the initial wave height of 3 feet. For the thick stands of tall grass, 

 the predicted increase in wave height is only 11 percent using the 

 approximate method of solution discussed in this report. 



* * * * . 



V. WAVE DECAY IN SHALLOW WATER 



Values of wind-generated significant wave heights and wave periods 

 as a function of windspeed and water depth are shown in Figures 1 to 12. 

 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, 

 the effects of the bottom friction may be assumed to exceed the effects 

 of the wind stress. Therefore, the wave is assumed to decay, lose height, 

 and over a long distance to approach a height equal to the maximum sig- 

 nificant wave height. 



The higher waves at the beginning of the fetch will actually repre- 

 sent a spectrum of waves. Waves of various periods could break at dif- 

 ferent points approaching the shallow-water fetch area from deep water. 

 As shown in Figure 16, the method used here predicts that the longer 

 period wind waves would decay faster than shorter period waves passing 

 through the shallow water (see Fig. 18). As discussed previously, the 

 effects of vegetation near the surface may have a strong influence on 

 very short-period waves. However, these waves are considered to be less 

 important for design purposes, and this influence is ignored. 



Assumed Exponential Decay of Significant Wave 

 Decay of Short-Period Waves 



Decay of Long-Period Waves 

 Exponential Growth of Significant Wave 



Fetch Length, X — 



Figure 18. Exponential wave growth and decay. 



36 



