Conventional breakwaters 



Conventional nibble-mound breakwaters (Figure 46) have been designed 

 for "zero" damage (less than 5 percent structural damage) under design wave 

 conditions. In the case of offshore breakwaters, this usually means specifying 

 the crest elevation such that little to no overtopping occurs, since the volume 

 of water overtopping the crest has been found to be an important parameter in 

 determining rear slope stability (Sheppard and Hearn 1989). Zero damage 

 and minimal overtopping are two assumptions incorporated into the Hudson 

 stability formula (Shore Protection Manual 1984), 



W = 



wH 3 



K D (S, - l) 3 cote 



(20) 



where W is the weight of the individual armor unit; w r is the unit weight of 

 the armor unit; H is the design wave height; K D is the stability coefficient; S r 

 is the specific gravity of the armor unit; and $ is the angle of structure slope 

 measured from horizontal. 



/-CREST WIDTH 

 BREAKWATER CREST-^ f- — "1 





MAX DESIGN SWL— ^ ^^^w ^^^^ 



SWL (MINIMUM)-^ ^^0^<^^ W//10 ^^^\^^ 



2r^r-y l — ^^l^>^W/200 TO W/400"^ ^^ 



\SWL (MINIMUM) 



_-— \ 





Figure 46. Cross section for conventional rubble-mound breakwater with 

 moderate overtopping {Shore Protection Manual 1984) 



The Hudson formula has been used extensively in the United States for 

 breakwater design. However, apparent shortcomings of Hudson's formula, 

 including lack of influence of wave period and the fact that it is based on 

 regular wave tests, have been the subject of much discussion in recent years. 

 Additional research aimed at such concerns has been conducted by a few 

 investigators (Van der Meer 1987, Carver and Wright 1992). 



Van der Meer (1987) derived two stability equations, one for plunging 

 (breaking) waves and one for surging (nonbreaking) waves. These equations 

 are as follows: 



Chapter 4 Structural Design Guidance 



81 



