FIND: The spectral significant wave height H , the period of the spectral 



o 

 peak T , and the significant period T^ for 



(a) Fetch = 10 kilometers. 



(b) Fetch = 100 kilometers. 



(c) Duration = 3 hours for each of the above fetches. 



SOLUTION : The winds are observed over land, so the procedure of Chapter 3, 

 Section IV, 3 will be followed. 



(a) Assume that there are no topographic convergences and that the winds 

 are from a large-scale pressure system. 



(b) The winds must be converted to a 10-meter level (Ch. 3, Sec. IV, a): 



1/7 in °-^^2 

 U,^ = U (10/Z)^'' = (20) [^) = 21.5 m/s (48.1 mi/hr) 



10 O ^ D 



(c) Since the anemometer is located 5 kilometers inland, the location 

 factor adjustment may be needed (Ch. 3, Sec. IV,l,d). For the 10-kilometer 

 fetch Chapter 3, Section IV, 3, c indicates R-^ = 1.2 . For the 100- 

 kilometer fetch, Figure 3-15 is used to obtain R^ = 1.2 also. So in both 



cases the windspeed is increased by 20 percent to 26 meters per second. 



(d) The stability factor (Fig. 3-14) is 1.14 for an air-sea temperature 

 difference of -6° C. Thus the windspeed is further increased by 14 percent 

 to 30 meters per second. (If the temperature difference had been + 6°, the 

 windspeed would have been reduced to 84 percent of its value or 22 meters 

 per second.) 



(e) Since the fastest mile windspeed is given, the duration-averaged 

 windspeed must be estimated. From Chapter 3, Section IV,l,b find Uj-^^q min* 



160 9 

 29, 



t = ^^ , = 54.7 seconds, say 55 



and 



"t/"3600 = 1-25 



The 1-hour windspeed is 



U 



"3ftnn = TTT^ = T^ = ^3.5 m/s (52.6 mi/hr) 



3600 "33/03^^^ 1.25 



3-52 



