108 



Scope 



During SUPERTANK, as many as 25 OBS for monitoring suspended 

 sediment concentration were deployed. Typically, these sensors were de- 

 ployed in vertical stacks of five sensors each, at five positions on a cross- 

 shore transect spanning the surf zone. Data for 166 experimental runs were 

 collected, encompassing a wide range of wave and breaking conditions. This 

 chapter contains background information on sediment concentration measure- 

 ment equipment and procedures employed during the SUPERTANK project 

 and provides coefficients for the conversion of the raw sensor output to scien- 

 tific units, instrument positions (horizontal - x, elevation - z), and basic statis- 

 tics of sensor output for all runs. 



Experiment Apparatus 



Instrument description 



Suspended sediment concentration measurements were made using OBS 

 which were first developed at the University of Washington and are presently 

 fabricated by Downing and Associates Instrument Company, Port Townsend, 

 Washington. The OBS used in this project had two configurations, but utilize 

 the same principle of operation. A typical sensor consists of a stainless steel 

 housing, 19 cm in diameter by 11 mm in length (Figure 6-1). It houses an 

 infrared emitting diode (IRED) with peak radiant intensity at 950 A, a silicon 

 photo-voltaic cell with peak spectral response at 900 A, and an appropriate 

 filter to limit transmittance from incident light. The optical components are 

 encapsulated in the sensor head with clear epoxy resin. When operating, a 

 scattering volume of approximately 1.3 cc is irradiated through a 5.6-mm- 

 diam aperture at the geometric center of the photo-detector by an IR-beam 

 with a half cone angle of 14 deg (0.24 rad). Backscattered radiation (110- 

 165 deg (1.92-2.88 rad)) from suspended solids is converted to photocurrent 

 by the detector oriented in a plane normal to the emitter beam axis and located 

 close to it (Downing, Sternberg, and Lister 1981). The quantity of 

 backscattered light is linearly proportional to the mass concentration of 

 sediment present (see following section for calibration results). 



Two OBS configurations were used during the project; sensors which are 

 pre-configured in vertical stacks and individual sensors. 



OBS: A01-A04, B01-B04, C01-C04. These three arrays of four 

 sensors each are the original OBS developed for the Nearshore Sediment 

 Transport Study (NSTS) in 1979 (Downing, Sternberg, and Lister 1981). The 

 four individual sensors are housed in a vertical stack with inter-sensor dis- 

 tances of 3, 3, and 5 cm for a total vertical coverage of 11 cm. Figure 6-2 

 shows one of these sensor arrays deployed during SUPERTANK. 



Chapter 6 Suspended Sediment Concentration 



