The bottom friction, £-?, can be obtained from Figure 13 for a 

 known type of vegetation. The adjusted fetch distance, V a , for a 

 segment distance, Ax, is then obtained using values of the decay 

 factor, K^, from Figure 14 (after Bretschneider, 1954) 3 . An adjust- 



ment factor 



where FL 



is defined as 



1 



K/.OI 



(4) 



1 " K/b 



where Kf.oi i- s tne decay factor for a bottom-friction factor, f f = 0.01, 

 and Kj? is the decay factor for the actual bottom-friction factor. The 

 adjusted 



fetch length, F , is then given as 



a Ax 



(5) 



An adjustment factor, a r , where H-^ > H sm , is defined as 



a.r 



1 - K 



12. 



1 - Kf.01 



(6) 



and, for a decaying wave, 



III. WAVE GROWTH 



(7) 



For any given water depth, windspeed, and fetch length, a maximum 

 significant wave height, H sm , which would be generated can be defined 

 from Figure 1. If the initial wave height, ti-i, at the seaward or 



beginning edge of the fetch segment is less than 

 the wave will increase in height. 



He 



it is assumed that 



To find the wave growth, first determine an equivalent fetch length, 

 F e , for the initial wave (obtained directly from Fig. 1 using the given 

 windspeed and water depth). Secondly, the adjusted fetch, F a , is deter- 

 mined using equations (4) and (5) and Figure 14. The total fetch is then 

 given as 



+ F, 



(8) 



Re-entering Figures 1 and 2 with the fetch length, F, and the windspeed , 

 U, and water depth, d, the wave height and period at the end of the 

 fetch segment, H^ and T, are determined. 



3 BRETSCHNEIDER, C.L., "Modification of Wave Height Due to Bottom 

 Friction, Percolation, and Refraction," TM-45, U.S. Army, Corps of Engi- 

 neers, Beach Erosion Board, Washington, D.C., Oct. 1954. 



23 



