FLOW CHARACTERISTICS IN LOCAL SCOUR AT BRIDGE PIERS

FLOW CHARACTERISTICS IN LOCALSCOUR AT BRIDGE PIERS

Based on experiment done by Prof. BRUCE W. MELVILLE and Prof. ARVED J. RAUDKIVI in 1976.

Abhilash Jana

PG student (2014-16) of Water Resources And Hydraulic Engineering

School Of Water Resources Engineering, Jadavpur University Introduction

Many bridges are failed due to scour around bridge pier. So, an accurate measure of the depth of scour is essential to safe the life and moneyThe literature review carried out here - summaries the major results of an investigation of flow patterns in the scour zone of a circular pier under clear water scour conditions, which was done by BRUCE W. MELVILLE and ARVED J. RAUDKIVI (Professors of Civil Engineering, University of Auckland, New Zealand) in 1976.The research was confined to uniform cohesion less material and clear-water flow conditions.

Objective

The principal objective of present study is to review the experiment done by Prof. Bruce W. Melville and Prof. Arved J. Raudkivi in 1976 and gather knowledge about flow patterns in the scour zone of a circular pier under clear water scour conditions.Vortex shedding

vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff (as opposed to streamlined) body at certain velocities, depending on the size and shape of the body.vortices are created at the back of the body and detach periodically from either side of the bodyThe fluid flow past the object creates alternating low pressure vortices on the downstream side of the object.The object will tend to move toward the low-pressure zone

Vortex shedding

The experimental set-up used by Melville and Raudkivi (1976)

A glass- sided flume 19 m long, 456 mm wide and 440 mm deep was used for the experiments.The working section was 15 m from the upstream end and a cylinder of 50.8 mm diameter was used as the pier.The flume had a fixed flat bed coated with the sand used at the working section.The sand used in the experiments had a uniform grading curve with d35 =0.300 mm, d50 = 0.385 mm, d65 = 0.500 mm and a specific gravity of 2.65.A flow of 17.12 l/s was used and it yielded on the bed slope of S0 =0.0001 a uniform flow depth y0 = 0.15 m and a mean approach velocity V0 = 0.25 m/s.

The shear velocity for this uniform flow was

The critical shear velocity for the threshold of grain movement from Shields' criterion was

Three stages of scour

The initial flat bed,

The intermediate scour hole after 30 minutes from commencement of erosion

The final equilibrium scour hole

The fixed bed model of the equilibrium scourThe upstream part of the scour holes was approximately conical in shape with slopes of 370 to 380The static angle of repose of the sand under water is 320The mean directions of flow were recorded with the aid of a short cotton tuft glued to the end of a thin rod. In addition the hydrogen-bubble technique was extensively used to trace the flow pattern.The velocity magnitudes and turbulence intensities within the scour hole were measured with the Dansk Industri SyndikatAnemometer (DISA).

The estimates of bed shear stress were normalized using the local critical bed shear stress, sc, as given by the Shields' criterion and adjusted for bed slope. The slope adjustment was by the relationship derived by Brooks (1963):

Where, sc = critical shear stress on the slope, c = critical shear stress on flat bed as given by the Shields' function,

= slope angle,= angle between flow direction and slope direction measured in the plane of the slope, and

Experimental results observed by Melville and Raudkivi (1976)

Direction (left) and magnitude (right) of mean velocity past a 50.8 mm diameter pier for the initial flat bed condition (top), the intermediate scour hole and the equilibrium scour hole (bottom). All measurements are at 2 mm perpendicular distance from the bed. The dotted lines of the flat-bed model are for the potential flow past a cylinder.Experimental results observed by Melville and Raudkivi (1976)

Direction (left) and magnitude (right) of mean velocity past a 50.8 mm diameter pier in the vertical plane of symmetry ahead of the cylinder for the initial flat bed condition (top), the intermediate scour hole and the equilibrium scour hole (bottom).

Conclusion made by Melville and Raudkivi (1976)At the bottom of the scour hole a rim, concentric with the cylinder, was observed for most of the time upstream of the cylinder. This rim was formed by the deflection upwards of the down flow in front of the cylinder where this up flow meets with the horseshoe vortex. The erosion occurs below the rim, which collapses irregularly in sand avalanches, forcing the material up the slope and into the flow. The eroded material is carried by the flow into the wake region. Future Scope

To observed this experimental result by using by using ADV(Acoustic Doppler Velocimetry).To see the flow pattern in live bed scour water instead of clear water scour.To observe the change of flow and scour if two pier are place serially along the flow direction.To experimental use of scour to increase the river bed except the use of dazing ship.

References

MELVILLE B.W. & RAUDKIVI A.J. (1977): Flow Characteristics in Local Scour at Bridge Piers, Journal of Hydraulic Research, 15:4, 373-380.Melville B.W. & Coleman S.E. (2000): Bridge Scour, Water Resources Publication, 2000 - Technology & Engineering, USA.Kothyari, U. C., Garde, R. J., and Ranga Raju, K. G. (1992). Temporal variation of local scour around circular bridge piers. Journal of Hydraulic Engineering, 118:8(1091), 10911106.BROOKS, N. H. (1963), Discussion of "Boundary Shear Stresses in Curved Trapezoidal Channels", by A. T. Ippen and P. A. Drinker, Proc. A.S.C.E., Journal Hydraulics Division, Vol. 89, HY3, pp. 327-333.HJORT, P. (1972), Lokal erosion och erosionsverkan vid avloppsledning i kustnaa omroden, Bulletin Serie B, nr. 21, Institutionen Fr Vattenbyggnad Takniska Hgskolan i Lund.MAULL, D. J. and T. A. YOUNC; (1973), Vortex Shedding from Bluff Bodies in a Shear Flow, Journal Fluid Mechanics,Vol. 60, Pt. 2: 401-409.MELVILLE, B. W. (1975), Local Scour at Bridge Sites, Thesis submitted for the degree of Doctor of Philosophy at the University of Auckland School of Engineering. Also available as University of Auckland, School of Engineering,Report No. 117.Dey, S. (1999). Time-variation of scour in the vicinity of circular piers. J. Wat. Maritime Eng., Proc. Inst. Civ. Eng., 136(June), 67-75.Dey, S., and Raikar, R. V. (2005). Scour in long contractions. J. Hydraul. Eng.,131(12), 1036-1049.Dey.S. and Barbhuiya. Abdul Karim, (2005), Turbulent flow field in a scour hole at a semicircular abutment . NRC Research, Can. J. Civ. Eng. 32: 213232.Dey.S, Mutlu Sumer.B, and Fredsoe.J, (2006) Control of Scour at Vertical Circular Piles under Waves and Current J. Hydr.Engg. ASCE, 121(12), 869 - 875.Dey. S ,and Barbhuiya. A.K, (2003) Time Variation of Scour at Abutments J.Hydr. Engg . ASCE, 131(1), 11-23.Dey .S and Raikar , Rajkumar. V, (2005), Clear-Water Scour at Piers in Sand Beds with an Armor Layer of Gravels. J. Hydr. Engg . ASCE, 133(6), 703-711.Wikipedia