used to provide a mean sea (or tide) level. Cit was suggested that the Baylor 

 staff gages along the FRF pier be used to measure water levels across the surf 

 zone to investigate the water's slope. This was not pursued because the gage 

 zero value showed both a random variation due to the difficulty in measuring 

 the zero offset and a time-dependent change due to amplifier drift.) 



23. The procedure used to monitor the gage zero level was to measure 

 the water level on the gage and gage output, then compare that to the corre- 

 sponding gage output for the measured water level based on the gage calibra- 

 tion curve. Differences implied a drift of the gage zero. In practice, this 

 was accomplished as follows: 



a. The distance from the pier deck to the still-water level was 

 measured by lowering a weighted surveyor's tape (i.e. lead 

 line) from the FRF pier deck (on a calm day) to the visually 

 determined still-water ]evel next to the gage. 



b. The distance from the bottom of the gage to the still-water 

 level was determined by accounting for the distance from the 

 top of the gage to the pier deck in the above measurement and 

 taking the difference between that value and the gage length. 



c. The gage output value was determined as the average of the few- 

 minute sample of gage measurement output while the weighted 

 tape measurement was made. 



d. The lead-line-determined level and the measured gage output 

 were then compared to determine the zero offset of the gage. 



24. This procedure is believed to be accurate to no better than ±10 cm; 

 errors arise from estimating the still-water level and from movement, bending, 

 and expansion of the surveyor's tape used in the lead-line measurement. This 

 accuracy is not sufficient for the detection of water slopes across the surf 

 zone, which may only amount to a few centimeters difference at the measurement 

 locations. The gage zero drift or uncertainty is random (see Figure 3). 



25. Although this variability seems artificial, precautions were taken 

 in the analysis of the 20-minute data records when computing wave statistics 

 (see Part IV). 



Waverider buoy gages 



26. The Waverider buoys were manufactured by the Datawell Laboratory 

 for Instrumentation, Haarlem, Netherlands. Each 0. 7-m-diameter buoy floats 

 on the water's surface and (a) measures the vertical acceleration produced by 

 the passage of a wave, (b) doubly integrates this signal to produce a dis- 

 placement signal, and (c) telemeters this signal to an onshore receiver and 

 associated electronics which extract the displacement signal for data logging 



- - 14 



