R 



o 3.0 



= 0.64 



\ A.7 



Enter Figure 3-8 with known values of d/gT^ and H /H, . 



o 

 Note that H/H^ in Figure 3-8 is a ratio of wave heights which may be 

 approximately estimated as ^ /Hi^ for irregular waves; however, only the 



energy-based parameter H is known in this example. If the computed 



value of H differs greatly from H , it may be necessary to return to 



H ° 

 g 



Figure 3-8 with a revised ratio — — . Using the vertical axis of Figure 

 3-8, estimate m 



o 



H 

 s 



H 

 m 

 o 



= 1.16 



This answer seems reasonable in light of the envelope of the field data 

 shown in Figure 3-8. It does not seem necessary to repeat the analysis with 

 a revised ratio H /H, . Thus 



H = 1.16(3.0) = 3.5 m (11.5 ft) 

 s 



(b) H, = 1.67 H from example in Qiapter 3, Section 11,2 



H^ = 1.67(3.5) = 5.8 m (19.0 ft) 



Note that this value is greater than Hv . Since R^ is the maximum 

 allowable individual wave height, the computed value for H, is too high in 

 this example. Use instead H =H,=4.7m(H =H,= 15.4 ft). 

 *************************************** 



III. WAVE FIELD 

 1 . Development of a Wave Field . 



Descriptions of the mechanism of wave generation by wind have been given, 

 and significant progress in explaining the mechanism was reported by Miles 

 (1957), Phillips (1957), and Hasselmann et al. (1973). 



The Miles-Phillips-Hasselmann theory, as extended and corrected by experi- 

 mental data, permits the formulation of a differential equation governing the 

 growth of wave energy. This equation can be written in a variety of ways 

 (Inoue, 1966, 1967; Barnett, 1968; Hasselmann, et al., 1976). Numerical 

 models have been developed that solve these equations for oceanic and Great 

 Lakes conditions (Inoue, 1967; Barnett, 1968; Hasselmann, et al., 1976; Resio 

 and Vincent, 1977a; Resio, 1981). This approach will not be discussed in 

 detail because the applications of such models require specialized exper- 

 tise. A brief discussion of the physical concepts employed in the computer 

 wave forecast, however, is presented to show the shortcomings and merits of 

 simpler procedures that can be used in wave forecasting. 



Growth and dissipation of wave energy are very sensitive to wave frequency 

 and wave direction relative to the wind direction. Thus it is desirable to 



3-19 



