The third condition for fetch segment is satisfactory, and 



AH < 0.5 H^ = 2 - 1.46 < 0.5 (2) 



Thus, at the end of the fetch segment, the wave height and period are 

 approximated by 



H^p = 1.46 meters (4.78 ft) 



T = 4.7 seconds 



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



VII. HURRICANE WAVES 



When predicting wave generation by hurricanes, the determination of fetch 

 and duration from a wind field is more difficult than it is for more normal 

 weather conditions discussed earlier. The large changes in wind speed and 

 direction with both location and time cause the difficulty. Estimation of the 

 free air wind field must be approached through mathematical models because of 

 the scarcity of observations in severe storms. However, the vertical 

 temperature profile and atmospheric turbulence characteristics associated with 

 hurricanes differ less from one storm to another than for other types of 

 storms; thus the relation between the free air winds and the surface winds is 

 less variable for hurricanes than for other storms. 



1 . Description Of Hurricane Waves . 



In hurricanes, fetch areas in which wind speed and direction remain 

 reasonably constant are always small; a fully arisen sea state never 

 develops. In the high wind zones of a storm, however, long-period waves which 

 can outrun the storm may be developed within fetches of 15 to 30 kilometers 

 (10 to 20 miles) and over durations of a few hours. The wave field in front, 

 or to either side, of the storm center will consist of a locally generated sea 

 and a swell from other regions of the storm. Samples of wave spectra, 

 obtained during Hurricane Agnes (1972) are shown in Figure 3-39. Most of the 

 spectra display evidence of two or three distinct wave trains; thus, the 

 physical interpretation of a significant wave pevi,od is not clear. 



Other hurricane wave spectra computed with an analog spectrum analyzer from 

 wave records obtained during Hurricane Donna ( 1959) have been published by 

 Bretschneider (1963). Most of these spectra also contained two distinct 

 peaks. However, near the center of a hurricane, very large single-peak 

 spectra can occur as well (Fig. 40). Significant wave heights may exceed 15 

 meters (50 feet) in deep water, as in Hurricane Camille. 



An indication of the distribution of waves throughout a hurricane can be 

 obtained by plotting composite charts of shipboard wave observations. The 

 position of a report is determined by its distance from the storm center and 

 its direction from the storm track. Changes in storm intensity and shape are 

 often small enough to permit all observations obtained during a period of 24 

 to 36 hours to be plotted on a single chart. Several plots of this type from 

 Pore (1957) are given in Figure 3-41. Additional data of the same type have 

 been presented by Arakawa and Suda (1953), Pore (1957), and Harris (1962). 



3-77 



