COMPUTER ALGORITHM TO CALCULATE LONGSHORE ENERGY FLUX AND 

 WAVE DIRECTION FROM A TWO PRESSURE SENSOR ARRAY 



by 

 Todd L. Walton, Jr. and Robert G. Dean 



I. INTRODUCTION 



The documented (FORTRAN IV programing language) computer program discussed 

 in this report was originally written as part of the Coastal Engineering 

 Research Center's (CERC) Longshore Sand Transport Research Program and was used 

 in analysis of wave data collected at Channel Islands Harbor in conjunction 

 with a study of sand transport at Channel Islands Harbor as discussed in Bruno, 

 et al. (1981). 



The program performs the basic analysis of two wave gage pressure records 

 necessary to compute wave direction and wave energy at a given frequency and 

 computes the longshore energy flux used in sand transport for the entire energy 

 spectrum of the wave record. This program uses linear wave theory for the wave 

 transformation process and includes the assumption of straight and parallel 

 bottom contours necessary for application of Snell's law of refraction. 



Necessary steps in the analysis of the wave data are presented in Sections 

 II and III of this report. Subroutines are discussed and sample outputs for 

 some wave records from the Channel Islands wave gage pressure sensor pair are 

 given. 



The program presently accepts data in the standard CERC magnetic-tape for- 

 mat where record lengths consist of 4,100 values. The first four values are 

 the gage number and the date-time group, and the remaining 4,096 values are 

 the pressures recorded in thousandths of a foot (head) of water at 0.25-second 

 intervals. Should other input data be available, the program could easily 

 be modified to accept the data by simple changes in the main program and in 

 subroutines BUF and SWITCH. 



Sample outputs have been presented for real wave data; some wave direc- 

 tional information cannot be obtained for all frequencies because the spectral 

 information at some frequencies is ill-conditioned. The percent of energy for 

 which this problem occurs is a small part of the energy (usually <3 percent) of 

 the entire spectrum and is insignificant in energy-flux computations. Reasons 

 for this feature are discussed later. 



II. METHODOLOGY 



Calculating the longshore energy flux at breaking required the following 

 steps : 



(1) Calculation of the frequency-by-frequency wave direction and 

 energy at the location of the wave gages; 



(2) determination of the breaking wave depth; 



(3) transformation of the wave spectrum to the "breaker" line, 

 including shoaling and refraction effects; and 



(4) computation of "P, ," the longshore energy flux at the 

 surf line. 



Each of the steps is described below. 



