laboratory using sediment at the field site. Jaffe, 

 Sternberg, and Sallenger (1984) used a vertical array 

 of OBS sensors originally described by Downing et al 

 (1981) mounted on a sled that moved through the surf 

 zone at Duck, North Carolina. Downing (1984), Stern- 

 berg, Shi, and Downing (1985), Hanes and Huntley 

 (1986), Sternberg et al . (1986b), Huntley and Hanes 

 (1987), and Beach and Sternberg (1987) all used either 

 OBS or Miniature Optical Backscatter Sensors (MOBS) in 

 the nearshore or surf zone to measure concentrations 

 of suspended sediment. Sternberg, Shi, and Downing 

 (1985) calibrated the instrument before and after the 

 field experiment in a calibration tank using sand from 

 the site; the instrument response was linear from 0.1 

 to 150 parts per thousand (ppt) concentration. The 

 introduction of air bubbles in the calibration tank 

 varied the calibration very little. However, suspen- 

 sion of fine sand in the field due to storm drain 

 flooding during a storm increased the background 

 signal levels and obscured the suspension of nearshore 

 sand. Sternberg et al . (1986a) used OBS sensors to 

 measure silty suspended sediment in the bottom boun- 

 dary layer of a tidal channel within San Francisco 

 Bay, California. The OBS and MOBS sensors can provide 

 an instantaneous and continuous measurement of the 

 suspended sediment concentration in the nearshore and 

 surf zone. The instruments are less sensitive to air 

 entrained in the water column than transmissometers . 

 However, disadvantages associated with the OBS sensor 

 include: (a) the instrument must be calibrated with 

 the grain size of the suspended sediment; (b) if left 

 in the field for an extended period of time, organic 

 growth on the sensors must be removed; (c) fluid 

 velocities must be known at the sensor elevation to 

 calculate sediment transport; and (d) scour may be 

 induced around the sensor mounts . 



(c) Radiation Absorption Meter 



Basinski and Lewandowski (1974) describe a radiation 

 absorption meter that determines the concentration of 

 suspended sediment in the water column by the absorp- 

 tion of gamma radiation. The apparatus consists of a 

 radioactive source, counter, photomultiplier , input 

 amplifier, discriminator, and output amplifier. The 

 calibration of the instrument depends on the suspended 

 sand concentration, natural gamma radiation, the 

 seawater chemical composition, and the chemical compo- 

 sition of sediment. During use of the instrument in 

 the Baltic Sea, electrochemical corrosion occurred on 

 the internal surface. This instrument has not been 

 widely used. Disadvantages of the device include: 

 (a) fluid velocity must be known at the point of 



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