Discussion of Irregular Wave Tests 



197. Movable-bed model tests using irregular wave conditions showed better success at reproducing 

 regular wave results when the significant wave height (i/1/3) of the irregular waves had nearly the same 

 value as the monochromatic wave height. This was well demonstrated by the comparison shown in 

 Figure 26. Incresising the energy level of the irregular waves by 41 percent resulted in excessive erosion 

 relative to the base case T03 (Figure 27). 



198. Although irregular waves with significant wave height equal to the regular wave height contain 

 approximately 30-percent less total energy than their regular wave counterpart, the two conditions are 

 similar in terms of the waves that move the sediment. In the irregular wave case, the waves in the 

 distribution larger than the significant wave height are expected to move more sediment than what would 

 be moved by waves in the monochromatic case, whereas waves in the distribution less than the significant 

 height should move less sediment than in the monochromatic case. Irregular waves much smaller than the 

 regular wave height might be expected to have minor effects on the sediment transport. 



199. The fact that significant wave height emerged as a good parameter for reproducing observed 

 regular-wave tests indicates that the higher 1/3 waves in the distribution are the most important for 

 sediment transport. Experiments conducted with equivalent energy levels (//^i/a is 1.4 times greater than 

 Hmono) contain a proportionally higher number of waves greater than the monochromatic wave height and, 

 thus, should cause significantly more erosion of the nearshore profile. Therefore, it is believed that 

 maintaining equivalent energy levels between regular and irregular waves is not proper guidance for the 

 situation of beach profile development due to cross-shore sediment transport. 



200. For purposes of analysis, assume for the moment that the only waves in the irregular wave height 

 distribution that contribute to net sediment transport are confined to the highest 1/3 waves; waves with 

 lower heights are present, but have no appreciable effect. Under this assumption, it might be expected that 

 profile development in the irregular wave case should take approximately three times as long as the regular 

 wave counterpart. Three times as many irregular waves would need to impinge on the beach to produce the 

 number of irregular waves in the highest-1/3 category equal to the number of regular waves required for 

 the same profile development. However, it is quite unreasonable to assume that all waves smaller than the 

 highest 1/3 waves make no contribution whatsoever to sediment transport. It is more likely that profile 

 development under regular waves is somewhere between one to three times more rapid than the irregular 

 wave case with some smaller waves actually having an accretionary effect as discussed by Mimura, Otsuka, 

 and Watanabe (1986). 



201. Figure 29 shows time-shifted comparisons between the irregular wave test T09 and its regular 

 wave equivalent, test T03. In Figure 29, regular wave profiles (dashed) are compared with profiles that 

 took approximately twice as long to develop in the irregular wave test (solid). Generally, a slightly better 



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