Since the observations indicate that the wind was constant over the 8 hours, 

 the 8-hour average windspeed can be assumed to be 23.5 meters per second. 

 (If the windspeed were variable, an iteration on the duration-averaged winds 

 would have to be made. For example, a 23.5-meter-per-second windspeed will 

 give a wind-stress factor of 34.5 (see below). Entering Figure 3-23 with a 

 fetch of 10 kilometers and a wind-stress factor of 34.5, the duration to 

 reach the fetch-limited case is about 1 hour and 20 minutes, so the wind 

 duration that should be considered is 1 hour and 20 minutes. 



(f) The wind-stress factor is computed by equation (3-28a) , 



U. = 0.71 (U)^*^-^ = 0.71 (23.5)^'^"^ = 34.5 m/s (77.2 mi/hr) 

 for both the lOO-kilometer and 10-kilometer fetches and 3-hour duration. 



(g) For F = 10 kilometers and Ua = 34.5 meters per second , Fig. 3-23 

 gives H =1.75 meters and 1^ = 4.4 seconds, (1^/3 = 0.95 T^ and T^ = 



4.2 seconds) . 



(h) For F = 100 kilometers and U^ = 34.5 meters per second, 



H = 5.5 m, T = 9.4 s and T, ,„ = 8.9 s 

 m m 1/3 



o 



(i) For t = 3 hours and U^ = 34.5 meters per second, 



H = 3.3 m, T = 6.7 s, and T, ,„ = 6.4 s 

 m m 1/3 



o 



However, if the fetch was only 10 kilometers, the wave growth would become 

 fetch limited after about 1 hour and 25 minutes and the wave height would be 

 limited to the values obtained in (g) above. If the fetch is 100 kilometers 

 and the wind duration is 3 hours, then the values in (h) above will not be 

 reached because the duration is too short. Therefore, it is essential to 

 check that what is expected to be a fetch-limited case is not duration 

 limited. 



If for a given wind-stress factor and fetch or duration the point of 



intersection on Figure 3-23 or 3-24 lies in the maximum condition fully 



arisen sea area, the maximum wave height for that wind-stress factor is the 

 wave height. 



*************************************** 

 5 . Ef fe c ts of M oving Storm s and a Variable Wind Speed and Directi on . 



The case of a variable windfield in space and time over a waterbody of 

 irregular geometry is complex and must be treated using advanced numerical 

 wave prediction models such as those of Resio (1981) and Hasselmann et al. 

 (1976). 



3-53 



