the waves approaching the coast. There is some evidence that the stability
of the armour units is affected by the wave groups, so named because of
the tendency of large waves to travel together.
The thickness, t 1 , of the cover layer consisting of nlayers of armour
units is calculated fromt 1 nKD(Wr/ (^) sg)1/3 (15.33)
where KD is the dimensionless coefficient of the layer. The required
number,N, of units for a given surface area Ais
NAn (^1)
2/3
(15.34)
wherepr(%) is the porosity. Porosity and layer coefficients are given, for
various armour units, in Table 15.4.
The crest width of the breakwater depends on the amount of over-
topping that might occur. It is also dictated by the construction method.
The recommended minimum width is three times the thickness of the
cover layer, i.e.
bmin 3 KD(Wr/ (^) sg)1/3. (15.35)
There are many variations in the size and proportion of materials used in
rubble-mound breakwaters. A breakwater, with the grading of stones used,
is shown in Fig. 15.9(a). The forces exerted on the vertical part of the
breakwater shown in Fig. 15.9(b) may be determined by the methods
described in Section 15.3. The vertical structure of a rubble mound may fail
owing to its sliding or overturning, toe erosion, pressures generated in the
mound, and previously mentioned problems with the rubble mound itself.
In the design of a cover layer, a certain amount of damage of armour
units in the region of wave attack may be allowed for by reducing their
size (and cost). Damage is defined as a percentage of the volume of
(^) sg
Wr
pr
100
Table 15.4 Layer coefficient KDand porosity for various armour units
Armour unit Number of Layer coefficient, KD Porosity, Pr(%)
layers, n
Smooth quarrystone 2 1.02 38
Rough quarrystone 2 1.0 37
Rough quarrystone 3 1.0 40
Tetrapod 2 1.10 50
Tribar 2 1.02 54
dolos 2 0.94 56