4
Risk Considerations
Wave Conditions
The sponsor-predicted return period of the higher waves generated on the
model was 50 years, i.e., the storm event is predicted to occur only once every
50 years. Experiments conducted on the model showed that the design waves
were depth-limited and broke seaward of the structure. Therefore, it was con-
cluded that waves exceeding the design condition would also break seaward of
the structure and would not increase damage.
The most severe storms approach the site from the northwest and stability
experiments were performed only from this direction. However, the structure
also is subjected to southern swell; therefore, the entire structure, including the
head section, should use 11-tonne armor units and the same toe protection
scheme as the breakwater trunk.
Armor Unit Selection
The Core-Loc, a recently developed armor unit (Melby and Turk 1995), was
selected for use in the armor layer for the rehabilitation. Construction of the
Port Saint Francis breakwater in South Africa and experiments by Smith and
Hennington (1995), and Smith (1996) have shown the Core-Loc is reliable and
an improvement to the Accropode, which is an armor unit that has been used
extensively and successfully for 20 years worldwide. Features of the Core-Loc
included improved stability to the Accropode by increasing the porosity of the
armor layer; no tendency for units to rock on slope; reserve stability for wave
conditions exceeding the design event; hydraulic stability when placed as a
repair with other armor shapes (in Vale de Cavaleiros, some tetrapod units will
remain in the northern part of the jetty); and low internal stresses. The Core-Loc
is presently the most efficient armor unit for rubble-mound breakwaters, because
of these properties
Displaced armor units were counted after each wave series to assess the
reliability of breakwater protection. Several units were displaced during the
initial plans, but only one Core-Loc was displaced over the entire breakwater for
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Chapter 4 Risk Considerations