The PC clock was continually updated to the same WWV time standard 

 used by all SUPERTANK investigators. A problem in the data collection 

 software, however, prevented the correct starting time from being recorded in 

 the header of each data file. This problem was not detected during 

 SUPERTANK and was found later upon inspection of the data. The data col- 

 lection software requires two key strokes to start sampling data. Unfortu- 

 nately, the software program recorded the start time based on the first 

 keystroke, whereas data collection did not begin until the second keystroke. 

 There was an elapsed time of several minutes to more than 1 hr between these 

 keystrokes. As a result, all start times recorded for the swash data files are in 

 error. The 10 capacitance gauges are properly sequenced in time from one to 

 another; however, the capacitance gauges are not linked in time to other 

 SUPERTANK gauges. 



Data Analysis 



Analysis methods 



Analysis of the swash zone wave records was nearly identical to that per- 

 formed on the other hydrodynamic data, as summarized in Chapter 3. Both 

 frequency- and time-domain analyses were performed. Data were also filtered 

 so that these analyses were performed on three signals: (a) the total water 

 surface record (denoted "T"), (b) the low-frequency water surface record 

 based on low-pass filtering of the data (denoted "L"), and (c) the high- 

 frequency water surface based on high-pass filtering of the data (denoted 

 "H"). The only major difference between the analysis of the swash data and 

 the other hydrodynamic data involved the treatment of the sand bed if the bed 

 was exposed during the wave record. 



Identification of offset. As noted, the reference (zero) datum for each 

 gauge is the initial water surface elevation for gauges seaward of the shoreline 

 or the initial wetted sand surface for gauges landward of the initial shoreline. 

 At the beginning of a run, the signal for each wave gauge was adjusted to give 

 approximately zero volts output for this initial reference elevation. These 

 initial "zero-voltage" readings were only approximate, however, and in prac- 

 tice each gauge had a small initial voltage offset. In the data analysis soft- 

 ware, the signal for each gauge was averaged over the initial 200 data points, 

 before waves arrived at the gauges, to determine the offset. This average 

 offset was then subtracted from the entire wave record to rectify the signal to 

 a reference datum. 



Preprocessing. In preliminary testing of the capacitance gauges, it was 

 found mat high-frequency noise was introduced as a result of gauge 

 interference. At a gauge spacing of 6 ft (1.83 m) this noise was almost 

 undetectable. At a spacing of 3 ft (0.91 m), the noise was noticeable but of 

 minimal importance and about the equivalent of 0.05 to 0.1 in. (0.13 to 



Chapter 4 SUPERTANK Swash Measurements 



81 



