to obtain good wedging or interlocking action between individual units, 

 or they may be placed at random. Present technology does not provide 

 guidance to detennine the forces required to displace individual armor 

 linits from the cover layer. Armor units may be displaced either over a 

 large area of the cover layer sliding down the slope en masse, or indivi- 

 dual armor units may be lifted and rolled either up or down the slope. 

 Empirical methods have been developed that, if used with care, will give 

 a satisfactory determination of the stability characteristics of these 

 structures, when under attack by storm waves. 



A series of basic decisions must be made in designing a rubble struc- 

 ture. Those decisions are discussed in succeeding sections. 



7o372 Design Factors . A primary factor influencing wave conditions at 

 a structure site is the bathymetry in the general vicinity of the struc- 

 ture. Depths will partly determine whether a structure is subjected to 

 breaking, nonbreaking, or broken waves for a particular design wave con- 

 dition. (See Section 7.1, WAVE CHARACTERISTICS.) 



Variation in water depth along the structure axis must also be con- 

 sidered as it affects wave conditions, being more critical where breaking 

 waves occur than where the depth may allow only nonbreaking waves or waves 

 that overtop the structure. 



When waves impinge on rubble structures, they may: 



(a) break completely, projecting a jet of water roughly perpen- 

 dicular to the slope, 



(b) partially break with a poorly defined jet, or 



(c) establish an oscillatory motion of the water particles up 

 or down the structure slope, similar to the motion of a clapotis at a 

 vertical wall. 



The design wave for a rubble structure is usually the significant 

 wave. Damage from waves higher than the significant wave is progressive, 

 but the displacement of several individual armor units will not neces- 

 sarily result in the complete loss of protection. A logic diagram for 

 the evaluation of the marine envrionment is presented in Figure 7-6, and 

 summarizes factors involved in selecting the design water depth and wave 

 conditions to be used in the analysis of a rubble structure. 



7,373 Hydraulics of Cover Layer Design . Until about 1930, design of 

 rubble structures was based only on experience and general knowledge of 

 site conditions. Empirical formulas subsequently developed are generally 

 expressed in terms of the stone weight required to withstand design wave 

 conditions. These formulas have been partially substantiated in model 

 studies. They are guides, and must be used with experience and engineer- 

 ing judgment. 



7-168 



