March 29, 2012

Vermelding onderdeel organisatie

1

Chapter 8: Dynamic Stability

ct5308 Breakwaters and Closure Dams

H.J. Verhagen

Faculty of Civil Engineering and GeosciencesSection Hydraulic Engineering

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What is dynamic stability ?

• Do not design on “damage”• Try to make the breakwater in such a way that it gets

a “stable” form• Extra material is needed• “Natural” dynamically stable breakwaters seem to exist

on Iceland• However, these breakwaters are not permeable

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Types of berm breakwaters

• Statically stable non-reshaping structuresIn this condition only some few stones are allowed to move similar to a conventional rubble mound breakwater

• Statically stable reshaped structuresIn this condition the profile is allowed to reshape into a profile, which is stable and where the individual stones are also stable

• Dynamically stable reshaped structuresIn this condition the profile is reshaped into a stable profile, but the individual stones may move up and down the slope

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Selection process for rubble mounds

• Is it economical to design an conventional rubble mound ? Can all quarried material be used ?

• If not all material can be used, and Hs < 2, use stable non-reshaping berm breakwater. If 2 < Hs < 3 m this might be a good option in case of dedicated quarry.

• If the stones are too small, use statically reshaped type

• If this also is not possible, use more stone and make dynamically stable berm breakwater

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Types of berm breakwaters

Dynamically stable reshaped BB

• Two stone classes• Homogeneous berm• Wide stone gradation• Low permeability• Reshaping structures• Allowed erosion < berm width• More voluminous• No interlocking

Statically stable non-reshaping BB

• Several stone classes• Berm of size-graded layers• Narrow stone gradation• High permeability• Non-reshaping structures• Allowed recession < 2*Dn50

• Less voluminous• Interlocking prescribed

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Berm breakwaters in the worldCountry Number of berm

breakwaters Year first

breakwater was completed

Iceland Canada USA Australia Brazil Norway Denmark (Far Oer) Iran Portugal (Madeira) China (Hong Kong)

27 5 4 4 2 4 1 8 1 1

1984 1984 1984 1986 1990 1991 1992 1996 1996 1999

Total 57

Data from Pianc report on berm breakwaters 2003

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schematised profile for sand and gravel beaches

500.041c s m nl H T g D=

1.8s r cl l l= =

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Influence of wave climate on a berm breakwater profile

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Berm breakwater Berlevåg (Norway)

Figure from Jacobsen et al, PIANC congress 2002

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Reshaped profile of Berlevågbreakwater

Figure from Jacobsen et al, PIANC congress 2002

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Reshaping calculations

• Van der Meer (1988 - 1990 Breakwat• Van Gent (1995) Odiflocs• Archetti and Lamberti (1996) (See Copedec Cape Town)

• new research by Tørum (1998, 2001)

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Recession according to Tørum (1)

Special parameter for recession: H0T0

50

050

so

n

zn

HHD

gT TD

=Δ

=

Tz is mean period

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Recession according to Tørum (2)

3 2 20 0 0 0 0 0

506

6

( ) ( ) ( ) ( 9.9 23.9 10.5)

2.7 109 100.11

g g hn

Rec A H T B H T C H T f f fd

ABC

−

−

= + + − − + − −

=

==

85 15/ 1.3 1.8g n n gf d d f= < <

50

0.1 3.2hn

hfd

⎛ ⎞=− +⎜ ⎟

⎝ ⎠

gradation factor

depth factor

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Recession according to Tørum (3)

50 50 50

0.2 0.5 12.5 25f

n

h h hford d d

= + < <

Place of the intersection of profiles:

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Longshore transport of stone

• apply same type of formula as longshore sand transport

• to prevent excessive transport, apply

• for heads use a value of 3

• Curve fitted transport formula:

504.5s

n

HD

≤Δ

2

50 50( ) 0.00005 105s

pn n

H gS x TD D

⎛ ⎞= −⎜ ⎟⎜ ⎟Δ⎝ ⎠

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cross section of the Sirevåg breakwater

20-3010-204-101-4

IIIIIIIV

Wmin-Wmax (tonnes)Stone class

€ 20.000/m (2000/2001)

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Design conditions• 100 years return period Hs = 7.0 m, Tp=14.2 s

(based on hindcast + refraction study)• Storm of December 1998:

Hs=7.0 m, Tp=14 s• Storm of February 1999:

Hs=6.7 m, Tp=15 s• Storm of January 2002:

Hs=9.3 m at deep water (450 m offshore)

Hs=7.9, Tp=10 s

•Damage to breakwater: 8 stones removed, 6 stones moved

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Prototype and model

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Stone classes and Quarry yieldSirevåg

1.59

1.36

1.26

1.114

dmx/dmin

19.3%4.02.01-4IV

13.7%2.56.04-10III

9.9%2.013.310-20II

5.6%1.523.320-30I

Expected quarry yield

Wmax/WminWmeanWmin-WmaxStone Class

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Sirevåg yield cure

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Sirevåg breakwater

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Sirevåg breakwater

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Sirevåg breakwater