Saville (1954) proposed a method to determine the effect of fetch 

 width on wave generation. Figure 3-13, based on this method, indicates 

 the effective fetch for a relatively uniform fetch width. The following 

 problem demonstrates the use of Figure 3-13. 



************** EXAMPLE PROBLEM ************* 



GIVEN : Consider a channel with a fetch length F = 20 miles, a width 

 W = 5 miles, an average depth d = 35 feet, and a windspeed U = 50 mph 

 along the long axis. 



FIND : Estimate the significant wave height Hg, and the significant 

 wave period Tg. 



SOLUTION : Compute W/F = 5/20 =0.25 



From Figure 3-13 for W/F = 0.25, F^/F =0.45 



Compute F^. = 0.45 X 20 = 9 miles or 47,500 feet. 



Using the forecasting relations given in Section 3.6, Wave Forecasting 

 for Shallow Water, for a fetch of 47,500 feet and a wind speed of 

 50 mph and an average uniform depth of 35 feet, the significant wave 

 height may be determined from Figure 3-27 to be Hg = 5.2 feet, say 

 5 feet and the significant wave period will be T = 4.6 seconds, say 

 5 seconds . 



The preceding example presents a simplified method of determining the 

 effective fetch. Shorelines are usually irregular, and the uniform-width 

 method indicated in Figure 3-13 is not applicable. A more general method 

 must be applied. This method is based on the concept that the width of a 

 fetch in reservoirs normally places a very definite restriction on the 

 length of the effective fetch; the less the width-length ratio, the shorter 

 the effective fetch. A procedure for determining the effective fetch 

 distance is illustrated in Figure 3-14. It consists of constructing 15 

 radials from the wave station at intervals of 6° (limited by an angle of 

 45° on either side of the wind direction) and extending these radials 

 until they first intersect the shoreline. The component of length of 

 each radial in a direction parallel to the wind direction is measured and 

 multiplied by the cosine of the angle between the radial and the wind 

 direction. The resulting values for each radial are summed and divided 

 by the sum of the cosines of all the individual angles. This method is 

 based on the following assumptions: 



(a) Wind moving over a water surface transfers energy to the 

 water surface in the direction of the wind and in all directions within 

 450 on either side of the wind direction. 



(b) The wind transfers a unit amount of energy to the water along 

 the central radial in the direction of the wind and along any other radial 

 an amount modified by the cosine of the angle between the radial and the 

 wind direction. 



3-30 



