Scope 



The purpose of this chapter is to describe the collection and analysis of data 

 from the resistance wave gauges and electromagnetic current meters and to 

 document the analyzed data. The following sections describe the data collec- 

 tion and analysis procedures. Wave and current parameters resulting from the 

 analyses are given in Appendix C, which is contained in Volume II of this 

 report. 



Experiment Apparatus 



Instrument description 



Resistance wave gauges. Sixteen resistance wave gauges measured the 

 elevation of the water surface to determine mean water levels and wave pa- 

 rameters. The resistance wave gauges were fabricated by the staff of the 

 O. H. Hinsdale Wave Research Laboratory, Oregon State University (OSU) 

 (Dibble and Sollitt 1989), where the SUPERTANK project was conducted. 

 Each gauge was 7 ft (2.1 m) tall and consisted of two 0.027-in. (0.69-mm), 

 seven-strand stainless steel cables, spaced 2.5 in. (6.4 cm) apart. The resis- 

 tance gauges use the conductivity of the water to determine the water eleva- 

 tion. The resistance between the two wires is inversely proportional to the 

 depth of water. By monitoring the change in resistance, the water surface 

 elevation can be determined. Each gauge is driven by a constant amplitude 

 AC voltage source and the current drawn by the gauge is measured. By 

 Ohm's law, with the voltage held constant, the current is inversely proportion- 

 al to the resistance or directly proportional to the water elevation. Current 

 sensing is accomplished by measuring the voltage drop across a 1-ohm resistor 

 placed in series with the gauge. The conversion from volts to engineering 

 units is determined by calibration of the gauges, as described under "Experi- 

 mental Procedures." 



Electromagnetic current meters. Cross-shore and vertical water 

 velocities were measured with 18 Marsh-McBirney Model 512 electromagnetic 

 current meters. Each current meter sensor consists of a 1.5-in.- (3.8-cm-) 

 diam sphere, which contains an electromagnet. Four electrodes are equally 

 spaced around the sphere. The electromagnet generates a magnetic field, and, 

 as the water (a conductor) flows through the magnetic field, it produces a 

 voltage that is proportional to its velocity (Faraday principle). The voltage is 

 detected with the electrodes. A rod containing the instrument cable is attached 

 to the sphere, which in turn is attached to a flat metal mounting bar. 



56 



Chapter 3 SUPERTANK Hydrodynamics 



