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February 1, 2012

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Chapter 9: Stability of monolithic breakwaters

ct5308 Breakwaters and Closure Dams

H.J. Verhagen

Faculty of Civil Engineering and Geosciences Section Hydraulic Engineering

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important aspects of caisson walls

• Quasi static load

• Impact forces

• sliding

• turning

• placing on a mound

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historical development of breakwater in Japan

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historical development of breakwater in Japan (2)

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historical development of breakwater in Japan (3)

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schematic pressure distribution for non-breaking waves

1

1

22 cosh

igHrp

hL

wd = gh

??

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load and equilibrium diagram

o

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example of a pressure record

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changes to incoming wave front induced by high mound breakwater

H=h

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Risk of local impact forces

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Brighton Marina, UK

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Parapet damage

Damage due to typhoon Tokage (Oct 2004) Muroto Kochi, Japan

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Sketch of upright breakwater for stability analysis

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wave pressure distribution

Hiroi

Sainflou

Minikin

p = 1.5 w0H

01 02

0

02

20

cosh

/ coth

Hp p w h

h H

w Hp

kh

H L kh

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Design wave for Goda’s method

• Wave height complicated method (formula of a magician)

• Wave period Tmax = T1/3

• Angle of incidence Assume an approach of 15 degrees to the normal

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Design wave height according to Goda

1/ 3 0

max * ' * * '0 0 1 max 0 1/ 3 0

1.8 : / 0.2

min , ,1.8 : / 0.2

H h L

HH h H H h L

'0 0

1/ 3 ' ' '0 0 1 max 0 0 0

: / 0.2

min , , : / 0.2

s

s

K H h L

HH h H K H h L

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Goda’s coefficients

0.38'

1.500

0

1

0.29'0

max0

0.028 exp 20 tan

0.52exp 4.2 tan

max 0.92,0.32 exp 2.4 tan

HL

HL

0.38'

* 1.500

0

*1

0.29'

* 0max

0

0.052 exp 20 tan

0.63exp 3.8tan

max 1.65,0.53 exp 2.4 tan

HL

HL

is the

inclination of the

seabed

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Nonlinear shoaling coefficient

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surface elevation

rise of the water above the caisson:

* = 0.75 (1+cos) Hmax

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wave pressure with Goda’s formula

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pressure values according to Goda

21 1 2 0 max

12

3 3 1

0.5(1 cos )( cos )

cosh

p w H

pp

kh

p p

2

1

2

max2

max

3

20.6 0.5

sinh 2

2min ,

3

' 11 1

cosh

b

b

kh

kh

h d H d

h d H

h

h kh

hb is waterdepth at 5 H1/3

seaward from breakwater

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uplift pressure

1 3 0 max0.5(1 cos )up w H

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safety factors

against sliding (W-U)/P >1.2

against overturning (Wt-MU)/MP >1.2

MP moment of total wave pressure around the

heel

MU moment of total uplift pressure around the heel

P total thrust of wave pressure

t horizontal distance between centre of gravity

and the heel

U total uplift pressure

W weight of the unit

coefficient of friction (order of 0.6)

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Example

• “runup” = 17.2 m

• safety factors “just sufficient”

H0 = 7 m

T1/3= 11 s

= 10 º

h = 18 m hcc=11.5 m

hc = 4.5 m

seabed slope 1:50

Wc

=

FH

examples: H 7-15; Fslide; T(6) 10-15

H 7-15; Fslide; hcc(6) 10-15

H 7-15; Fslide; h(6) 15-20

H 7-15; Fslide; wc(6) 15-25

H 7-15; Foverturn; wc(6) 15-25

d=10 m

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Damage due to typhoon Togage (Oct 2004)

Susami Wakayama

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Overflow over a caisson (1)

3 4.30.2 exp b

ss

Fq gH

H

formula of Van der Meer and Franco

Fb freeboard

parapet shape factor

0.7 normal parapet

1.0 perforated caisson

1.15 is maximum value

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Overflow over a caisson (2)

TAW-report “waterkerende kunstwerken”

based on research at HR Wallingford

qgem = 5.013 10-3 kwkagHsTzexp(-20.12 kb Fb/(Tz(gHs))

kw wind effect surcharge

ka,kb correction for oblique waves

Fb freeboard

example:

Hs = 1.8 m; Tz = 3 s; Vwind = 15 m/s

Fb 0.25 - 2; q; Ftype(3)1-3

1= Van der Meer & Franco

2= Wallingford/TAW

3= Van der Waal, Delft Hydraulics

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Overflow over a vertical wall

North Wirral Coast, near Liverpool

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Hanstholm caisson

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Jarlan Caisson

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Cylindrical breakwater, Nagashima

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breakwater with wave power generating system

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semi-circular caisson (for extremely high breakers)

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The Miyazaki breakwater

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Curved slit breakwater Funakawa Port

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Composite, curved breakwater

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Honey comb wall breakwater

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erosion pattern depending on the grain size

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Do not forget the filter layers ct4310