Kashima (1965) developed a calibration curve for a 

 photoelectric concentration meter used in the labora- 

 tory; the instrument did not appear to disturb the 

 flow or suspended sediment concentration, but accuracy 

 of the meter was found to decrease at higher con- 

 centrations of sediment. Thornton and Morris (1977) 

 installed a series of instruments at Torrey Pines, 

 California, including a nephelometer designed to 

 measure the suspended sediment concentration at any 

 one of three elevations in the surf zone . As men- 

 tioned previously, three intake nozzles were also in 

 place to sample sediment -laden water and in situ 

 calibrate the nephelometer. The nephelometer was 

 designed with scattered and reference light detectors; 

 therefore, any fluctuations in the light source could 

 be accounted for by a ratio of the two values. 

 Unfortunately, unanticipated line loss in the power 

 cable meant that the excitation line voltage for the 

 nephelometer had to be varied to match the output 

 level of the telemetry /recording system. Absolute 

 calibration of the nephelometer was not possible. In 

 addition, Thornton and Morris (1977) note that the 

 nephelometer may have sensed entrained air in the 

 water column. Brenninkmeyer (1974, 1976) describes 

 the use of almometers in the surf zone. The instru- 

 ments consist of 64 photo-electric cells opposite a 

 high intensity fluorescent lamp; both parts are 

 encased in a plastic cylinder and mounted on poles 

 that can be placed up to 1 m apart. Leonard and 

 Brenninkmeyer (1979) installed two almometers at 

 Nauset Light Beach, Cape Cod, Massachusetts; data were 

 collected from 2400 and 0600 hours so that sunlight 

 could not affect the incident light sensed by the 

 photocells. Nakato et al . (1977) calibrated and used 

 the Iowa Sediment Concentration Measuring System 

 (ISCMS) (Locher, Glover, and Nakato 1976) in an oscil- 

 latory-flow flume. These types of optical instruments 

 have several disadvantages: (a) calibration is 

 dependent on grain size and may vary vertically; (b) 

 almometers using photocells are sensitive to sunlight; 

 (c) bubbles may be interpreted by the instrument as 

 suspended sediment; (d) water opacity and organic 

 material may influence the calibration of the instru- 

 ment; and (e) scour may be induced by the rods that 

 mount the source and sensor . 



(b) Optical Backscattering Sensors (OBS) 



An Optical Backscattering Sensor (OBS) radiates a cone 

 of light and detects the intensity of backscattered 

 light. The intensity of backscattered light is a 

 function of sediment concentration and grain size; 

 therefore, the instrument can be calibrated in the 



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