Application of the Pier son-Neumann method, as developed at New 

 York University, to the October 19!?0 storm showed that the first waves 

 generated by this storm would arrive at Point Arguello at about 

 1800 October 26, more or less at the same time as predicted by 

 Bretschneider(2), and by the Darbyshire method. (See Figure 2). For 

 the following 18 hours, the waves predicted by the "Wave Spectra 

 Method" were considerably lower than those predicted by Bretschneider 

 and those shown by the recorder; but were almost identical with those 

 predicted by the Darbyshire method. During the following h& hours, 

 the waves predicted by the Pier son-Neumann method were considerably 

 higher than either the recorded or those predicted by Bretschneider, 

 averaging about twice as high as those predicted by Bretschneider; this 

 average was taken over 6-hour intervals (see Figure 2). The peak 

 storm waves occurred at 1200 October 27 according to both Bretschneider 

 and the recorder and were about lk feet in height; however, the peak 

 waves obtained by the Pierson-Neumann method averaged about 21 feet 

 in height and arrived 6 hours later at 1800 October 27. 



An hourly average taken over the 66-hour period from 1800 October 

 26 to 1200 October 29, showed that the wave height determined by the 

 Pierson-Neumann method was 10.it feet as contrasted with 7.8 feet for 

 Bretschneider (for a more exact comparison see Figure 2). Due to the 

 variance in wave height between Bretschneider, Darbyshire and the 

 recorder as against the Pierson-Neumann results, it was believed 

 possible that errors had been made in the application of this method, 

 and a second independent hindcast analysis was made by another Beach 

 Erosion Board staff member. The results were about the same; the 

 wave heights obtained by the Pierson-Neumann method again being about 

 twice the others for the period from 1200 October 27 to 1200 October 29, 

 and the storm peak height being about 23 feet. 



There would appear to be several possible explanations for the 

 difference between the two results. One is, of course, error in applica- 

 tion of the newer method. Another is possible error in analysis of the 

 observed waves — these were obtained from a pressure recorder, and 

 analysed by the significant wave concept, which generally gives some- 

 what (5> to V~> percent) lower values than actual. Still another is 

 difficulty in using the Co-Cumulative Spectra Curves of the Pierson- 

 Neumann method. In this connection it was often almost impossible to 

 determine with any accuracy the frequencies, hourly durations and the 

 corresponding energy (E) values located near the curve extremes (and 

 frequently values determined for these regions from one set of curves 

 would not agree with those from another). A portion of the differences 

 in height may certainly be ascribed to this reason (i.e. difficulty 

 in accurately reading the curves for high velocities and low durations). 

 It is understood that these curves have now been redrawn and are in the 

 process of publication by the Navy Hydrographic Office, so that this 

 difficulty will no longer exist in the future. 



