A COMPARISON OF DEEP WATER WAVE FORECASTS BY THE PIERSON-NEUMANN, 

 THE DARBYSHIRE, AND THE SVERDRUP-MUNK-BRETSCHNEIDER METHODS WITH 

 RECORDED WAVES FOR POINT ARGUELLO, CALIFORNIA FOR 26-29 OCTOBER '^0 



by 



Robert F. Dearduff, Hydraulic Engineer 

 Research Division, Beach Erosion Board 



A brief comparison of deep water wave forecasts made by the 

 Darbyshire and the Sverdrup-Munk-Bretschneider methods with recorded 

 waves at Point Arguello, California for October 26-29, 19^0 has pre- 

 viously been made (1,2)*. A further comparison for this same period 

 with the method recently developed at New York University(3) is believed 

 to be of further interest; this method is referred to herein as the 

 Pierson-Neumann or Wave Spectra method. 



This latter method differs from the Sverdrup-Munk-Bretschneider 

 method in that: 



(1) Greater cognizance is taken of the entire wave spectrum 

 which is treated as the sum of numerous simple sine waves. In the use 

 of the method, each simple component is treated individually, and the 

 resultant characteristics obtained by summing up the components per- 

 tinent to the particular place and time. (The Darbyshire method also 

 does essentially the same.) 



(2) Wave decay is considered as being due entirely to dis- 

 persion (the stretching out of a wave train as the longer period, 

 higher velocity waves pass through and beyond the shorter period waves) 

 and to angular spreading (the spreading out of the area of wave 

 activity due to the difference in direction of the various wave com- 

 ponents). This latter means that fetch width is considered as an 

 important factor in the determination of wave height after decay. For 

 example, given two fetches, one being twice as wide as the other, the 

 waves created by the wider fetch will be Af~2" higher than those of the 

 narrower fetch. Important consideration is thus given to the angular 

 spread of the waves as they are propagated from the fetch front to a 

 coastal point. That is, the greater the angle between a line drawn 

 from the same point of observation to the extreme end of the fetch front 

 and a line extended from the fetch side, the relatively higher the waves 

 will be at the point of observation (see Figure l). 



* Numbers in parentheses refer to References at the end of ihe article, 



