Analysis of waves
A GEDAP batch file was developed to automate the procedure of post-
processing several transects of wave data. The general procedure consists of the
following steps. First, the program FILT_FFT is used to remove the low-
frequency wave energy in the surf zone which results from a slowly fluctuating
mean water level. This step is especially important in shallow water near the
shoreline. This program uses a rectangular bandpass filter to remove all wave
energy that occurs at frequencies below F1 Hz and at frequencies above F2 Hz.
The fast Fourier transform (FFT) of the input signal is computed and the
rectangular bandpass filter is applied in the frequency domain. An inverse FFT is
used to obtain the filtered output signal. During recent experiments conducted
using irregular waves with a peak frequency of 0.67 Hz, F1 = 0.33 Hz was
selected as the low-frequency energy cutoff. The upper cutoff frequency, F2,
was set equal to the Nyquist frequency.
The program VSD, an acronym for Variance Spectral Density analysis, is
used to calculate the peak frequency and the energy-based significant wave
height of the filtered output signal. The program ZCA, an acronym for Zero
Crossing Analysis, also is used to calculate the average and maximum wave
height as well as the average wave period of the filtered output signal. In
addition, the program STAT1 is used to calculate the root-mean-square wave
height and the mean water level for the filtered output signal.
Finally, the GEDAP program PLUCK is used to collect the corresponding
values of the seven statistical parameters discussed above from the headers of
each of the output files. The final result is an ASCII file consisting of 10
columns of data. The first three columns identify the wave gauge number and
the x- and y-locations. The last seven columns contain the values of the seven
statistical parameters discussed above for the 14 wave gauges. As many as 12
transects have been measured during a 2-hr segment of an experiment. The
ASCII file lists the statistical results obtained from the 12 transects, one transect
below the other. All of the wave data can be analyzed in approximately 15 min.
Examples of the cross-shore distribution of the measured wave height during
a regular wave and irregular wave experiment are shown in Chapter 9. In
addition, the cross-shore distribution of the measured mean water surface
elevation for the same two experiments, are shown in Chapter 9, as well.
Current Data
As mentioned previously in Chapter 2, the 10 ADVs are used to measure 3-D
flow velocities (currents) in the facility. The ADVs are mounted along the
downstream side of the instrumentation bridge and are co-located with the wave
gauges, as shown in Figure 40 and Figure B-4, Appendix B. The ADVs are
configured to operate at 10 MHz because measurements are required in shallow
water and a high spatial resolution is needed to quantify the vertical velocity
profile in shallow-water depths.
77
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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