(1) Transparency indicator 



As volume of suspended sediment increases, the transparency 

 of the water decreases. This type of instrument measures 

 the decrease in fluid transparency through which suspended 

 sediment concentration may be inferred. A disadvantage of 

 such a device is that it must be calibrated for every 

 situation because of the variability of fluid, sediment, 

 and lighting. However, such a device could be designed to 

 minimize flow disturbance, thereby accurately measuring the 

 quantity of suspended material. Coastal instrumentation 

 using this principle are discussed in detail in the 

 "Coastal Suspended Sediment Apparatus" section. 



(2) Resistance indicator 



This type of instrument measures the variation in resis- 

 tance as sediment passes through an aperture. 



(3) Ultrasonic indicator 



The attenuation of sound waves passing through a fluid- 

 sediment mixture varies as the quantity of sediment in- 

 creases and decreases. With calibration, this type of 

 instrument can indirectly measure the concentration of 

 suspended sediment. 



Bed -load samplers 



10. Methods available for measuring bed-load transport can be classified 



as either direct or indirect. These methods and the types of riverine 



samplers associated with each are discussed below. 



a. Direct-measuring bed-load samplers. These samplers consist of 

 a container or excavated region into which sediment moving along 

 the bed is deposited. Because of the cyclic nature of bed-load 

 transport as bedforms pass a fixed point, the quantity of 

 sediment collected with a bed-load trap depends on the location 

 of the sampler at the start of sampling relative to the bedform. 

 Four types of direct-measuring samplers for riverine use were 

 identified by Hubbell (1964) and Graf (1984), and are described 

 below. 



(1) Box or basket sampler 



Box or basket samplers consist of a rectangular frame with 

 screen material on all sides except the front and, possib- 

 ly, the top. These samplers rest on the bed and collect 

 sediment during a sampling period; then they can be retri- 

 eved and the sample retained for later analysis. An 

 example of a basket trap designed to collect large-grained 

 sediment is the Nesper sampler (Figure A7). This sampler 

 has a bottom of loosely woven iron rings that conforms to 

 the shape of the bed. Einstein (1937) measured sediment- 

 trapping efficiencies of this sampler ranging from 0.9 to 



All 



