IV. WAVE DECAY 



If the initial significant wave height, at the seaward or beginning 

 edge of a segment of fetch, exceeds the maximum significant wave height 

 for the given water depth of the segment of fetch and the given windspeed, 

 it may be assumed that the effects of the bottom friction will exceed the 

 effects of the wind stress. Therefore, the wave will decay, will lose 

 height, and over a long distance will approach a wave height equal to the 

 maximum significant wave height. 



The method of determining the decayed wave height is shown in Figure 

 15. The following steps are used to predict the decay of a wave: 



(a) Determine the maximum significant wave height that 

 would be generated for a given windspeed and water depth, 

 assuming an unlimited fetch and using Figure 1. 



(b) Determine the fractional reduction, R^, repre- 

 sented by the initial wave at the seaward edge of the 

 segment of fetch under consideration. 



This is given by 



H = 



H m - H c 



(9) 



where H^ is the maximum stable wave height given as 



1^ = 0.78 d . (10) 



(c) Determine the equivalent initial wave height, H^ e , 

 for wave growth by 



H^ e = *t H sm ■ (ll) 



(d) Determine the equivalent fetch length, F e , for the 

 wave height, H-£ e . 



(e) Determine an adjusted fetch length, F a , for the 

 segment length, Ax, using equations (6) and (7) . 



(f) Determine the total fetch, F, from equation (8). 



(g) Determine an equivalent wave height, H e , for the 

 total fetch and the given windspeed and water depth. 



(h) Calculate the fractional growth by 



c _ H e (12) 



Z H„„, 



26 



