For the two days of tests dedicated to investigating the sand cloud created 

 under plunging breakers, all 33 OBS and four EMCMs were attached to a 

 frame that was suspended underneath the carriage. Four stacked arrays of 7 

 to 9 sensors were spaced at intervals of 0.75 m, and the wave gauge was 

 installed adjacent to the shoreward-most array. 



Instrument characteristics 



The specifications for the OBS in the mobile array are the same as those in 

 the stationary OBS arrays described in Chapter 6. Signals from the five 

 sensors in the mobile OBS array were recorded on the same system used for 

 the stationary arrays. 



For the wave gauge, zero and span adjustments allow this instrument to be 

 used under a wide range of conditions. The analog output signal can be 

 adjusted to a maximum of +. 5 V, and the still-water level can be set to read 

 V. The capacitance wave gauge output was transmitted to a personal com- 

 puter (PC) operated by the researchers from the U.S. Naval Academy (Chap- 

 ter 4). The computer contained a DAS-16 analog-to-digital (A/D) conversion 

 board. The voltage data output from the capacitance wave gauge, collected at 

 a sampling rate of 16 Hz, was immediately converted to free surface elevation 

 (referenced to still-water level) and saved as a time series in ASCII format. 



Experiment Procedures 



Sequence of events 



Before each run began, a cross-shore position was selected for the sensors 

 on the carriage. Some of the factors that determined the positioning of the 

 carriage were: 



a. Breaker height. 



b. Breaker type. 



c. Wave period. 



d. Shape of the bottom profile. 



e. Objectives of the upcoming test. 



Once the carriage was in position, the OBS array was lowered until the 

 bottommost sensor was a few centimeters from the bed. All necessary elec- 

 trical connections were made and the parameters characterizing the run were 

 recorded in a logbook. 



1 26 



Chapter 7 Sediment Suspension Measurements from a Platform 



