124 



To address the second objective, a two-day abbreviated experiment was 

 conducted with an OBS matrix, fabricated using all 33 available sensors. The 

 OBS were mounted in four vertical arrays, spaced at 0.75-m intervals. Four 

 electromagnetic current meters (EMCMs), one for each array, the wave 

 gauge, and the video camera were also employed. The matrix was suspended 

 from the carriage, which was positioned at different locations in the zone of 

 initial breaking. Data were collected across a range of incident wave heights 

 and periods. 



Experiment Apparatus 



Instrument description 



Mobile OBS array. A stacked array of five OBS was used to detect 

 suspended sediments in the outer surf zone. The main components of an OBS 

 are an infrared emitter that irradiates the water near the sensor and a 

 co-located photoelectric cell that is filtered to detect only infrared radiation. 

 The emitter produces a beam with an elliptical cone shape which has half- 

 power points at 50 and 30 deg (0.87 and 0.52 rad) from the beam axis. Sedi- 

 ment particles within this conical region scatter the infrared radiation. That 

 which is reflected 140 deg (2.44 rad) or more from the beam axis (i.e., 

 backscattered) is received at the photoelectric cell (Downing, Sternberg, and 

 Lister 1981). 



The five OBS sensor heads were screwed into PVC cylinders, which were 

 back-filled with epoxy resin to provide a watertight housing. Electrical con- 

 nections were made from each sensor head to a neoprene jacketed multi- 

 conductor cable, which transmitted the voltage produced by the photoelectric 

 cell to a housing containing signal conditioning boards and a power supply. 

 Each OBS head must be paired with its own signal conditioning board, which 

 amplifies the signal for transmission, compensates for temperature, and 

 provides synchronous switching of multiple sensors. 



An aluminum sailboat mast of elliptical cross section (9.52 cm x 5.72 cm) 

 was used to deploy the mobile OBS array (Figure 7-1). Forty holes at 3.8-cm 

 spacing were drilled through the mast along the major sectional axis, so that 

 sensors would mount horizontally with only the sensor head protruding. The 

 sensor beam axis was oriented across the channel and set screws held each of 

 the sensor heads in their respective positions. Unoccupied holes were covered 

 with tape to provide a smooth surface. Cables from the sensors ran up the 

 inside of the mast to the electronics housing. 



The aluminum mast was mounted to the shoreward side of an existing 

 motorized carriage that spanned the walls of the wave channel, thus allowing 

 the cross-shore position of the sensors to be changed with relative ease. The 

 mast was held vertically by clamping it between two steel I-beams that were 



Chapter 7 Sediment Suspension Measurements from a Platform 



