80 



By adopting a standard calibration for all gauges, data conversion from volts 

 to engineering units of inches of water surface was performed during 

 measurement, and data were stored in units of inches in the archived data 

 files. 



Initial depth and zero reference 



Zero reference levels for the gauges were established in two ways during 

 the SUPERTANK project. Initially, gauge zeros were selected during the 

 gauge calibration at the midpoint of the sensing wire. Once installed in the 

 channel, however, this zero point was often several inches above the actual 

 still-water surface. As a result, this method was modified. In subsequent 

 tests, gauges were calibrated, installed in the wave channel, and then the 

 offset potentiometers were adjusted to give approximately zero volts output 

 based on the initial sand bed or water surface elevation. For gauges placed 

 seaward of the still-water shoreline, gauge zero reflects the approximate initial 

 still-water level. For gauges placed landward of the shoreline on the beach 

 face, gauge zero reflects the approximate initial saturated sand bed elevation. 



Reference elevations for the capacitance gauges are therefore dependent on 

 the local water depth or sand bed elevation at each gauge. At the start of each 

 run, sand bed elevations were measured by holding a survey rod next to each 

 gauge and reading the distance between the sand bed and the top of the alumi- 

 num box beam that ran the length of the measurement region. Since the top of 

 this beam was at a known elevation of -0.63 ft (-0. 19 m) below the top of the 

 wave tank, sand bed elevations could be determined in the SUPERTANK 

 coordinate system. Measurements were also taken of the still-water surface 

 elevation and referenced to the top of the tank wall. From these measure- 

 ments, the initial bed elevation was determined relative to the still-water level. 

 These values are listed in Table Dl, where positive values indicate that the 

 sand bed was above the still-water level and negative values indicate the initial 

 water depth at the gauge was below the water level datum. 



Sampling and time reference 



The PC-based data acquisition system, using the LabTech Notebook soft- 

 ware package, sampled all 10 capacitance wave gauges at a 16-Hz rate, the 

 same as that used by the WRL data collection system. Because of data stor- 

 age limitations on the PC, data sampling run lengths varied throughout 

 SUPERTANK. Normally, sampling lengths of 10 to 20 min were used to 

 create data files that were small enough to store and transfer by diskette on the 

 PC. Because of this procedure, runs of 40 or 70 min required two to four 

 data collection runs and produced two to four data files. The typical sequence 

 of events was to collect data for 20 min, save the data file, and then re-run the 

 software to collect another data file. This resulted in some "downtime" 

 between the end of one file and the beginning of another (typically 1 or 

 2 min). 



Chapter 4 SUPERTANK Swash Measurements 



