A COMPARISON OF HOMOGENEOUS AND MULTI-LAYERED BERM BREAKWATERS WITH RESPECT TO OVERTOPPING AND STABILITY Lykke Andersen, Skals & Burcharth ICCE2008, Hamburg,

A COMPARISON OF HOMOGENEOUS AND MULTI-LAYERED BERM BREAKWATERS WITH RESPECT TO OVERTOPPING AND STABILITY

Lykke Andersen, Skals & Burcharth

ICCE2008, Hamburg, Sep., 20081 of 17

A Comparison of Homogeneous and Multi-Layered Berm Breakwaters with Respect to Overtopping and Stability

ICCE2008, Hamburg, Germany

Lykke Andersen, T., Aalborg University, Denmark

Skals, K. T., Niras, Denmark

Burcharth, H. F., Aalborg University, Denmark




Homogeneous Berm Breakwater Investigations by Lykke Andersen (2006)

Recession(Rec)

Erosion

Deposition

Initial profile

Reshaped profile

hf

Rc

hs

B

-hb

d

R

Gc

hb is negative when the berm level is higher than the still water level.

Berm recession, wave overtopping and wave reflection measured for a large number of cross sections and sea states. In total approximately 700 model tests performed.

Recession and overtopping formulae were also established.




Lykke Andersen (2006) Recession Formula for Homogenous Berm Breakwaters

Geometrical

considerations

Geometrical

considerations

Parameters included:• Influence of stability index and wave period• Influence of vertical wave skewness• Influence of water depth• Influence of front slope• Influence of berm elevation. • Influence of number of waves• Influence of stone gradation




Lykke Andersen (2006) Recession Formula for Homogenous Berm Breakwaters

• Some of the other data sets available show same amount of scatter as observed for the present data.

• Other data sets show much more and in some cases also a bias. This deviation could not be addressed.

90% confidence band:

Lykke Andersen (2006) data

All data (12 additional data sets, mainly homogenous)




Multi-Layer Berm Breakwaters (Icelandic Type)

Not covered in detail in study by Lykke Andersen (2006)

I

II

IIIIV

II

V

III

VI

• In a multi-layer berm breakwater the armour stones are separated in several stone classes.

• The largest stones are placed where they add most to the total stability of the structure.

• On a multi-layer berm breakwater very limited damage is accepted.




Multi-Layer Berm Breakwaters (Icelandic Type)

• The parameter depends on

the stone size.

• Lykke Andersen (2006) proposed to apply the stone size (Dn,50) of the largest class in the berm when using the recession and overtopping formulae.




Lykke Andersen (2006) Recession Formula

Comparison of calculated recession from the Lykke

Andersen (2006) formula and measured recession data from

other researchers.

Includes multi-layer data




Lykke Andersen (2006) Overtopping Formulafor Homogenous Berm Breakwaters

fH0 defined as in the recession formula

Initial profile parameters B and Gc have to be adjusted to match a profile with initial front slopes 1:1.25 as given by Lykke Andersen (2006).




Evaluation of Overtopping Formula – Other Data2 Additional data sets homogenous berm breakwater:

3 Additional data sets multi-layer berm breakwater:

Good agreement for the few other data sets available as well.




New Model Tests Studying Stability and Overtopping of Different Berm Breakwaters Types (Low Crest)

Prototype conditions (model tests at scale 1:50):• Water depth: 20 m

• Tested wave heights (Hm0): 4-11 m

• Deep water peak wave steepness (s0p): 0.02 & 0.04

• Based on a real quarry yield from Keilisnes in Iceland.

• Maximum stone size: 30t

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Un

its P

assi

ng [%

]

W [t]

Layer I

Layer II

Layer III

Layer IV

Layer V

Same stones but study influence of amount of sorting on stability and overtopping!! 5

m

12 m

I

IIIII IV

IVII

III

VI

V

Multilayer Berm and Low Crest

Seeward Landward

10

m

V

1:1.5

1:1.5

1:1.3

1:1.3

1:1.3

1:1.3

1:1.5

I+II+III

IV+V VI

1:1.3

1:1.5

I+II+III

IV+V

IV+V

I+II+III+IV+V VI

1:1.3

1:1.5

I+II+III+IV+V

6.3 m

5 m

12 m

Hybrid Berm and Low Crest

Seeward Landward

10

m 1:1.5

1:1.5

1:1.3

1:1.3

1:1.3

6.3 m

5 m

12 m

Seeward Landward

10

m 1:1.5

1:1.5

1:1.3

1:1.3

1:1.3

6.3 m

Homogeneous Berm and Low Crest




New Model Tests Studying Stability and Overtopping of Different Berm Breakwaters Types (High Crest)

5 m

12 m

I

IIIII IV

IVII

III

VI

V

Multilayer Berm and Low Crest

Seeward Landward

10

m

V

1:1.5

1:1.5

1:1.3

1:1.3

1:1.3

1:1.3

1:1.5

I+II+III

IV+V VI

1:1.3

1:1.5

I+II+III

IV+V

IV+V

I+II+III+IV+V VI

1:1.3

1:1.5

I+II+III+IV+V

6.3 m

5 m

12 m

Hybrid Berm and Low Crest

Seeward Landward

10

m 1:1.5

1:1.5

1:1.3

1:1.3

1:1.3

6.3 m

5 m

12 m

Seeward Landward

10

m 1:1.5

1:1.5

1:1.3

1:1.3

1:1.3

6.3 m

Homogeneous Berm and Low Crest

I

IIIII IV

IVII

III

VI

V

V

1:1.51:1.27

1:1.5

3.5 m

VI

1:1.27

1:1.5

I+II+III+IV+V

I+II+III+IV+V

VI

1:1.27

1:1.5

I+II+III+IV+V

I+II+III+IV+V

5 m

12 m

Seeward Landward1

3.3

m1:1.5

1:1.3

1:1.3

1:1.3

Multi-layer Berm and High Crest

1:1.5

3.5 m

5 m

12 m

Seeward Landward

13.

3 m

1:1.5

1:1.3

1:1.3

1:1.3

Homogeneous Berm and High Crest

1:1.5

3.5 m

5 m

20 m

Seeward Landward

13.

3 m

1:1.5

1:1.3

1:1.3

1:1.3

Wide Homogeneous Berm and High Crest (Add. Volume)




Recession ResultsInfluence of Berm Type and Comparison to Lykke Andersen (2006) formula

• The multi-layer and hybrid berm breakwaters performs quite similar in terms of recession, i.e. the number of stone classes in the berm can be reduced from 5 to 2 without significantly influencing the stability. This is important as it is easier and cheaper to construct.

• The homogenous berm breakwater gives significantly more recession.

• Wave height for which berm recession starts is well predicted by the formula.

• The test results for the homogenous berm breakwater shows a slower development of recession with wave height compared to that given by the formula while the multi-layer and hybrid is well described by the formula.




Recession ResultsComparison with Lykke Andersen (2006) formula

• Recession calculations are very reliable for the tested multi-layer and hybrid berm breakwaters with start of berm recession estimated very well. Note that front side erosion might start before recession of the berm.

• Recession calculations for the homogenous berm breakwaters are unexpectedly somewhat conservative in many cases. Might be due to a small fraction (around 10%) of very large stones which is not described by fg = Dn,85 / Dn,15.

Low Crest High Crest High Crest & Wide Berm




Overtopping ResultsInfluence of Berm Type

• Investigated range of overtopping discharges corresponds to prototype discharges in the range 0.03 l/sm to 100 l/sm.

• Homogenous berm breakwater gives slightly more overtopping than the more stable multi-layered types.




Overtopping ResultsComparison with Lykke Andersen (2006) formula

Low Crest High Crest High Crest & Wide Berm

• Overtopping prediction by the formula seems reliable for all tested cross-sections but with a slightly conservative bias.




Overall Conclusions Related to Application of Lykke Andersen (2006) Design Formulae

• Recession seems well predicted by the Lykke Andersen (2006) formula for multi-layer berm breakwaters.

• Recession for the tested homogenous berm breakwaters shows in many cases less recession than indicated by the Lykke Andersen (2006) formula. However, start of damage is well predicted by the formula. The reason for this deviation was not completely clear but could be due to a small fraction of very large stones included in the berm stones.

• Overtopping is for all cases in good agreement with the formula of Lykke Andersen (2006).




Overall Conclusions Related to Sorting of Armour Stones

What do we get out of sorting the stones in the berm?

• The homogenous berm breakwater gives slightly more overtopping than the more stable multi-layer and hybrid types.

• Recession is significantly smaller for the multi-layered berm breakwaters compared to the homogenous berm breakwater but the failure mode is less ductile.

• It seems as the number of stone classes in the berm can be reduced from 5 to 2 without significantly influencing berm recession and overtopping performance. This means a structure that is significant easier to construct and cheaper.

Thank you for your attention

Documents

A COMPARISON OF HOMOGENEOUS AND MULTI-LAYERED BERM BREAKWATERS WITH RESPECT TO OVERTOPPING AND STABILITY Lykke Andersen, Skals & Burcharth ICCE2008, Hamburg,