The toe of a rubble-mound structure in water shallower than about twice the design-wave 

 height may be subjected to severe scouring currents caused by wave turbulence. It is 

 important that the bedding layer be carried well beyond the toe stones to prevent these 

 currents from scouring sand or other soft bottom material from under them. If the toe 

 stones are dislodged, they may allow the armor units above them to sUde down the slope. 

 For this same reason, a berm of secondary armor stone about two stones wide is usually 

 placed at the toe of the armor units. 



When a rubble-mound breakwater is subjected to massive wave overtopping, the armor 

 units in the crown and back slope of the structure are as much in danger of being dislodged 

 as those in the front slope. Therefore, the crown should be three and preferably four armx)r 

 units wide and the units on the back slope down to about wave height distance below the 

 surface should be as large and as well placed as those on the front slope. In designing a 

 rubble breakwater for overtopping, this fact must be pointed out in the project reports so 

 that there will be no misunderstandings or recriminations if minor damage occurs in later 

 stages of the project. It should be noted that damage to rubble mounds is not usually 

 sudden but progressive, and even a badly damaged structure retains much of its original 

 wave breaking capability and can be easily repaired. Van de Kreeke (1969), and the Shore 

 Protection Manual (U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 

 1973) present data obtained from model studies that can be used to estimate damage to 

 rubble-mound structures under design-exceedence wave attack. 



Timber, steel, and concrete sheet-pile or masonary-block breakwaters, or composite 

 structures made with these materials, differ from rubble -mound structures in that they may 

 fail or be severely damaged by a single wave of more than design proportions. It is essential 

 that rubble-mound breakwaters be designed to withstand either the breaking wave or the 

 highest 1 percent nonbreaking wave, although the wave may overtop the structure by several 

 feet. A wall must be designed for hydrodynamic shock forces if it presents a continuous flat 

 surface to the sea with no pressure-release features, and is in water to a depth where waves 

 break directly on its face. Hydrodynamic shock forces may be highly locaUzed and of 

 extremely short durations, but pressures within the shock zone may exceed by several fold 

 the normal hydrostatic pressures exerted by nonbreaking waves. 



Design for timber, steel, and concrete breakwaters take many forms. The most common 

 in low wave areas is the sheet-pile structure, but its use is limited to soft bottoms into which 

 piles can be easily driven. Sheet-pile structures must be strengthened by wales or a 

 substantial cap, and unless the design wave is small, it must be braced to prevent 

 overturning. A variation of the sheet-pile breakwater is the diaphragm wall supported by 

 king piles (Fig. 19). In hard bottom, the king piles may have to be drilled and grouted in 

 place. 



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