Electromagnetic current meters. The manufacturer's specifications state 

 that the electromagnetic current meters have an accuracy of +2 percent. The 

 range of the meter is +.5 V, which translates to +5 m/sec (CM1 to CM16) or 

 3 m/sec (CM17 and CM18) (the difference between the meters is due to dif- 

 ferent factory calibrations). The meters were deployed in groups of two 

 (CM 17 and CM 18) or four (CM1 to CM 16) using the close proximity option. 

 This reduced interference between closely spaced meters by driving all four 

 meters with one timing pulse. 



Experiment Procedures 



Sequence of events 



The sequence of a typical run was: 



a. Specify wave conditions and data collection parameters. The wave 

 conditions were either monochromatic (specified wave height and 

 period) or random (specified target zero-moment wave height H^, 

 peak period T p , and peakedness parameter 7 to generate a TMA spec- 

 trum). Data collection parameters included sampling frequency and 

 run length. Wave conditions and run lengths are listed in Table 1-2. 



b. Check gauges and water level. Checks were made of the gauges 

 (checking gauge voltages) and the water level. 



c. Position the carriage. The carriage with the current meters was posi- 

 tioned at a strategic location (e.g., in the expected incipient breaking 

 zone, adjacent to wall-mounted meters, in the trough, etc.). 



d. Begin wave action and data collection. One computer program con- 

 trolled both the wave generation and the collection of the resistance 

 wave gauge and electromagnetic current meter data. The data collec- 

 tion began as soon as the wave generation program began (although 

 wave action did not begin immediately) and ended when the wave 

 action ceased. Typically, wave action lasted 10 to 70 min. 



e. Lab notes. Notes were kept documenting significant features of the 

 run, e.g., breaker type and approximate position, instrument status, 

 and visual observations of sand ripples. 



/ Beach profile survey. At the beginning of the day and following each 

 run, the beach profile was surveyed (the post-run survey typically 

 served as the pre-run survey for the next run). Divers also measured 

 the elevation of the lowest current meter in each vertical array and 

 made any necessary meter adjustments. Current meter positions were 

 surveyed once a week and after major changes in configuration. 



Chapter 3 SUPERTANK Hydrodynamics 



59 



