meteorological conditions considered result in a point lying below 

 the appropriate curve, then the wave height will not be expected to 

 reach the limiting value (or height of interest); similarly if the 

 weather conditions result in a point on or above the appropriate curve, 

 waves greater than the specified values may be expected to occur (and 

 the unanswered question, as to whether the waves exceed the limiting 

 value by 1 foot or by 50, is not important to the determination). 



A set of curves showing limiting values for generation of 3-foot 

 waves is shown in Figure 1, The curves show values of wind velocity 

 and duration, and fetch and decay lengths necessary to produce the 

 limiting waves of 3 feet. They are used in exactly the same manner 

 as the usual generation curves used in wave forecasting, only instead 

 of reading off the height of the generated waves, the decay distance 

 resulting in the specified limiting wave height (3 feet) is obtained. 

 If the actual decay is less than this value, then the waves will be 

 greater than the (3-foot) limiting value; and if it is more, then the 

 wave height will be less than the (3-foot) limiting value. For example, 

 from Figure 1, with a wind speed of 20 knots, a duration of 12 hours, 

 a fetch of 175 miles, and a decay of 350 miles it is seen that the 

 wave growth is limited by duration, and that the waves will be reduced 

 to 3 feet after a decay of about 200 miles; since the actual decay 

 distance is 350 miles, the waves there will be expected to be less than 

 3 feet. If now the duration in this example had been 2ii hours rather 

 than 12, the wave generation would be limited by the 175-mile fetch 

 length, and (from the figure) the waves would reduce to 3 feet- after 

 a decay of 500 miles; since the actual decay was only 350 miles, the 

 waves there would be expected to be greater than 3 feet. 



Although curves are shown for a 3-foot wave height only, it is a 

 relatively simple matter to obtain +hem for any desired height. The 

 method is shown below for the case of the 3-foot height, and a 100-mile 

 fetch and 500-mile decay. Using the Bretschneider decay curves it may 

 be seen that for a 500-mile decay with a minimum fetch of 100 miles, 

 the relative decrease in wave height over the decay distance (Hjy'Hp) 

 may range between 0.26 and 0.33; this means that the wave height at the 

 end of the fetch (Hp) which will produce 3-foot waves after the 500-mile 

 decay must be between 9.1 and 11.5 feet (these values being obtained 

 by dividing the Hj) of 3 feet by the ratio Hj)/Hf). Going again to the 

 decay curves and using these values for Hp, it may be seen that Hd/Hj? 

 must be between about 0.285 and 0.295. Repeating the above process 

 with these new values of Hp/Hp would again reduce the range, though 

 this is probably not warranted as it may be seen that the desired value 

 of Hd/Hf will be very close to 0.29. Hence the wave height at the end 

 of a 100-mile fetch which will produce a 3-foot wave after a 500-mile 

 decay is 3/0.29 or 10.3 feet. This point may then be plotted on the 

 Bretschneider generation figure at the intersection of the 100-mile 



