APPENDIX A 



WAVE'GAGING TECHNIQUES 



Both electrical and "direct" measurement techniques were used to 

 record water levels in the laboratory tests. The electrical gages 

 recorded, on a strip chart, the electrical signal indicating the immersed 

 length of a probe, either by changing resistance (CERC gages) or by 

 changing inductance (the Marsh-McBirney gage). In the direct techniques, 

 wetting of a paper sleeve or erosion of a powder deposit on a circular 

 pile preserved the peak waterline, which was marked with waterproof 

 ink or crayon and then manually measured. This technique measured the 

 entire pile circumference with no important flow obstruction from a 

 sensing probe, but recorded no information on-water level variation in 

 time. 



The electrical gages were calibrated and used according to well- 

 defined CERC laboratory procedures (Stafford, Ray, and Jones, 1973). 

 Before use, a calibration in still water ensured that the electrical 

 gage had a suitable linear sensitivity through the expected range of 

 water level. After use, the probe was again dipped in still water to 

 ensure there was no appreciable drift of the gage datum or change in 

 sensitivity. There were no major differences in using the resistance 

 or inductance principles of water level measurement; the exact electron- 

 ics are unimportant. Figure A-1 shows an electronic signal -conditioning 

 unit, a strip-chart recorder, and a standard water level probe. Hori- 

 zontal cross sections of all the electrical probes used are shown in 

 actual size in Figure A-2. 



White and Miller (1971) investigated the physical considerations in 

 an accurate measurement of fluid level by a surface-piercing probe. 

 They determined that the water and probe surfaces must be kept clean, 

 and that the diameter of the probe and the static meniscus between probe 

 and fluid should be minimized. A probe of minimum diameter approximates 

 a needle piercing the fluid surface, rather than a bluff cylinder; this 

 minimizes the possibility of erroneous measurement due to significant 

 flow obstruction by the probe. James (1974) presented a solution for 

 the shape and height of the static meniscus occurring at an extremely 

 thin circular cylinder due to interfacial tension. For probes of about 

 0.01- to 0.1-inch (0.3 to 3 millimeters) diameter (see Fig. A-2) in 

 water, the effects due to the static meniscus and flow obstruction 

 should be minimal. 



Confinement of a probe within a narrow channel was observed to have 

 a significant effect on its electrical characteristics, but each probe 

 used within a channel was statically calibrated while mounted there. 

 Significant measurement error may arise from the static probe calibra- 

 tion. Water draining off the probe may not exactly follow the water 

 surface; this can be significant when the water level is fluctuating 

 rapidly. Dean and Ursell (1959) reported errors in using static cali- 

 bration factors in wave measurements with a surface-piercing gage. 



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