some specified exceedance value, but it must be acknowledged that both height 

 and period will vary in any storm. Consequently, some incident waves will be 

 breaking on the structure, and others will not. The succession of high and 

 low waves (wave groupiness) and of breaking and nonbreaking waves can be a 

 critical factor. The potential effects of wave groupiness or multiple con- 

 verging wave trains (multi-peaked spectra) are difficult to assess without a 

 substantial amount of field data and scale model testing with irregular waves. 



27. The alternative stability coefficients for the Hudson and Iribarren 

 formulae discussed above which include wave steepness H/L provide one means 

 of making a more explicit description of incident wave conditions. Other de- 

 scriptive parameters that have been investigated include the surf similarity 

 parameter in Equation 4 (Bruun and Gunbak 1978, Burcharth 1979, Losada and 

 Gimenez-Curto 1980, Van der Meer and Pilarczyk 1984, and Bruun 1985) and the 

 Stokes or Ursell parameter HL /h3 (Carver 1983). Estimated values of these 

 wave form parameters can be used as a more systematic means of classifying 

 individual waves as breaking in the critical plunging mode, as spilling, or as 

 nonbreaking. Irregular sea states require further definition in terms of ei- 

 ther time domain characteristics or spectral (frequency domain) parameters. A 

 number of useful parameters for characterizing irregular waves are discussed 

 by Rye (1977). 



28. Wave transformation effects caused by the proposed construction 

 works themselves cannot be neglected. Breakwaters with shallow slopes or with 

 extensive toe development can also change the wave conditions at the waterline 

 on the seaward face by "tripping" the waves. Scale-model tests are necessary 

 to quantify these effects on the armor layer (Jackson 1968b). Relatively 

 steep and impermeable structures may partially reflect incident waves such 

 that resonance of incident and reflected waves causes scour near the toe. De- 

 termination of the sensitivity of a structure to these effects from oblique 

 waves requires scale-model testing in a three-dimensional wave basin. These 

 potential problems make physical modeling critical for reliable estimation of 

 the stability of a proposed rubble-mound breakwater. 



29. The duration of a storm at sea is a real world parameter that 

 should be considered in any design effort or laboratory stability analysis. 

 Figure 5 illustrates the time-history of significant wave height, peak spec- 

 tral wave period, and predominant direction of wave propagation for a storm 

 in the Gulf of Alaska simulated from synoptic weather data at 6-hr intervals. 



19 



