changed. The following or opposing wind may 

 blow over only a part of the distance of decay. 

 The problem is to determine how much more or 

 how much less the wave height decreases in any 

 given distance as compared to its decrease in 

 the absence of wind. An ejfective decay dis- 

 tance can be defined and the problem solved 

 with plate VI by means which will be explained 

 when discussing the practical procedure. 



Distance from which Observed Swell Comes; 

 Travel Time; Speed of Wind which Pro- 

 duced the Swell 



If the height and period of swell are observed 

 it is possible to find approximate values of the 

 distance to the end of the generating area from 

 which the swell came, of the travel time of the 

 swell, and of the wind speed in the generating 

 area. Plate VII shows these values as func- 

 tions of the observed height and period of the 

 swell. 



The values which can be derived from the 

 plate are only approximate because the height 

 and period of the swell depend also upon the 

 relation between wave and wind speeds (C/U) 

 at the end of the fetch. For the present report 



the ratio is assumed to vary in accordance with 

 an assumed relationship between wind speed 

 and duration. In plate VII two specific rela- 

 tionships between wind speed and duration are 

 shown in the inset. In the upper and lower 

 parts of the diagram corresponding values of 

 decay distance, travel time, and wind speed are 

 shown. 



Choice of the better relationship between 

 wind speed and duration must be based on a 

 knowledge of weather situations which prevail 

 in the area under consideration and on the 

 common experience that high winds are usually 

 of short duration while weaker winds may 

 blow for a long time. For some combinations 

 of observed height and period only one of the 

 two parts of plate VII will apply. 



Results obtained from plate VII are only 

 approximate since the wind speed and duration 

 relationship may vary considerably from one 

 weather situation to another. Lack of knowl- 

 edge as to changes caused by following or op- 

 posing winds and inaccuracies in the observa- 

 tions of height and period of the swell also in- 

 troduce errors. Values of decay distance and 

 travel time obtained from plate VII are more 

 accurate than values of wind speed. 



13 



