measurement to calculate a sediment transport rate; 

 (b) calibration varies with grain size, and grain size 

 may vary vertically in the water column; and (c) scour 

 around the mounts may increase suspended sediment 

 concentrations . 



(2) Acoustic instruments 



Acoustic instruments measure the absorption or backscatter 

 of sound due to sediment entrained in the water column. 

 The acoustic concentration meter (ACM) (Young et al. 1982, 

 Tamura and Hanes 1986, Hanes and Vincent 1987, Hanes et al . 

 in press) senses the intensity of backscattered acoustic 

 energy and can measure the suspended sediment profile up to 

 200 cm away from the instrument with 1-cm spatial resolu- 

 tion and 0.5 -sec temporal resolution. In addition, the 

 instrument measures the location of the bed within 0.5 cm. 

 The instrument works well if air is not entrained in the 

 water column; bubbles are obvious by spikes in the signals 

 (Hanes et al . in press). An ultrasound doppler meter (Jan- 

 sen 1978) measures the sediment concentration and fluid 

 velocity in the water column. Acoustic instruments are 

 useful for continuous measurement of vertical suspended 

 sediment concentrations. Disadvantages of the instrument 

 include: (a) bubbles, non-sand particles, and organisms in 

 the water column cause scattering of sound, thereby alter- 

 ing the instrument's calibration; (b) calibration of the 

 instrument is dependent on grain size; and (c) a low sig- 

 nal -to -noise ratio exists near the bottom, creating a 1-m 

 "dead zone" between the instrument and bed. 



(3) Electrical resistance meters 



Hattori (1969, 1971) describes the use of electrodes on the 

 heads of two mounts to detect the number of suspended 

 sediment particles passing through a slit. Electrical 

 resistance between the mounts varies as the concentration 

 of material increases or decreases. Horikawa, Watanabe, 

 and Katori (1983) used an electro-resistance detector close 

 to and within the bed in a laboratory study with sheet-flow 

 to measure the near -bed suspended sediment concentration 

 and bed-load transport. The instrument was calibrated in a 

 cylindrical tank, and instrument response with concentra- 

 tion was linear. These types of instruments have several 

 disadvantages: (a) because of their fragility, they can 

 only be used in the laboratory; and (b) material other than 

 sediment will obscure results. 



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