breakwaters and closure dams 8
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April 12, 2012
Vermelding onderdeel organisatie
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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