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