b. Influence of wave groupiness in natural irregular seas (Bur- 

 charth 1979) 



c. Effect of the foreshore or the breakwater toe on wave transfor- 

 mation (Bruun 1979 and Kjelstrup 1979) 



d. Effect of oblique waves (Losada and Gimenez-Curto 1982 and 

 Christensen et al. 1984). 



e. Interaction of waves with monolithic crest elements or densely 

 packed underlayers, such as resonance of reflected waves with 

 incident waves (Jensen 1983). 



f. Friction of outer armor material with underlayers (Hedges 1984) 



g. Mechanical strength (resistance to tension, compression, im- 

 pact, fatigue, etc.) of individual armor units (Poole et al . 

 1984 and Groeneveld, Mol, and Zwelsloot 1983). 



h. Potential settlement, foundation failure, and related geotech- 

 nical problems (Thorpe 1984). 



i. Seismic stability. 



17. The Hudson formula can be applied to interpret scale tests of pro- 

 posed designs to measure the "actual" K^^ of an armor unit in a particular 

 breakwater configuration, in which case many of the above factors would be ad- 

 dressed. A series of successive tests on the same configuration with varying 

 monochromatic wave period can determine the critical period when waves of that 

 height would break directly on the face of the armor slope. Likewise, this 

 "sensitivity analysis" approach (vary one parameter while holding others con- 

 stant) can provide estimates for the reliability of the point of incipient 

 damage and damage rates for more severe wave height and period combinations. 

 Tests with irregular waves are also possible and should be considered, even 

 though the procedures involved and interpretation of results in terms of Hud- 

 son formula parameters are less standardized. Physical modeling is an essen- 

 tial step in the cost-effective design of rubble-mound breakwaters and should 

 not be neglected for any except the smallest, most inconsequential structures 

 (Paape and Ligteringen 1980). Some specific techniques for verifying armor 

 stability and damage rates by scale model testing will be discussed later in 

 this report. 



Alternative Stability Relations 



18. The Iribarren formula, as mentioned earlier, has recently been re- 

 ceiving renewed attention worldwide because of some spectacular failures of 



13 



