ICCE2008, Hamburg, Germany Lykke Andersen, T., Aalborg University, Denmark

1. ICCE2008, Hamburg, Germany Lykke Andersen, T., Aalborg University, Denmark Skals, K. T., Niras, Denmark Burcharth, H. F., Aalborg University, Denmark A Comparison of Homogeneous and Multi-Layered Berm Breakwaters with Respect to Overtopping and Stability

2. Homogeneous Berm Breakwater Investigations by Lykke Andersen (2006) • 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.

3. 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

4. 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)

5. Not covered in detail in study by Lykke Andersen (2006) Multi-Layer Berm Breakwaters (Icelandic Type) • 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.

6. 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. Multi-Layer Berm Breakwaters (Icelandic Type)

7. 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

8. 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).

9. 2 Additional data sets homogenous berm breakwater: 3 Additional data sets multi-layer berm breakwater: Evaluation of Overtopping Formula – Other Data Good agreement for the few other data sets available as well.

10. New Model Tests Studying Stability and Overtopping of Different Berm Breakwaters Types (Low Crest) Same stones but study influence of amount of sorting on stability and overtopping!! • 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

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

12. 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.

13. Recession ResultsComparison with Lykke Andersen (2006) formula Low Crest High Crest High Crest & Wide Berm • 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.

14. 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.

15. Overtopping ResultsComparison with Lykke Andersen (2006) formula LowCrestHighCrestHighCrest & Wide Berm • Overtopping prediction by the formula seems reliable for all tested cross-sections but with a slightly conservative bias.

16. 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).

17. 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