(c) Site-specific laboratory scale model studies are recommended, 

 when feasible, to finalize design. The advantages of a model study are 

 that structural stability, wave transmission, and reflection can all be 

 examined in a single series of model tests (Hudson et al., 1979). 



f. Ponding of Water Landward of Breakwaters . Wave transmission of break- 

 waters causes water to build up landward of breakwaters. If a breakwater 

 completely encloses an area, the resulting ponding level can be estimated from 

 Figure 7-57. Note that, for the special case of F = , ponding level is a 

 weak function of deepwater steepness (Fig. 7-58). Irregular waves have lower 

 ponding levels than swell because of reduced overtopping and seaward flow that 

 occurs during the relatively calm intervals between wave groups. 



If gaps or a harbor entrance are present, the ponding level will be lower 

 than given in these figures due to a new seaward flow through the gaps. A 

 method of predicting this flow rate is given in Seelig and Walton (1980). 



g. Diffraction of Wave Spectra . The diffraction of monochromatic waves 

 around semi-infinite breakwaters and through breakwater gaps of various widths 

 is made up of numerous waves having various frequencies, each propagating 

 within a range of directions. Goda, Takayama, and Suzuki (1978) have 

 calculated diffraction diagrams for the propagation of irregular, directional 

 waves past a semi-infinite breakwater and through breakwater gaps of various 

 widths. The diagrams are based on the frequency-by-frequency diffraction of a 

 Mitsuyasu-modif ied Bretschneider spectrum (Bretschneider, 1968; Mitsuyasu, 

 1968). The results, however, are not very sensitive to spectral shape; 

 therefore, they probably also pertain to a JONSWAP spectrum. The results are 

 sensitive to the amount of directional spreading of the spectrum. This 

 spreading can be characterized by a parameter, S x • Small values of 

 S indicate a large amount of directional spreading, while large values of 

 S ^ indicate more nearly unidirectional waves. For wind waves within the 

 generating area (a large amount of directional spreading), S y = 10 ; for 

 swell with short to intermediate decay distances, S x ~ 25 ; and for swell 

 with long decay distances (nearly unidirectional waves) , S„„„ = 75 . The 

 amount of directional spreading for various values of S v is shown in 

 Figure 7-59. The value of S x > o'^ equivalently the amount of directional 

 spreading, will be modified by refraction. The amount that S x ^^ changed 

 by refraction depends on its deepwater value and on the deepwater direction of 

 wave propagation relative to the shoreline. For refraction by straight, 

 parallel bottom contours, the change in S x is given in Figure 7-60 as a 

 function of d/L for deepwater waves making angles of 0, 30, and 60 degrees 

 with the shoreline. 



The diffraction of waves approaching perpendicular to a semi-infinite 

 breakwater is shown in Figures 7-61a and 7-61b for values of S x ~ 10 ^^^ 

 S = 75 , respectively. In addition to diffraction coefficient contours, 

 the figures show contours of equal wave period ratio. For irregular wave 

 diffraction there is a shift in the period (or frequency) of maximum energy 

 density (the period or frequency associated with the peak of the spectrum) 

 since different frequencies have different diffraction coefficients at a fixed 

 point behind the breakwater. Thus, in contrast to monochromatic waves, there 

 will be a change in the characteristic or peak period. 



7-89 



