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 



28 Chapter 4 Risk Considerations 



