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



GIVEN : A wind speed U = 35 knots (40 mph) and a duration t = 10 hours . 



FIND : The significant wave height H^, and the significant period T^^, 

 at the fetch front for: 



(a) A fetch length, F = 200 nautical miles and 



(b) a fetch length, F = 80 nautical miles. 

 SOLUTION : 



(a) Enter Figure 3-15 from the left side at U = 35 knots and move 

 horizontally across the figure from the left toward the right, until 

 intersecting the dashed line for a duration of 10 hours that comes 

 before the line indicating a fetch length of 200 nautical miles. At 

 the 10-hour duration line F = 92 nautical miles; this is the minimum 

 fetch F^ for this case. 



With U = 35 knots, t = 10 hours, and F = 200 nautical miles then 

 H^ = 13.1 feet, T^ = 8.0 seconds, t^ equals 10 hours, and F^ = 92 

 nautical miles. 



(b) Entering Figure 3-15 as above, when F = 80 nautical miles 

 and t = 10 hours, then the heights, periods, minimum duration and 

 fetch would be H^ = 12.6 feet, T^ = 7.8 seconds, and t^ = 9.0 hours. 

 The minimum duration t^ is 9 hours, corresponding to the miles 

 which limit generation, and comes before a duration of 10 hours. 



In this example problem, the wave pattern in (a) is limited by the 

 duration; the wave pattern in (b) is limited by the fetch. 



When a series of surface synoptic weather charts (Fig. 3-9) are used 

 to determine wave patterns, the values of U, F and t can be tabulated 

 for the first chart. For the same fetch on a later chart drawn for a time 

 Z, after the first chart, U, F, and t are again tabulated. Using 

 the subscript 2 to refer to those of the second chart and subscript 1 

 to refer to those of the first chart, if U2 = U-^ , the above procedures 

 should be followed using either t2 = t^i + Z or F2. If, however, U2 f U^ , 

 certain additional assumptions must be made before using the forecasting 

 curves . 



A change in wind speed from Uj^ to U2 in a time Z between charts may 

 be assumed to take place instantaneously at a time Z/2. Waves due to Ui 

 may then be calculated by assuming that the first chart minimum duration 

 time has been lengthened by an amount Z/2 or that its minimum fetch has 

 been changed by AF/2, where AF represents the change in fetch length 

 between weather charts. Since at the assumed abrupt change in wind speed. 



3-38 



