78 



runup limit and were seldom (or never) activated by wave action during a 

 particular test. 



Capacitance wave gauges were supported by an aluminum frame which 

 was attached to the west wall of the wave tank. This frame consisted of canti- 

 levered supports which extended 2 ft (0.61 m) out from the tank wall every 

 12 ft (3.66 m) at Stations 1 through 5. A 2-in. (5.1-cm) square aluminum 

 box beam was then placed on top of these cantilevered supports parallel to the 

 tank wall. This beam formed a continuous horizontal support for the wave 

 gauges that extended from just landward of Station 1 to just seaward of Sta- 

 tion 5. The centerline of this beam was located 1.94 ft (0.59 m) out from the 

 west tank wall. The top of this aluminum beam formed a convenient refer- 

 ence datum for swash measurements and was located at an elevation of 

 -0.63 ft (-0. 19 m) below the top of the wave tank wall. 



Instrument characteristics 



Capacitance wave gauges were designed and constructed at the Coastal and 

 Oceanographic Engineering Laboratory of the University of Florida. The 

 gauge design has been used for many years in the ocean engineering laborato- 

 ries at both the University of Florida and the University of Delaware. The 

 gauges are reliable, in that they have a linear response over almost the entire 

 length of the sensing wire and they maintain their calibrations over many days 

 without changes due to temperature or other factors. 



Output voltage from the gauges is proportional to the total capacitance of 

 the system. This is composed of the capacitance of the wetted sensing wire, 

 which is linearly proportional to the immersed length of the sensing wire, plus 

 the internal capacitance in the gauge electronics, which is based on installation 

 of three capacitors in series and in parallel with the sensing wire. During 

 preliminary tests, sensing wires with lengths of 3 to 6 ft (0.91 to 1.83 m) 

 were tested and internal capacitors were then selected to provide a desired 

 gauge sensitivity for all sensing wire lengths. 



The electronics housing of the wave gauges contained internal gain and 

 offset potentiometers that were preset based on bench-testing of the wave 

 gauges. The gauges were then connected to a power supply and signal condi- 

 tioning unit. This unit contained a 40-Hz filter for each channel to remove 

 high-frequency electrical noise from the signals. Each channel had separate 

 gain and offset potentiometers located in the power supply and signal condi- 

 tioning unit that were used to routinely adjust the sensitivity of the gauges and 

 the zero of the signals. 



Prior to deployment at SUPERTANK, all gauges were tested in the Hydro- 

 mechanics Laboratory at the U.S. Naval Academy. This regimen involved 

 static tests in which the gauge calibrations were checked over a period of up 

 to 2 weeks to ensure absence of long-term gauge drift. Tests were then con- 

 ducted in the Naval Academy large wave tank to evaluate the dynamic re- 



Chapter 4 SUPERTANK Swash Measurements 



