Weir and Barrages

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Plan of Barrage

COMPONANTS

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Guide Bund

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Types of Wear Sloping Weir of Concrete

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ll. Vertical Drop Weir

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lll. Sloping Weir of ConcreteThis 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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lv. Parabolic WeirA 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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v. Dry Stone Sloping WeirIt 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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B. 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

*BARRAGES

Comparison of Barrage Vs WeirBarrage WeirLow set crest.High set crest.Ponding is done by means of GatesPonding is done against the raised crest or partly against crest and partly by shuttersGated over the entire lengthShutters in part lengthGates are of greater heightShutters are of low height (2 m)Gates are raised to pass high floodsShutters are dropped to pass floodsPerfect control on river flowNo control of river in high floodsGates convenient to operateOperation of shutters is slow, involve labour and timeHigh floods can be passed with minimum affluxExcessive afflux in high floods

*BARRAGES

Barrage WeirLess silting Upstream due to low set crest.Raised crest causes silting UpstreamLonger construction periodShorter construction periodSilt 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

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Site Selection

The following considerations should be kept in mind when deciding on the site for a Barrage;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.The width of the river at the site should preferably be the minimum with a well defined and stable river approaches.A good land approach to the site will reduce the expense of transportation and, therefore, the ultimate cost of the Barrage.A good Catchment Area having minimum infiltration and appropriate gradient to generate sufficient discharge with minimum rainfall.

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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.The material required for construction should preferably be available close to the site to minimize the construction cost.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.A rock foundation is the best but in alluvial plains the bed is invariably sandy. Easy diversion of the river after construction.

*BARRAGES AND WEIRS

Investigations for Site SelectionTopographic SurveyTopographical survey comprises;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.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.Cross section of the river at 2 km intervals up to pondage effect on upstream

*BARRAGES AND WEIRS

Investigations for Site SelectionLongitudinal 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. 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

Investigations for Site SelectionCollection of Hydrological DataThe Hydrological data are collected to;Compute the Design Flood.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

Investigations for Site SelectionSurface and Sub Surface InvestigationsTrial 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.Observations of water table in the area adjacent to the location of the barrage is also carried out for three-dimensional electrical analogy studies. 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

Investigations for Site SelectionConstruction MaterialsSurvey 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.Diversion RequirementsDiversion requirements are worked out in accordance with the need of the project.Communication SystemInvestigation includes dislocation of existing facilities and their relocation and additional facilities required during construction and operation.

*BARRAGES AND WEIRS

Investigations for Site SelectionOther Miscellaneous StudiesThese 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.Environmental and EcologicalThe 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.Flood Plain Aerial map of the flood plain indicating dominant River Course.

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Purpose of Barrage/ Headworks

Headwork serves the following purposesIt raises the water level in the river so that the commendable area can be increased.It regulates the intake of water into the canal.It controls the silt entry into the canal.It reduces fluctuations in the level of supply in the river.It stores water for tiding over small periods of short supplies.It facilitates the flood management as well as smooth entry of river supply into the off-taking canal.It provides a road way over the river crossing for public facilitations.

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A) Components of Diversion Headworks (Plan)

Main WeirUnder Sluice portionDivide WallFish LadderCanal Head RegulatorU/S Guide BundD/S Guide BundCanal Head RegulatorU/S Marginal BundD/S Marginal BundRiver Training Works

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Plan of Barrage

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

U/S Flexible ProtectionU/S Sheet PileU/S Concrete FloorIntermediate Sheet PileThe Main Weir StructureU/S Glacises 1:4CrestD/S Glacises 1:3D/S Vertical Sheet PilesInverted FilterD/S Flexible ApronD/S River Bed

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Cross Section of Barrage

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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.Main Barrage Portion;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.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.U/S glacis having the necessary slope to join the U/s floor level to the highest point, the crest.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). 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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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.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.Sheet Piles a) U/S Sheet PilesU/S sheet piles is situated at the U/s end of the U/s concrete floor. The piles are driven into the soil beyond the maximum possible scour that may occur. The functions are;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 sheet piles to increase the bearing capacity.

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b) Intermediate Sheet PilesIntermediate 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 PilesD/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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Inverted FilterAn 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 9gravel. 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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Flexible ApronA flexible apron is placed D/S of the filter and consists of boulders large enough not to be washed away by the highest likely water velocity. The protection provided is such as to cover 1.5 x depth of scour on the U/s side and 1.5 to 2 x depth of scour on the D/S side at a slope of 3:1. figure below;

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UndersluiceA 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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Divide WallThe 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;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.To generate a parallel flow and thereby avoid damage to the flexible protection area of the undersluice portion.

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Fish LadderFish ladder built along the divide wall, is a device designed to allow fish to negotiate the artificial barrier in either direction.Guide BanksGuide 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

RetrogressionRetrogression 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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AccretionAccretion 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-IDetermined 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 = CIAWhere 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-IIWidth of WeirThe width of the Barrage should be adequate enough to pass the design discharge amicably for the given pond level. Laceys 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 DischargeThis is the clear water way required for passing the Design Discharge. However, using the Laceys looseness coefficient which varies 1 1.6. The width between the abutment = Wa =Pw x 1.6

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Step-IIIProfile of BarrageThe 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-IVFixing of the Crest LevelThe 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 Laceys scour formula. R = 0.9 (q2/f)1/3 or R = 1.35 (q2/f)1/3The 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.3/2Where 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 MKSHence H can be determined. Subtract this H from the Total Energy Line (TEL) which will fix the crest level.

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ExampleCalculate the crest level for a gated diversion structure for the following data;Maximum discharge = 1000 m3/sec, High Flood Level = 100 mLength of the Barrage = 200 m, f = 0.1Solution 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 mUsing 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 mCrest Level of Barrage = 100 1.82 = 98.18 m

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Step-VHydraulic Jump Formation and Fixation of D/S Floor LevelThe 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;Blench CurvesCrumps CurvesConjugate Depth method

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Hydraulic Jump Formation and Fixation of D/S Flow Level

Blench curvesThis 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 TELFor 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 Ef2Repeat 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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Hydraulic Jump

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Rating Curves D/S of Barrage

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Hydraulic jump formation and fixation of d/s floor level

Crump's CurveThis 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)1/3Also find out the U/S and D/S Energy Lines for one set of Flow Condition and Calculate (HL)/dcFor 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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Crumps Curves

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Step-VI

Inverted FilterAn 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 9gravel. 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 ApronThe protection provided is such as to cover 1.5 x depth of scour on the U/s side and 1.5 to 2 x depth of scour (d2 ) on the D/S side at a slope of 3:1. The apron in the launched position over the slope of 3:1, the apron must have a thickness of 90-100 cm. knowing the inclined length and the thickness, the total volume of the stone can be calculated and hence the thickness in the horizontal position in a length of 2.5 d2 can be calculated.

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Design of Stone Apron

U/S SideLength according to Lacy = 2.20 CHH = 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.D/S SideLength according to Lacy = 2.20 CH/13H = Depth of water above the apron level.C = Lacey's coefficient.

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

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Step-VIIIDivide WallA 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.FunctionsDivide 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-IXFish LadderRivers 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-XScouring Sluices or Undersluices, Silt Pocket and Silt ExcludersThe 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

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Step-XIGuide BundGuide bund is shown in the enclosed figure. Guide banks are constructed on both side of the Headworks to protect the structures and guide the flow so as the confine it in a reasonable width of the river. It was first designed by Bell and therefore, it is also called Bell Bund. It consists of a heavily built embankment in shape of bell mouth on both sides. Enclosed figure shows the length proportion upstream and D/S of the weir. If L is length of weir or waterway, Upstream length portion is taken 1.25L to 1.5L and length of Downstream of the weir is equal to 0.25L.Angles made by bell mouths both at Upstream and Downstream are also shown in enclosed figure.

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Components of Guide Banks are;Upstream curved headDownstream curved headShank portion which joins upstream and downstream curved end.Sloping apronLaunching apronPile Protection

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Guide Bund

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Step-XIIMarginal EmbankmentThese are earthen embankments constructed parallel to river bank. It starts upstream from the head of the guide bank as shown in the enclosed figure. It serves the purposes of;Preventing flood water from entering the surrounding area.It retains the extra water due to flood within a specified section.It protects the important cropland upstream of the project from flooding.Due to construction of weir an afflux of water upstream is created and marginal embankments are always necessary to confine this afflux well within the river.

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Guide / Marginal Bund

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Guide/ Marginal Bund (Thickness of Apron/ Stone Pitching)

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Step-XIIICanal Head RegulatorCanal 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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Step-XIVSilt 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 TheoryBligh 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 TheoryConsidering 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 +2d2It 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 CriteriaBligh gave two design Criteria

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Sub Surface Flow (Blighs Creep Theory)

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Sub Surface Flow (Blighs Creep Theory)

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Sub Surface Flow

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ExampleThe 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

Lanes Weighted Creep TheoryAccording to this theory, greater weight should be given to vertical cut-off than to horizontal floors. The reasons are;In practice the contact between the vertical and steeply sloping surface is likely to be closer than along horizontal or slightly sloping surfaces.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.Vertical cut-off are more effective against horizontal stratification, and check the free flow through the layers of low permeability.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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Lanes Weighted Creep TheoryLane 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 +3d2Although his theory is a modification over Blighs Theory, it is still empirical. There is no rational basis to be used for design.

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Lanes Weighted Creep TheoryTo 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 Lacys theory is shown as below;-

MaterialCj (Lanes Values)C (Blighs Values)Very fine sand and silt8.518Fine sand7.015Coarse sand5.712Gravel and sand3.5 to 39Boulders gravel and sand2.5 to 34 to 6Clayey soils3.0 to 1.6--

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Khoslas Theory

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Khoslas Theory (Theory of FlowNet)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.

DIVERSION HEADWORKSKhoslas Theory

DIVERSION HEADWORKSSubsoil HGL and Piping

DIVERSION HEADWORKSPercolation below Weirs on Sand

Critical Exit Gradient and Safe Exit Gradient.

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Khoslas Method of Indepndent Variables

Figure 6.10

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Khoslas TheoryFor 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.Figure-I shows a relationship between uplift pressure and 1/.The Khoslas curve is used for calculation of D, E and D for the piles and the ends.Figure-II is used for calculation of Uplift pressure for the intermediate sheet piles.Figure-III is used for calculation of Exist Gradient.

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Khoslas Theory (Figure-I)

DIVERSION HEADWORKSKhoslas Theory (Figure-II)

DIVERSION HEADWORKSKhoslas Curve (Figure-II)

DIVERSION HEADWORKSKhoslas Theory (Figure-III)

DIVERSION HEADWORKS = b/d b = Total Length

d = Depth of D/S Sheet pile

Calculation of pressures on Khoslas Theory

DIVERSION HEADWORKSKhoslas TheoryMethods of Reading Khoslas Curve

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Khoslas Theory

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Khoslas Theory

Figure 6.10

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Khoslas 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 C, D and E etc.The correction due to, floor thickness, 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 FloorLet 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 Khoslas 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.

DIVERSION HEADWORKSFigure 24.22

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)

DIVERSION HEADWORKSFigure-V

C) Correction for Slope

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Correction for SlopeCorrection of slope of the floor has also been recommended by Khosla. The following table gives the recommended slope correction;

DIVERSION HEADWORKSCorrection for Slope

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Khoslas Method for Calculation of Depth of D/S PileAs already discussed, Exit Gradient expression is available from potential theory. It is also shown there that in the case of flush floors, the Exit Gradient value is theoretically infinity without a D/S Sheet Pile.According to Khosla it is the D/S sheet pile which controls the Exit Gradient value. Hence in Khoslas method the entire floor and D/S Pile is taken as the elementary profile for the computation of the Exit Gradient. For this case an analytical solution is available. Exit Gradient = H 1 d where = 1+ 1+ 2 2 and = b/d where b = Total Floor Length (L) d = Depth of D/S Sheet pile (d) H = Head acrossThe limiting value of Exit Gradient will fix the D/S Sheet Pile.

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Example-ICalculate the safe exist gradient with the following data;Depth of end sheet pile = 7 mSeepage Head = 4 mLength of the impervious floor = b = 50 m = b/d = 50/7 = 7.14For = 7.14 1/ = 0.165Hydraulic gradient GE = H/d 1/ = 4/7 x 0.165 = 0.094 = 1/10.6

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

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Alternative Solution = b/d = 50/7 = 7.14For = 7.14

= 1 + 1 + 2 2 = 1 + 1 + (7.14)2 2 = 1 + 1 + 50.97 = 51.97 2 = 1 + 7.209 = 8.209 = 4.10 2 2 = 1/ = 0.165 GE = H/d 1/ = 4/7x 0.156 = 0.089 = 1/11 which is within the safe limit.

DIVERSION HEADWORKSExample-II

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DIVERSION HEADWORKSExample-III

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DIVERSION HEADWORKSThe above calculator values are either subtracted or addedfor Calculation of Net pressure at the key points.

DIVERSION HEADWORKSExample-IV

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Calculation of up left Pressures at the key points by analytical methods

Composite Profile of Floor : Khoslas Solution :

Specific Cases

Pile at some intermediate point

Fig 12.13

Pile at downstream Refer Fig 12.12 a

Pile at the upstream end

Exit gradient

Permissible Exit Gradient

SolutionFor Upstream Pile Line No.1

Example :Determine the uplift pressures at the key points by Analytical method for the Fig show below .

Intermediate Pile Line No. (2)

From previous

For Downstream Pile line No. 3.

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Causes of Failure of Weirs and their Remedies

PipingWater seeps under the base of the weirs founded on permeable soils. When the flow lines emerges out at the D/S end of the impervious floor of the weir, the Hydraulic Gradient or the exit gradient may exceed a certain critical value 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 further concentration of flow lines resulting into the depression and still more soil is removed. This process of erosion thus progressively works backward towards the upstream and results in the formation of a channel or a pipe underneath the floor of the weir, causing its failure.Remedies; Piping failures can be prevented by;Providing sufficient length of the impervious floor so that path of percolation is increased and the exit gradient is decreased.Providing pile at downstream ends.

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Causes of failure of weirs and their remedies

Rupture of Floor Due to UpliftIf 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;Providing impervious floor of sufficient lengthProviding impervious floor of appropriate thickness at various points andProviding pile at the upstream end so that the uplift pressure to the d/s is reduced.

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Rupture of Floor Due to Suction Caused by Standing Wave/ Hydraulic JumpThe 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;Providing additional thickness of floor to counterbalance the extra pressure due to the standing wave.Constructing the floor thickness in one concrete mass instead of in masonry layers.

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Scour on the Upstream and Downstream of the WeirWhen 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;Taking the piles at upstream and downstream ends of the impervious floor, much below the calculated scour level. 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;Uplift PressureThis 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.Therefore in the case of gravity floors, the thickness of the aprons or the glacis must be of greater weight than the uplift pressure.Hence it is very important to determination the exact uplift pressure at each point under the Barrage profile.

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UnderminingWhen 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.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.The river discharge over the weir further aggravates the situation by washing away the loosened soil due to the excessive exit gradient.The problem consists therefore in controlling the seepage force so that it cannot carry away the foundation material.

Computation of Seepage Discharge *

Computation of Seepage DischargeComputation of Rate of Seepage from Flow Net

Computation of Seepage Discharge

Design of a Barrage (Example)*

*Example on the design of a BarrageExample 11.6.Example 11.6.

* Fixing the Crest Levels and WaterwayCrest Levels.Waterway :

*Assume the waterway as below(a) Undersluice portion :(b) Other barrage bays portion

*Design of Undersulice Potion(1) High flood condition

*(2) Pond Level Flow Condition

*Depth of Sheet Pile Lines from Scour Considerations

*Total Floor length and Exit Gradient

*Uplift Pressures

*Upstream Pile No. (1)D/s Pile LineLet us correct these pressures

*Table 11.9

*(a) Pre-jump profile:Table 11.10. Pre-jump Profile Calculations

(b) Post jump ProfileTable 11.11. Post Jump Profile Calculation

Protection Workssay 1.5 m in length.say 16 m in length.

Design of Other Barrage Bays PortionHere the crest level is 258.3 m.

Table 11.12. Other Barrage Bays Portion

IMPORTANT

Depth of Sheet Piles from Scour

Total Floor Length and Exit Gradient

Uplift PressuresUpstream Pile No. (1)Downstream Pile No. (1)Let us correct these pressures

Table 11.13Pre-jump Profile

Table 11.14. Pre-jump Profile Calculations

Post Jump Profile.Table 11.15 Post Jump Profile Calculations

Protection Works

Canal Head Regulator DesignExample 11.7.Solution.

Provide 6 bays of 7.5 m each giving a clear waterway of 45 m.

Hydraulic Calculations for Various Flow Conditions

Table 11.16

Depth of Sheet Piles from Scour ConsiderationsTotal Floor Length and Exit Gradient

Adopt total floor length = 38 meters.

Uplift PressuresUpstream pile No. 1.

Downstream Pile No. (2)

Table 11.17

Floor Thicknesses

Protection works

Weir and Barrages

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