850_Seminar Cheng Yunshuo

CIVL 850Cheng Yunshuo, 12/1/ 2015

Scour of Bridge Pier

What is scour? Most of the bridge piers are located on the cohesionless river sediment (sand). A flow separation occurs at the junction of pier and bed, with the vortex generated causing scour. The scouring vortex is also called the primary vortex.

Effect of Scour Lowering the river bed level around Pier Destabilize the Foundation(Pier)

since 1950 about 500 bridges failed in USA and a majority of them were the result of hydraulic conditions primarily due to the scour of foundation material.

Si Chuan, Yueya City Scour reaches 2.7m after the lower of river bed Unprotected piles can be seen

Gan Su,Dunhuang City Foundation sink result from Flood Scour Damage of bridge cause 600 people trapped

Mechanism of ScourDevelopment scour hole: Vortex system formed in front of the obstruction,and has the form of horseshoe River flow and boundary condition give rise to the energy of the vortex Increased shear stress commence local sediment transport

Mechanism of Scour

Equilibrium : Flow pattern changes in developing causing a reduction in shear stress Gravity,Friction and Shear stress of sediment reach a equilibrium state

Experimental Vortex description

D = Diameter of pier modelDv = Mean vortex size=1/2(a+b)Dc = diameter of caisson pier (m)d = Mean size of sedimenth = Depth of flowN = Vortex rotationReD = Pier Reynolds number = UD/vU = Mean approach velocityUe = Mean approach velocity corresponding to initiationof scourUj = Mean approach velocity corresponding to initiationof sediment motion

Vortex on Rigid falt bed

Vortex in development(soildfied scour hole)

As long as the scour depth is smaller to the initial vortex size, there is no change in the size of the vortex. However, when the vortex is sunk completely into scour hole, it starts expanding with the development of scour.

Velocity and strength of the vortex rise in the initial stage, and fall in the middle Peak value Vortex looses strength after reaching the peak value,equilibrium state reached

Vortex in development(mobile scour hole)

Velocity and strength of horseshoe vortex are related to:River flux,Mean diameter of sediment,Mean approach velocity according to sediment motion.

Scour depth: equilibrium power concept

Power available in the primary vortex

Utilization: Overcome friction due to sediment grains Overcome gravity of grains and transport Simple application of power concept: A higher slope of the scoured surface results from a higher vortex velocityerosion reduces and reaches the minimum at the downstream end of the caisson pier due to energy dissipation of upstream

Scour depth: equilibrium power concept

The power of the primary vortex =(vortex force).(velocity) 𝑉𝑜𝑟𝑡𝑒𝑥 𝑓𝑜𝑟𝑐𝑒:

vortex strength: Velocity: Flux :Substituting Dv, N, and VThe power of the primary vortex

Scour depth: equilibrium power concept

The power to dislodge and transport the sediment

• The number of grains =the surface area of the primary vortex divided by surface area of a single grain

• Therefore

Lift velocity taken as critical velocity that the sediment first begin to transport =

Scour depth: equilibrium power concept

∙ At equilibrium, Pv ≈ PU,therefore

∙ Which gives

Maximum scour depth are related to 1.water depth,2.mean grain size,3.critical velocity corresponding to sediment transport and 4.diameter of caisson pier

Scour depth: equilibrium power concept

Scour depth equation is based on combination of experimental data and theoretical analysis--semi empirical Equ. Test data matches Scour depth expectation.

Three- dimensional Numerical Simulation(CDF)

Governing equations(laminar flow):Continuity equationNavier-Stoke’s equation

Governing equations(turbulent flow):Continuity equationA R Navier-Stoke’s equation

Shearing Stress Calculation Scour shearing stress Critical shearing stress

ShearingModification for bed slope

Simulation ResultsMaximum shearing stress can be found in 75° to symmetric plane(darkest part in graph)Maximum depth reaches 3.5cm approximate to 4cm measured in experiment.

• References for lecture material: The mean characteristics of horseshoe vortex at a cylindrical pier-- MUZZAMMIL, T. GANGADHARIAH Circulation characteristics of horseshoe vortex in scour region around circular piers--Subhasish DAS*, Rajib DAS, Asis MAZUMDAR Vortex scouring process around bridge pier with a caisson--GANGARUDRAIAH VEERAPPADEVARU Flow pattern in the scour hole around a cylinder--BABAEYAN-KOOPAEI, K. Indian practice on estimation of scour around bridge piers—A comment--UMESH C KOTHYARI Three- dimensional Numerical Simulation for Local Scour Around Cylindric Bridge Pier--ZHU Zh-i w en, LIU Zhen-qing

Thank you