Local Scour Pile

8/3/2019 Local Scour Pile

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Influence of flow velocityU U cr / .< 0 5

0 5 1. / < <U U cr

U U cr / .> 1 0

Clear water scour:

Live-bed scour:

No scour

ds

DP



Formulae for Design

• Difficult to propose a scour formula fordesign (lacking of field data and theoretical

developments)

• Based on the current data, the scour depth

for a single pile, positioned in a wide

channel, can be estimated using thefollowing relationships



For clear-water scour (U/Ucr < 1, by Shen 1971):

619.0

00022.0

=

ν

p

s

UDd

For sediment-transport and clear-water scour (by Breusers et al.1977):

α ξ ξ s pcr p

s

D

h

U

U g

D

d

= tanh0.2

>=

<<

−=

<=

11

15.012

5.00

cr

cr cr

cr

cr

U

U

U

U

U

U

U

U

U

U g

(7.2)

(7.3)



For sediment-transport (live-bed) scour by Raudkivi (1991):

α ξ ξ ξ sg

p

s

D

d 0.2=

Valid for h/Dp > 1.5 and (Dp/d50) > 25

• All above equations can only be used as a guideline estimate.

Where

Ds is the scour depth

Dp is the diameter of the pile

ζg is the gradation factor

ζs is the shape factor

ζα

is the alignment factor

h is the water depth

(7.4)



Influence of water depth



Influence of sediments

( )σ g d d =84 16

/

Size Gradation



Influence of pier shape andalignment

Shape Alignment



Influence of two piles



Influence of pile height

)55.0exp(10 D

hSS −−=

Where h is the pile height and S0 is the scour depth for a long pile.

(7.5)



Summary

• Flow mechanisms of local scour around avertical pile

– Horse-shoe vortex in front of the pile

– Streamline contractions at pile shoulders– Vortex shedding in the wake of the pile

• Types of scour

– Clear water scour

– Live-bed scour



Summary of continued

• Estimates of maximum scour depth – designformulae

• Correction for pile height – for submerged

piles



Example 7.2

Estimate scour depth around submerged vertical pile anchor. The

diameter of the pile is 3 m and the height of the pile (above the

seabed) is 2 m. The local water depth is 20 m. The sediments found

at the site have d50 = 0.2 mm and density of ρs = 2720 kg/m

3

. Thedensity of sea water ρ = 1027 kg/m3 and viscosity of ν = 1.36×10-6

m2 /s. The metocean data give steady currents of 0.54 m/s at 1m

above the seabed.



Scour around a vertical pile in waves

• Vortex shedding around the pile is adominant flow mechanism inducing the scour

• Keulegan Carpenter number (KC = UwT/D)

is a key parameter



Design formula for scour depth

{ })]6(03.0exp[13.1 −−−= KC D

S

For KC > 6

(7.6)



Scour depth in combined waves andcurrents



Time development

• Laboratory experiments indicated that time-dependent scour

around a single pile subject to steady currents and waves can berepresented by the following function:

) / exp(10

T t S

S−−=

Where

S0

is the equilibrium scour depth

T is the time scale

t is the time

(7.7)



Time Scale for Scour

• Dimensional analysis shows that time scale T depends on the

following parameters:

[ ]*

2 / 13

50

2

)1(T

d sg

DT

−=

2.2*

2000

1 −= θ δ

DT

3

6* 10

= −

θ

KC T

For currents

Where δ is the boundary layer thickness, θ is the Shield’s parameter

For waves

(7.8)

(7.9)

(7.10)



Example 7.3

How long it will take for scour around a pile of 1 m in diameter to

reach 80% of its equilibrium scour depth:

(a) The pile subjects to a steady current of 0.6 m/s (depth-averaged)

(b) The pile subject to a wave of 2.5 m in height and 10s in period.

The water depth is 10 m.

Sediment median diameter is 0.2 mm.



Scour protection in currents



Scour protection in waves

• Not much work been reported in this area• The stone size can be determined from the

Shields criterion for the surface layer of the

stone protection• The extent of the protection can be roughly

estimated as the extent of scour hole (use

the scour depth and internal friction angle of the sediment)



Example 7.4:

Consider a 1 m pile exposed to waves of a height = 2.5 m and aperiod 10 s in 10 m water. Predict the scour depth if the sediment

size is 0.2 mm.

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Local Scour Pile