(b) the low-frequency wave record containing wave group and tank seiching 

 frequencies, and (c) the high-frequency wave record containing the wavemaker 

 frequencies and higher harmonic components. 



Spectral analysis. Spectral analysis of the three wave records followed 

 the same procedures used for the other hydrodynamic data as described in 

 Chapter 3. The only difference is that wave records obtained on the PC-based 

 data acquisition system for the swash measurements are typically shorter than 

 those obtained on the WRL system for the other wave gauges and current 

 meters. For this reason, up to four individual data files, each containing 

 20 min of data, were sometimes required during a 40- or 70-min run. For 

 spectral analysis, each shorter record was analyzed in blocks of 4,096 data 

 points (256 sec) so that, for example, four blocks of data would be analyzed 

 in a 20-min data file. Over the two to four data files required for a 40- to 

 70- min run, this provided 8 to 16 blocks of 4,096 data points for which 

 individual wave spectra were calculated and then averaged to give the final 

 result. 



Spectral parameters determined from the block-averaged spectra are given 

 in Equations 3-1 through 3-5 in Chapter 3 and are listed in Table D2 in 

 Appendix D (contained in Volume II of this report). Two changes were made 

 in the spectral analysis routines to compute the first and second spectral 

 moments of the spectrum, m 1 and rtu. These were used to compute the mean 

 wave period as Tm 1 = m 1 jm 1 and the mean zero-crossing period as Tm- = 

 (jnjm^ 1 --. These wave period parameters are reported in addition to the peak 

 spectral period because they better describe the changes in the wave spectrum 

 as waves propagate through the inner surf zone and into the swash zone. In 

 many cases, the peak period varies suddenly and inconsistendy from one 

 gauge to the next. The mean period and the mean zero-crossing period, on 

 the other hand, tend to vary in a systematic way from gauge to gauge. 



Time series analysis. The time series analysis was also nearly identical to 

 that performed on the other hydrodynamic data and equations describing the 

 calculated parameters are given in Equations 3-9 through 3-15. Results of the 

 time series analysis are given in Table D3 in Appendix D. The wave setup or 

 the mean-water level of the record was first determined. Statistical parameters 

 such as the standard deviation, skewness, and kurtosis of the water surface 

 were then calculated using all of the data points in the wave record (excluding 

 some points at the beginning and end of the records). Individual waves were 

 then identified from a zero up-crossing analysis, and statistical parameters for 

 wave height and period were determined. 



Chapter 4 SUPERTANK Swash Measurements 



S3 



