Barrage& its hydraulic design

By: Vishal Roll no.1130456

CONTENTS INTRODUCTION OBJECTIVE LITERATURE REVIEW HYDRAULIC DESIGN CONCLUSION REFERENCES

INTRODUCTION

What is Barrage :

A barrage is a type of low-head, diversion dam which consists of a number of large gates that can be opened or closed to control the amount of water passing through the structure, and thus regulate and stabilize river water elevation upstream for use in irrigation and other systems.

PRAKASAM BARRAGE (VIJAYWADA, AP )

LITERATURE REVIEW Historically, the first scientific attempt to study the science of subsoil

hydraulics in relation to the design of weirs on permeable foundations was made as early as 1895, in connection with the failure of Khanki Weir, in India. These historic experiments were carried out by Lt. Col. Clibborn, Principal of Thomson Civil Engineering College, Roorkee, during 1895–97. Because of these experiments, the flow of water through the subsoil below the weir/barrage with its attendant hydraulic gradients and uplift pressures has been recognized as the determining factor in the design of the barrage (Government of India 1902). Until 1930, Bligh’s Creep theory, which was no more than an imperfect adaptation of Col. Clibborn’s work, held the field. It was realized eventually that the subject was more complex than simple formula indicated. The years 1929 to 1935 were marked by special activity with respect to subsoil hydraulics in relation to weir design, both in India, the United States of America, Europe, and elsewhere and also various study conducted on barrages by Parsons 1929; Terzaghi 1929; Khosla 1930; Khosla 1932; Haigh 1935; Harza 1935; Lane 1935.

OBJECTIVES 1.THE SUB-SURFACE CONSIDERATION FOR THE

DESIGN OF A BARRAGE 2. STEPS FOR COMPUTING SEEPAGE PRESSURE,

SEEPAGE FORCE AND EXIT GRADIENT FOR A BARRAGE

3. DETERMINATION OF WATERWAY OF A BARRAGE 4. DIFFERENT PROTECTION WORKS FOR A

BARRAGE

DESIGN OF BARRAGE THE DESIGN OF ANY HYDRAULIC STRUCTURE COMPRISES

OF TWO STEPS: • HYDRAULIC DESIGN, TO FIX THE OVERALL DIMENSIONS

AND PROFILES OF THE STRUCTURE ( DIMENSIONS ARE USUALLY FIXED BY EMPIRICAL FORMULAE)

• STRUCTURAL DESIGN, WHERE THE VARIOUS SECTIONS ARE ANALYSED FOR STRESSES UNDER DIFFERENT LOADS AND REINFORCEMENT OR OTHER STRUCTURAL DETAILS ARE WORKED OUT.

For barrages, there are two different sets of hydraulic conditions exist :

1. The first is due to sub-surface or seepage flow conditions that occurs due to a water level difference on the upstream and down stream of a barrage and it is the maximum when the gates of a barrage are mostly closed as during the low flow period of the river.

2. The other is due to surface flow conditions which occur while the barrage gates are open during floods.

Hydraulics of sub-surface flow The sub-surface flow below a barrage causes two

definite instability problems, as listed below and illustrated in Figure 1.

1. Uplift forces due to the sub soil pressure that tends to lift up the barrage raft floor, and

2.Upward rising seepage forces through the river bed just down stream of the solid apron causes sand particles to erupt upwards and tends to ‘piping’ failure of the foundation.

As seen from Figure 3, the distribution of sub-surface pressure, that is, the pressure of the water held within the pores of the soil is such that it varies from a maximum along the upstream river bed to a minimum at the downstream river bed.

The pressure head differential between the upstream and downstream is expressed as a percentage and denoted by φ. A comparison of pressure distribution below the barrage floor from Figs. 3(a) and 3(b) indicate that the introduction of sheet piles reduce the pressure below the barrage raft floor. In fact, the seepage paths increase due to the introduction of sheet piles, consequently reducing the gradient of sub-surface pressure.

The pressure at any location of a certain equipotential line is given by the following expression

Hu is the head of water on the upstream

pool above datum HD

is the head of tail water above datum.

ESTIMATION OF SEEPAGE DISCHARGE

Total seepage flow q is: q = Nf .Δq Δq m3/s per metre width, discharge between

two stream line per meter Nf is the no of flow channels

The quantity Δq is governed by Darcy’s law Δq = k (Δh/ Δ s) Δ n k is the coefficient of permeability Δh is the potential drop between two equipotential

lines Δs is the potential length along the stream line of flow

net ‘square’ and Δn is the length normal to the stream line and pressures the other length of the ‘square’.

Δs = Δn Δh = Hdiff

/ Nd

q = Nf k Hdiff

/ Nd

Hdiff is the head difference between upstream

pool and downstream tail water level Nd

is the number of equipotential drops between the upstream and the downstream river bed.

SEEPAGE FORCE

An expression for the seepage force ΔF acting on the cylindrical elementary volume may be expressed as:

ΔF = p. ΔA - (p + Δp).ΔA This expression yields ΔF = -Δp. ΔA Thus, the seepage force per unit volume of soil is given as: ΔF / (ΔA.Δl) = -Δp /Δl =-ρ g.ΔH/Δl

At the exit end, where the stream line meets the river bed surface (B in Figure 5), the seepage force is directed vertically upwards and against the weight of the volume of solid held in the soil. If the seepage force is great enough, it would cause sand-boiling, with the ejection of sand particles causing creation of pipe-like voids through the river bed

In order to provide safety against piping-failure at the exit end, the submerged weight (w) of the solid should be at least equal to the seepage force. This may be expressed as:

w = (1-e)(ρs-ρ) g≥-ρg ΔH/Δl w is the submerged weight of the solids assuming a void

ratio e. ρs and ρ stand for the density of the solids and

water The equation then simplifies to -ΔH/Δl ≤(1-e) (G-1)

EXIT GRADIENT AT THE END OF IMPERVIOUS FLOOR.

FACTOR OF SAFETY FOR DIFFERENT TYPES OF SOILS SHALL BE AS FOLLOWS COARSE SAND : 5 TO 6 FINE SAND : 6 TO 7 GE = {H/d} X [1/π√] = (1 + √1+ 2 )/2 = b/d b = LENGTH OF BARRAGE d = DEPTH OF DOWN STREAM CUT OFF H = HEAD OF WATER THE DEPTH OF D/S CUT OFF ALONG WITH THE TOTAL LENGTH OF IMPERVIOUS FLOOR

SHOULD BE SUFFICIENT TO REDUCE THE EXIT GRADIENT TO WITHIN SAFE LIMITS.

COMPUTATION OF DISCHARGE THE DISCHARGE SHALL BE OBTAINED FROM THE FOLLOWING

FORMULA: Q = CLH3/2; WHERE Q = DISCHARGE IN CUMECS, C - COEFFICIENT OF DISCHARGE L = CLEAR WATERWAY OF THE BARRAGE IN M, AND H = TOTAL HEAD CAUSING THE FLOW IN M.

LENGTHOF WATERWAY LACEY’S WATERWAY IS GIVEN BY THE FOLLOWING

FORMULA: L = 4.83 √Q WHERE Q = DESIGN FLOOD DISCHARGE IN CUMECS FOR 50 YEAR FREQUENCY FLOOD.

SCOUR DEPTH: SCOUR IS LIKELY TO OCCUR IN ERODIBLE SOILS

SUCH AS CLAY,SILT,SAND. THE DEPTH OF SCOUR MAY BE CALCULATED

FROM THE LACEY’S FORMULA WHICH IS AS FOLLOWS.

AS PER Cl. 19.1 OF I.S:6966(PART I) -1989. R = 0.473 (Q/f)1/3

WHEN LOOSENESS FACTOR IS MORE THAN 1

R = 1.35 (q2/f )1/3 WHEN LOOSENESS FACTOR IS LESS THAN 1 Where R = DEPTH OF SCOUR BELOW THE HIGHEST FLOOD

LEVEL Q= HIGH FLOOD DISCHSRGE IN THE RIVER IN CUMECS

q = INTENSITY OF FLOOD DISCHARGE IN m3 per m

width f = 1.76 (mr)1/2

where mr is average particle size of soil grain

EXTENT OF SCOUR IN A RIVER WITH ERODIBLE BED MATERIAL VARIES AT DIFFERENT PLACES ALONG A BARRAGE. THE LIKELY EXTENT OF SCOUR AT VARIOUS POINTS ARE AS FOLLOWS• LOCATION RANGE MEAN

U/S OF IMPERVIOUS FLOOR 1.25 TO 1.75 R 1.5 R D/S OF IMPERVIOUS FLOOR 1.75 TO 2.25 R 2.0 R WHERE R = DEPTH OF SCOUR IF THE CONCENTRATION FACTOR IS TAKEN INTO ACCOUNT IN FIXING DEPTH

OF CUT OFFS.

THESE SHOULD BE SUITABLY EXTENDED IN TO THE BANKS ON BOTH SIDES UP TO AT LEAST TWICE THE DEPTH

CUT OFFS The Cut offs can be provided in RCC, Steel Sheet pile

or wells. These are the barriers provided below the floor both

on U/s and D/s The main purpose is to increase the seepage path

and also to prevent piping action below the floor. The depth of Sheet pile depends on the safety factor

adapted for the design THE U/S AND D/S CUT OFFS SHOULD GENERALLY BE

PROVIDED TO CATER FOR SCOUR UPTO 1.0 R AND 1.25R RESP.

PROTECTION WORKS U/s block protection: PERVIOUS PROTECTION COMPRISING OF C.C. BLOCKS OF

ADEQUATE SIZE LAID OVER LOOSE STONE SHALL BE PROVIDED JUST BEYOND THE U/S END OF THE IMPERVIOUS FLOOR.

THE C.C. BLOCKS SHALL BE GENERALLY BE OF 1500 MM X 1500 MM X 900 MM SIZE.

THE LENGTH OF U/S BLOCK PROTECTION SHALL BE EQUAL TO ‘D’.

D/S BLOCK PROTECTION: IT SHALL COMPRISE OF C.C.BLOCKS OF ADEQUATE SIZE

LAID OVER A SUITABLY DESIGNED INVERTED FILTER FOR THE GRADE OF MATERIAL IN THE RIVER BED.

THE C.C. BLOCKS SHALL GENERALLY NOT SMALLER THAN 1500 X 1500 X 900 MM SIZE TO BE LAID WITH GAPS OF 75 MM WIDTH, PACKED WITH GRAVEL.

THE LENGTH OF THE D/S BLOCK PROTECTION SHALL BE EQUAL TO 1.5 D

The downstream block protection is laid on a graded inverted filter designed to prevent the uplift of fine sand particles upwards due to seepage forces. The filter should roughly conform to the following design criteria.

Where d15 and d85

represent grain sizes. dx is the size

such that x% of the soil grains are smaller than that particle size. Where x may be 15 or 85 percent.

CONCLUSION Barrages are the most important diversion headworks

which are if appropriately designed will create lots of opportunities in terms of irrigation, regulation, power projects. But there seems to be a lot of uncertainties such as:Non- homogeneity of the foundation soil, Difference in the packing and pore space, Local intrusion of impervious material like clay beds or very porous material, Faults and fissures in sub-soil formation,

So mere design on the basis of empirical formulae will not lead us to effective design. Therefore it becomes necessary for us to conduct model studies of structure before prototype construction.

REFERENCES Khosla, A. N. (1930). ‘‘Stability of weirs and canal works: An application of the new

theory of hydraulic gradient.’’ Paper No. 140, Punjab Engineering Congress, Punjab, India.

Parsons, H. D. (1929). ‘‘Hydraulic uplift in previous soils.’’ Paper No.1713, Trans. ASCE.

Garg N.K. (2002). “Optimal Barrage Design based on Subsurface Flow Considerations” Paper No.128, ASCE

https://en.wikipedia.org/wiki/Barrage IS 6966: Part1 (1989) ‘Guidelines for Hydraulic Design of Barrages and Weirs: Part-1

Alluvial Reaches (First revision)’, pub. By Bureau of Indian Standards, Manak Bhawan, New Delhi

nptel.ac.in/courses/105105110/pdf/m4l02.pdf