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BARRAGE DESIGN

Group MembersShahbaz Manzoor BSCE 01113099Arslan Ali Mughal BSCE 01113130Uzair Akram BSCE 01113133Shahzad Mehmood BSCE 01113149

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

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Plan of Barrage

Guide Bund

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Terms used in Barrage Design

Discharge (Q) = m3/secIt is the volume metric flow of water during per unit time. Discharge Intensity (q) = m3/secDischarge flowing through per unit width of a structure which is; q = Q/B and q = 1.70E3/2Velocity of ApproachThe velocity of flowing water approaching to a metering section is called velocity of approach which is;Hap = V2/2giv) Energy Line (E)It is equal to depth of water + velocity of approach.E = D+ Hap

Critical Depth (dc)It is the depth of water at which Specific Energy is minimumdc = [q2/g]1/3Scour Depth (R)It is the maximum depth measured from the High Flood Level (HFL) to the lowest bed point which is eroded/ scoured as an outcome of water current.R = 1.35 (q2/f)1/3Wetted Parameter (P)It is the surface area of any cross section which is wetted by the flowing water.P = 4.75 Q Where P = B + 2D.

Conjugate Depth (d1d2)These are the depth of water it is before and after the formation of Hydraulic jump.Discharge over the Weir (Q)Q = CBH3/2B = Breadth of the Weir in meterH = Total Water Depth above the Weir CrestC = Constant depends upon the Drowning Ration (2.9 3.1) in FPS system and 1.7 in MKS system.Drowning RatioIt is the ratio between the depth of water above crest at the D/S to the depth of water above crest on the U/S.DR = h/D

Site Selection

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.

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.A rock foundation is the best but in alluvial plains the bed is invariably sandy.

SURFACE FLOW CONSIDERATION

Surface Flow Consideration

Retrogression:

Retrogression is a temporary phenomenon which occurs after the construction of weir or barrage in a river flowing through alluvial soil.

As a result of back-water effects and the increase in depths, the velocity of the water decreases resulting in the deposition of the sediment load.

Accretion

Accretion is the reverse of retrogression and normally occurs u/s although may also occur d/s after the retrogression cycle is completed.

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. BY Q = CIA

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) Laceys looseness coefficient which varies 1 1.6

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Step-iiiFixing of the Crest Level

The crest level is fixed by the requirements of the total head required to pass the designed flood over the crest.

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

The velocity of approach will be (q/R) and therefore the velocity head (V2/2g) can be calculated. This would fix the U/S energy line. Thus using the Discharge formula.

Q = C.L.H.3/2

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Step # iv: Fixation of crest level of under sluice*

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Step # v:Fixation of d/s floor using blench curve*

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vi) Hydraulic jump formation

Crump's CurveFirst calculate discharge intensity (q) 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)/dcCalculate the value of F for critical flow condition and check weather the Hydraulic Jumps moves on the D/S glacis.

Crump's Curve

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Step # vii) Scour protection:*

Step-viii

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.

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Step-ix

Inverted Filter:An inverted filter is provided between the D/S Sheet Piles and the flexible protection. It would typically consist of 6 fine sand, 9 coarse and 9gravel. The length of the filter should be 2 x D/S depth of the sheet piles.

Inverted Filter:

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Step # x) Design of guide bank* Guide Banks:

The guide banks provided in pairs, symmetrical in plan and may either be kept parallel or may diverge slightly up-stream.

Details of Guide Bund

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Step # xi)

Design of marginal bund:*Marginal Embankments or Levees. The marginal embankments retains the flood waters as a result of back water effect.

Details of Marginal Bund

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SUB SURFACE FLOW CONSIDERATION

Step # 1) Fixing the depth of sheet pil:*

Step # 2) Calculation of exit gradient:*

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Step # 3) Calculation of uplift pressure and correction:*

Khoslas Theory

Khoslas Theory

Correction for Mutual Interference of Piles

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Correction for Thickness of the Floor

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Correction for Slope

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