Overtopping and wave reflection
Due to the high porosity of the Xbloc armour layer (up to 63% voids), the wave overtopping and wave reflection are minimised.
The wave overtopping and wave reflection of Xbloc armour has been studied extensively using physical model tests. For the European Union CLASH program several different types of armour units have been model tested in the same model set up at the University of Edinburgh.
The CLASH model test results show that single layer Xbloc armour gives low wave reflection and overtopping compared to other breakwater armour.
Wave overtopping
In the table below the roughness factor γf is listed for a slope of 1:1.5. A smooth slope has a roughness factor of 1. The lower the roughness factor the lower the overtopping. The rougness factors have been derived from the CLASH studies and were published in Coastal Engineering 56 (2009) by T.Bruce et al.
| Type of armour | no. of layers |
γf |
| Smooth slope | - | 1 |
| Antifer cubes | 2 | 0.5 |
| Cube | 1 | 0.49 |
| Cube | 2 | 0.47 |
| Haro | 2 | 0.47 |
| Accropode® | 1 | 0.46 |
| Core-Loc® | 1 | 0.44 |
| Xbloc® | 1 | 0.44 |
| Dolosse | 2 | 0.43 |
| Tetrapode | 2 | 0.38 |
Note: Accropode® is a registered trademark of Sogreah, Core-loc® is a registered trademark of the US Government
Wave reflection
Zanuttigh and Van der Meer (2006) derived a new formula for the wave reflection coefficient:
Further explanation on the formula can be found by following the link to the Zanuttigh and Van der Meer paper below.
Coefficient a was derived by fitting the formula on the the Edingburgh CLASH dataset for different armour units and is given in the table below; for each unit the overtopping roughness γf is also included in the table; b is constant and equal to 0.87. The armour slope, if not mentioned is 1:1.5. The table is ranked on the value of a. The lower a, the less wave reflection occurs.
|
Armour unit |
no of layers |
γf |
a |
|
Cube |
1 |
0.5 |
0.120 |
|
Cube flat |
2 |
0.47 |
0.120 |
|
Antifer |
2 |
0.47 |
0.115 |
|
Accropode® |
1 |
0.46 |
0.115 |
|
Core-Loc® |
1 |
0.44 |
0.113 |
|
Xbloc® |
1 |
0.44 |
0.112 |
|
Cube rough (1:1.5) |
2 |
0.47 |
0.108 |
|
Cube rough (1:2) |
2 |
0.47 |
0.105 |
|
Tetrapode |
2 |
0.38 |
0.102 |
Results of the CLASH tests have been published on the following web sites:
www.itv.is/ics2005/Data/B1.3/Pearson_pa.pdf
www.vandermeerconsulting.nl/downloads/functional_c/2006_zanuttigh_vandermeer.pdf