made of large quarry-stones or concrete prefabricated units such as
tetrapods, cobs and dolos (Fig. 15.10), the latter specially formed to reduce
wave reflection and produce a good degree of interlocking.15.6.2 Stability of breakwaters(a) Shore Protection Manual formulae
Hudson (1961) presented a simpler formula for determining the stability
of the rubble-mound breakwater. It gives the required weight of the
armour units in the cover layer in the formWr (15.32)whereWris the weight of the individual armour unit, (^) sis the density of
the armour units, is the inclination of the structure to the horizontal and
KDis a dimensionless coefficient. Typically, breakwater slopes on the
seaward side vary from 1 in 1.5 to 1 in 3 (V:H). Hudson’s empirical
formula is applicable for slopes of the cover layer ranging from 1 in 1.5 to
1 in 5. The coefficients KDas recommended by the Shore Protection
Manual(US Army, 1984), are given in Table 15.3 for various armour units.
For the armour units used at the structure head, KDis found to depend on
the slope of the cover layer. The use of a single quarrystone cover layer is
not recommended unless special precautions are taken in placing. Of the
specially formed units, tetrapods have probably been the most popular.
From Table 15.3, it is clear that large values of KDare associated with the
specially formed units.
It should be noted that Hudson’s equation does not take account of
the wave period, the oblique approach of the waves, or the irregularity of
(^) sgH^3
KD( (^) s/ 1)^3 cot
Fig. 15.10 Examples of specially formed armour units (Chadwick, Morfett
and Borthwick, 2004)