is recorded, and if the average wave "period" is 10 seconds, then 

 such an observation requires nearly 20 minutes to make. A wave 

 observer nnust take sufficient time to make an adequate observation. 

 A good observation simply cannot be made in just a few minutes. 



6. Truncated Distribution 



As the wave observer looks at the waves, there are times when 

 low waves pass. For example, with E equal to 100 ft. 2' the significant 

 height would be 28.3 feet. One wave out of 10 would be less than 6.4 

 feet high; compared with the more dominant waves, there would be a 

 very strong tendency to ignore some of these low waves. 



When this is done, the problem is what to do with the observed 

 values which have now become a sample from an unknown population, 

 since the probability that the observer will ignore a given low wave 

 is an unknoAvn factor. 



The first thing that must be done is to truncate the theoretical 

 distribution sharply. That is, equation (1) must be set equal to zero 

 for all X less than a certain value and then correctly normalized. 

 If the low waves are to be ignored in the tabulation, then all low waves 

 must be ignored and not just some unknown and unspecifiable fraction 

 of the values within a certain class interval. 



There are two ways to truncate the distribution. The first way 

 is to discard a certain fixed percentage of the lowest waves of all the 

 waves that pass a given point. The second way is to discard all waves 

 less than a preassigned height value, 



7, Truncated Distribution at a Fixed Percentage 



The first way, naniely discarding a certain fixed percentage of 

 all the lowest waves to pass a fixed point of observation, is inherent 

 in the concept of the significant height. The significant height is the 

 average of the heights of the 33 percent highest waves to pass the 

 point of observation. 



To observe the significant height correctly, the follow^ing procedure 

 could be used. The observer would watch the waves pass the fixed 

 point. If a high wave passed, he would note down its height. If a low 

 wave passed he would simply make a check to note the passage of the 

 wave. A series of recorded heights and checks would be the result. 

 The total number of heights and checks, say M, would be counted up, 

 and the sum divided by 3. The observed heights would then be put in 



19 



