d. Influence of a Permeable Breakwater on Other Wave Characteristics . 

 Wave energy shifts to higher harmonics are found in the transmitted wave 

 records for monochromatic wave tests, as determined for overtopped impermeable 

 breakwaters (Fig. 41). The energy shift is primarily a function of incident 

 wave steepness and the ratio of the water depth to structure height. The 

 largest shifts of energy to higher harmonics occur for steep waves where the 

 structure crest is near to the Stillwater level (Fig. 41). 



100 



^-x — r- 



D 



o 



^ 



b. 



1 ■ 



■ 



1 



90 



- 







X 

 D 





• 



■ 



80 



- 



dj/h 





o 







A 



• ■ - 





A 



1.29 







1° 





* A 



70 



X 

 o 

 



1.2 1 

 1.13 

 1.04 







o 





XO 4 



O D * 



60 



• 



0.98 

 089 





d/gl^ 



= 0.016 





X 



50 



■ 

 1 



0.83 

 I 



1— 



BW4 



1 1 — 





1 



0.0002 



0.0004 00006 



OOOi 

 H/qT' 



0.002 



0.004 0006 



Figure 41, 



Percent of wave energy at the forcing 

 period for waves transmitted past a 

 permeable breakwater (monochromatic waves) 



In the case of irregular waves the higher frequency parts of the reflected 

 and transmitted spectra tend to be dampened out, so relatively more wave energy 

 is found at lower frequencies than in the incident spectrum (Fig. 42). This 

 means that on the average the spectral peakedness, %, of reflected and 

 transmitted spectra is greater than or equal to the spectral peakedness of 

 incident spectra (Fig. 43). 



A zero up-crossing analysis of wave records shows that on the average the 

 wave height distribution shape is approximately the same for incident and trans- 

 mitted waves for the irregular conditions tested for a permeable breakwater 

 (Fig. 44). 



The amount of wave grouping or the tendancy of large waves to follow large 

 waves and small waves to follow small waves is characterized by the autocorrela- 

 tion of zero up-crossing wave heights, p (see Sec. 111,4). Results from BW16 

 show that the autocorrelation transmitted waves is less than or equal to that 

 for incident waves in the case of irregular waves incident on a permeable break- 

 water (Fig. 45) . 



The joint distribution of transmitted wave heights and periods for an 

 irregular wave condition is similar to that found for smooth impermeable break- 

 waters. There is a tendancy for lower transmitted waves to have average periods 

 less than found in the incident joint height -period distribution (Fig. 46). 

 Both the incident and transmitted larger wave heights have average periods 

 approximately equal to the period of peak energy density. 



53 



