water depth. Chow (1959) provides values of n for a wider range of 

 conditions. 



2. Selection of Fetch Segment . 



The growth or decay of a wave at any point is dependent on the water 

 depth, the wave height and period at that point, the bottom friction, and 

 the windspeed. For the method lised here, the windspeed is assumed to be 

 constant across a fetch. To accurately predict the growth or decay of 

 waves traversing a particular fetch, it is necessary to divide the fetch 

 into segments according to water depth and bottom friction. The bottom 

 friction may vary substantially as a function of water depth for a par- 

 ticular type of vegetation (Fig. 15) . 



Figures 1 to 12 were derived for a constant depth; any variation in 

 depth is assumed to be very gradual, and these figures are applied to an 

 average depth across a fetch segment. Therefore, the total variation in 

 depth across a fetch must be considered. The type of vegetation may also 

 vary, with sections of marsh grass, brush, trees, or shallow lagoons. 

 Wave heights will normally vary across a fetch and, as the decay factors 

 will depend on the wave height, new decay factors must be calculated if 

 the wave height varies excessively. A fetch segment is also considered 

 to be much longer than a single wavelength. 



Dividing the fetch into segments, the segment distance (length in the 

 direction of wave travel) is determined so that, first. 



Ad < 0.25 d-i , (5) 



where Ad is the change in depth over the segment distance and d^ is 

 the depth at the seaward or beginning edge of the segment; second, 



Afj. < 0.25 f^^ , (6) 



where Afj? is the change in the bottom- friction factor over the seg- 

 ment distance and ff^ is the friction factor at the seaward or begin- 

 ning edge of the segment; and third, after the change in wave height 

 across the segment, AH, has been determined, 



AH < 0.5 % , (7) 



where H^ is the initial wave height at the seaward edge of the segment. 

 Wave decay is also assumed to be small (less than 0.1 H^) over a single 

 wavelength. The segment distance may require adjustment; i.e., a shorter 

 segment may be necessary if the computed value of AH is unacceptable. 

 The numerical coefficients used on the right-hand side of equations (5) , 

 (6), and (7) are arbitrarily chosen. Smaller coefficients would be ex- 

 pected to increase the calculations required for a solution; larger co- 

 efficients would be expected to produce a greater error in the estimated 

 wave height obtained. 



27 



