be +3.5 times the standard deviation about the mean. Data points outside the data
limits are discarded, and the mean value is recalculated for the remaining data set
within the sliding window. Smaller values of Alpha sometimes are used for
ADVs 1 and 2 because these signals contain many glitches that result from the
presence of air bubbles in shallow water. However, good judgement is required
in the selection of Alpha, because valid parts of the signal may be removed. The
remaining set of valid data points is sorted into increasing order by time, and
linear interpolation is used to replace points removed by the program. Therefore,
the length of the filtered time series is the same as the original time series, and
the 38 channels of current data remain synchronized. Normally, only ADVs 1
through 4 need to have glitches removed from the velocity signal, since these are
the sensors that are located in shallow water depths.
At this step in the analysis, the batch file pauses allowing the user to
superimpose the filtered time signal onto the original time series, which contains
spikes caused from the entrained air bubbles. This is integrated into the GEDAP
batch routine and is plotted using GPLOT, as shown in Figure 46. This figure
shows the results of filtering the y-axis (longshore) time series for ADV 2. These
results were obtained using a value of Alpha = 3.5, which works well if the true
signal has a Gaussian distribution. It sometimes requires several iterations to
determine the optimum value of Alpha. However, it should be mentioned that in
some cases, the number of spikes are so numerous that the signal cannot be
properly filtered to obtain a time series free of spikes. If this is the case, the time
series is discarded. A more complete description of this filtering routine is
available in the GEDAP on-line documentation.
After the signals in shallow water have been filtered, the program STAT1 is
used to calculate the minimum, mean, maximum, and standard deviation of the
velocity signals for both the x- and y-axes. The mean value obtained from the y-
axis of the 10 ADVs defines the cross-shore distribution of the longshore current.
Finally, the GEDAP program PLUCK is used to collect the values of the four
statistical parameters calculated using STAT1 from the header of each of the
output files. The final result is an ASCII file consisting of six columns of data.
The first two columns identify the channel number and ADV number and axis.
The last four columns contain the values of the four statistical parameters
discussed above for all 38 ADV channels.
As mentioned previously, as many as 12 transects have been measured
during a 2 hr segment of an experiment. In contrast to the GEDAP batch file
used for postprocessing the wave data, the batch file processes only one transect
of current data, because several iterations may be necessary to determine an
optimum value of Alpha. One transect of current data can be analyzed in
approximately 15 min. Therefore, approximately 3 hr would be usually be
required to process12 transects of current data.
Examples of the cross-shore distribution of the measured mean longshore
current during a regular wave and irregular wave experiment are shown in
Figures 66 and 68, Chapter 9.
83
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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