Local Scour Pile
Transcript of 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
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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
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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)
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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)
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Influence of water depth
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Influence of sediments
( )σ g d d =84 16
/
Size Gradation
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Influence of pier shape andalignment
Shape Alignment
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Influence of two piles
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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)
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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
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Summary of continued
• Estimates of maximum scour depth – designformulae
• Correction for pile height – for submerged
piles
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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.
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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
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Design formula for scour depth
{ })]6(03.0exp[13.1 −−−= KC D
S
For KC > 6
(7.6)
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Scour depth in combined waves andcurrents
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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)
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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)
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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.
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Scour protection in currents
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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)
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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.