April 8, 2023
Vermelding onderdeel organisatie
1
Chapter 15: Failure modes and optimisation
ct5308 Breakwaters and Closure Dams
H.J. Verhagen
Faculty of Civil Engineering and GeosciencesSection Hydraulic Engineering
Sri Lanka, Kudawella Tsunami damage of breakwater
2004
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What is the most important element of a breakwater or closure dam ??• the element which is the most expensive one• the section which is the most costly one• the element which is the most unreliable one• the element which is the most sensitive to
variations in the boundary conditions
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failure modes for dike-type structures
Failure of breakwater by earthquake
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failure modes for a rubble mound (Burcharth, 1992)
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Failure modes for a monolithic breakwater
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rock fill overflow dam failure modes
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fault tree
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fault tree for closure dam (cross section)
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fault tree for closure dam (equipment)
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fault tree for construction planning
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equipment utilisation in relation to fault tree
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The Dilemma
• A strong and heavy breakwater does not require maintenance … but is very expensive to construct
• A light breakwater is much cheaper to construct … but requires a lot of maintenance
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Wave climate
wave exceedance
0
2
4
6
8
10
0.00010.0010.010.11
exceedance (times per year)
wa
ve h
eig
ht
(m)
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development of damage
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Cost of construction
Initial cost for armour units: € 1320 * Hd
for core: € 8620
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annual risk
1 < H < 1.3 Hnd
n = 4% damage
1.3 Hnd < H < 1.45 Hnd
n = 8% damage
H > 1.45 Hnd
Collapse
Hnd
p w p.w p w p.w p w p.w
(m) (1/year) (€) (€/year) (1/year) (€) (€/year) (1/year) (€) (€/year)
4 1.02 420 430 4.6 10-2 860 40 3.8 10-2 13900 530
5 1.5 10-1 530 80 4.7 10-3 1060 5 2.6 10-3 15220 40
5.5 7.4 10-2 580 40 2.2 10-3 1160 - 8 10-4 15900 10
6 2.4 10-2 630 15 7.5 10-4 1260 - 1.5 10-4 16540 3
p probability of occurrence of the wave heightw cost of repair of the armour layer (2nA)
respectively cost of replacement (C)
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average annual risk
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capitalised maintenance cost
lifetime of 100 years; rate of interest 3.33%
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total cost
Adding up initial cost plus capitalised maintenance cost
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total cost for various strategies
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
3.5 4 4.5 5 5.5 6 6.5
H-design
cost
full repair
partly repair
no repair
initial cost
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Conclusions for Rubble Mound Breakwaters• There is an optimum design wave height• Accepting regular maintenance is the best
option• This implies that the design also should allow a
“repairable” breakwater
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differences in breakwater type
• In case of overload a Rubble mound breakwater will suffer from some damage, which can be repaired.
• In general for Rubble mounds:the amount of repair costs increase linear with the amount of overload:
cost = B * (Hstorm - Hdesign)
• In general for Vertical wall breakwaters:you have always a given fixed amount of damage, not depending on the amount of overload:
cost = A + B (Hstorm - Hdesign)
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Vertical wall breakwater
0
50000100000
150000200000
250000
300000350000
400000
5 7 9 11
Hs (design wave)
tota
l co
st p
er m
eter
building cost
yearly repair
capitalised repair
total
damage cost = 4000 + 1500*Hs
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Conclusions
0
10000
20000
30000
40000
50000
5 7 9 11
Hs (design wave)
tota
l co
st p
er m
eter
0
10000
20000
30000
40000
50000
5 7 9 11
Hs (design wave)
tota
l co
st p
er m
eter
Rubble mound breakwater Vertical wall breakwater
total
repair
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Conclusions (2)
• Rubble mound breakwaters are less sensitive to uncertainties in wave data
• If there is no overload, a Vertical wall breakwater requires less maintenance
• If there is overload, Vertical wall breakwaters cause much more problems
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Including “secondary damage”
• When a breakwater is damaged, the port cannot function well
• The cost because of loss of production should be included in the calculation
• In general secondary damage will not change the tendency of the conclusion before, but make them more even more stronger:• The optimum for a
rubble mound breakwater is allowing quite some damage, and doing a lot of repair
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