of canal.

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

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 water level so that water can be diverted by canal to crop field due to difference of head.

• Barrage is practically a low weir with an adjustable gate over this low weir. Heading up of water is affected by gate.

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 soil foundation as:

• Vertical Drop Weir• 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 apron are provided both at upstream and downstream ends of floor to safeguard against scouring action. 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.

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 and downstream riverbed is not more than 3 m. Hydraulic jump is formed when water passes over the sloping glacis. Weir of this type is of recent origin.

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 this weir is designed as low dam. A cistern is provided 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 dry stones are laid in the form of glacis with some intervening core wall.

Dry Stone Slopping Weir

Barrage• When the water level on the

upstream side of the weir is required to be raised to different levels at different time, then the barrage is constructed. Practically a barrage is an arrangement of adjustable gates or shutters at different tiers over the weir. The water level can 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 or cement concrete. On the upstream side, the wall is extended just to cover the canal regulator and on the down stream side, it is extended up to the launching apron. The functions of the divide wall are as follows,

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

• (b) It controls the eddy current or cross current in front of the canal 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. These openings are provided with adjustable gates. Normally, the gates are kept closed. The suspended silt goes on the depositing in front of the canal head regulator. When the silt deposition becomes appreciable the gates are opened and the deposited silt is loosened with an agitator mounting on a boat. The muddy water flows towards the downstream side through the scouring sluices. The gates are closed. But, at the period 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 are important source of fishes. There are various types of fish in the river. The nature of fish varies from type to type. But in general, the tendency of fish is to move from upstream to downstream in winters and from downstream to upstream in monsoons. This movement is essential for their survival.

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

movement gets obstructed, and is detrimental to the fishes. For the movement of the fishes along the course of the river, the fish ladder is essential. In the fish ladder, the baffle walls are constructed in the zigzag manner so that the velocities of flow within the ladder does not exceed 3 m/s. The width, length, and height of the fish ladder depends on the nature of the river and the type of the weir or barrage.

Fish Ladder

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

the canal regulator to regulate the flow of water is known as canal head regulator. It consists of a number of piers which divide the total width of the canal into a number of spans which are known as bays. The pier consists of a number of tiers on which the adjustable gates are placed. The gates are operated from the top by suitable mechanical device. A platform is produced on the top of the piers for the facility of operating the gates. Again some piers are constructed on the downstream side of the canal head to support the roadway.

Canal Head Regulator

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

canal head by constructing the divide wall, then it is found that the lower layer of water contains heavy silt and the upper layer contains very fine silt. The fine silt is very fertile and it may be allowed to enter the canal. But the heavy silt causes sedimentation in the pocket.. To eliminate the suspended heavy silt, the silt excluder is provided. It consists of a series of tunnels starting from the side of the head regulator up to the divide wall.

Silt Excluder• The tunnel nearest to the head regulator is

longest, and the successive tunnels decrease in length, the tunnel nearest to the divide wall is shortest. The tunnels are covered by R.C.C. Slab. The top level of the slab is kept below the sill level of the head regulator. So, the completely clear water is allowed to flow in the canal through the head regulator. The suspended heavy silt carried by the water enters the silt excluder tunnels and passes out through the scouring sluices.

Silt Excluder• Silt excluders are those works which

are constructed on the bed of the river, upstream of the head regulator. The clearer water enters the head regulator and silted water enters the silt excluder. In this type of works, the silt is, therefore, removed from the water before in enters the canal.

Silt Excluder

Silt Ejectors• Silt ejectors, also called silt

extractors, are those devices which extract the silt from the canal water after the silted water has

• traveled a certain distance in the off-take canal. These works are, therefore, constructed on the bed of the canal, and little distance downstream from the head regulator.

Silt Ejectors

Marginal Embankments or dykes

• The marginal embankments or dykes are earthen embankments which are constructed parallel to the river bank on one or both the banks according to the condition. The top width is generally 3 to 4 m and side slope is generally 1 ½ : 1 to 2: 1. The height of the embankment depends on the highest flood level. A suitable margin is provided between the toe of the embankment and the bank of the river. To resist the effect of erosion on the embankment, wooden piles are driven along the river banks throughout the length of dyke. The length of the dyke is protected by boulders pitching with cement grouting and the downstream side 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 must be constructed on both the approaches to protect the structure from erosion. It is an earthen embankment with 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• 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 may

cause 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 bottom of the foundation and emerges at the downstream end of the weir or barrage floor. The force of percolating water removes the soil particles by scouring at the point of emergence. As the process of removal of soil particles goes on continuously, a depression is formed which extends backwards towards the upstream through the bottom of the foundation. A hollow pipe like formation thus develops under the foundation due to which the weir or barrage may fail by subsiding. This phenomenon is known as failure 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 pressure on the foundation of the weir or barrage. If this uplift is not counterbalanced by the self weight of the structure, it may fail by rapture.

• 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, then hydraulic jump develops. This hydraulic jump causes a suction pressure or negative pressure on the downstream side which acts in the direction of uplift pressure. If the thickness of the impervious floor is not sufficient, then the structure fails by rapture.

• (b) By Scouring During floods: The gates of the barrage are kept open and the water flows with high velocity. The water may also flow with very high velocity over the crest of the weir. Both the cases can result in scouring effect on the downstream and on the upstream side of the structure. Due to scouring effect on the downstream and on the upstream 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 fail by piping, uplift force, suction caused by standing wave and scouring on both upstream and downstream of the weir.

• When hydraulic gradient or exit gradient exceeds the critical value of soil, surface soil at down end starts boiling first and is washed away by percolating water. This process of removal or washing out of soil continuous and eventually a channel in the form of pipe is formed by seepage water. This is called piping which may cause the failure of foundation. Similarly uplift force of percolating water is acting on ther floor from bottom and if the weight 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 sheet piles at upstream, downstream or at intermediate point to reduce the hydraulic gradient.

• Floor thickness should be increased to increase its self weight to balance the uplift force.

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 percolating water is increased consequently reducing the exit gradient.

• (b) Sheet piles should be provided on the upstream side and the downstream side of the impervious floor to increase to the length of percolating water so that the uplift pressure is considerable reduced.

• (c) The thickness of the impervious floor should be such that the weight of the floor is a sufficient to counterbalance the uplift pressure.


• (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 the percolating water does not wash out the soil particles.

• Deep foundation like well foundation should be provided for the barrage piers

Energy Dissipater Blocks

Increasing Floor Thickness

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

dimensional steady-state groundwater flow through aquifers.

• The method consists of filling the flow area with stream lines and equipotential lines, which are everywhere perpendicular to each other, making a curvilinear grid.

• Stream Lines: The streamlines represent the paths along 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 on the basis of Bligh’s theory between the periods 1910 to 1925. In 1926 – 27, the upper Chenab canal siphons, designed on Bligh’s theory, started posing undermining troubles. Investigations started, which ultimately lead to

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

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

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

intersecting each other orthogonally. The resultant flow diagram showing both of the curves is called a Flow Net.

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

• Every particle entering the soil at a given point upstream of the work, will trace out its own path and will represent a streamline. The first streamline follows the bottom contour of the works and is the same as Bligh’s path of creep. The remaining streamlines follows smooth curves transiting slowly from the outline of the foundation to a semi-ellipse, as shown below.

Khosla’s Theory• Equipotential Lines: (1) Treating the

downstream bed as datum and assuming no water on the downstream side, it can be easily started that every streamline possesses a head equal to h1 while entering the soil; and when it emerges at the down-stream end into the atmosphere, its head is zero. Thus, the head h1 is entirely lost during the passage of water along the streamlines.

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

path, there is certain residual head (h) still to be dissipated in the remaining length to be traversed to the downstream end. This fact is applicable to every streamline, and hence, there will be points on different streamlines having the same value of residual head h. If such points are joined together, the curve obtained is called an equipotential line.

Khosla’s Theory

Khosla’s Theory Every water particle on line AB is having

a residual head h = h1, and on CD is having a residual head h = 0, and hence, AB and CD are equipotential lines.

Since an equipotential line represent the joining of points of equal residual head, hence if piezometers were installed on an equipotential line, the water will rise in all of them up to the same level as shown in figure below

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 as shown in figure above. This force (F) has an upward component from the point where the streamlines turns upward. For soil grains to remain stable, the upward component of this force should be counterbalanced by the submerged weight of the soil grain. This force has the maximum disturbing tendency at the exit end, because the direction of this force at the exit point is vertically upward, and hence full force acts as its upward component.

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

submerged weight of soil grain should be more than this upward disturbing force. The disturbing force at any point is proportional to the gradient of pressure of water at that point (i.e. dp/dt). This gradient of pressure of water at the exit end is called the exit gradient. In order that the soil particles at exit remain stable, the upward pressure at exit should be safe. In other words, the exit gradient should be safe.

Critical Exit Gradient

• This exit gradient is said to be critical, when the upward disturbing force on the grain is just equal to the submerged weight of the grain at the exit. When a factor of safety equal to 4 to 5 is used, the exit gradient can then be taken as safe. In other words, an exit gradient equal to ¼ to 1/5 of the critical exit gradient ensured, so as to keep the structure safe against piping.

• The submerged weight (Ws) of a unit volume of 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 is necessary to plot the flow net. In other words, we must solve the Laplacian equations. This can be accomplished either by mathematical solution of the Laplacian equations, or by Electrical analogy method, or by graphical sketching by adjusting the streamlines and equipotential lines with respect to the boundary conditions. These are complicated methods and are time consuming. Therefore, for designing hydraulic structures such as weirs or barrage or pervious foundations, Khosla has evolved a simple, quick and an

• accurate approach, called Method of Independent Variables.

Khosla’s Method of independent variables for determination of pressures and exit gradient for seepage below a weir or a barrage

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

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

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

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

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

• 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 given by

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 at which interference is desired.

d = The depth of the pile on which the effect is considered b = Total floor length The correction is positive for the points in the rear of back water, and

subtractive for the points forward in the direction of flow. This equation does not apply to the effect of an outer pile on an intermediate pile, if the intermediate pile is equal to or smaller than the outer pile and is at a distance less than twice the length of the outer pile.


• Suppose in the above figure, we are considering the influence of the 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 direction of flow. Similarly, the correction at C2 due to pile no (3) is positive 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 is assumed to have negligible thickness. Hence, the percentage pressures calculated by Khosla’s equations or graphs shall pertain to the top levels of the floor. While the actual junction points E and C are at the bottom of the floor. Hence, the pressures at the actual points are calculated by assuming a straight line pressure variation.

(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 downward slopes, an negative for the upward slopes following the direction of flow. Values of correction of standard slop such as 1 : 1, 2 : 1, 3 : 1, etc. are tabulated in Table below


• The correction factor given above is to be multiplied by the horizontal length of the slope and divided by the distance between the two pile lines between which the sloping floor is located. This correction is applicable only to the key points of the pile line fixed at the start or the end of the slope.

Exit gradient (GE)

• It has been determined that for a standard form consisting of a floor length (b) with a vertical cutoff of depth (d), the exit gradient at its downstream end is 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