tinuous or near -continuous contact with the bed. The sura of the suspended and 

 bed-load quantities is termed the "total load." Bed-load transport is even 

 more sporadic than that of suspended load, varying cyclically with the move- 

 ment of bedforms (Ehrenberger 1931, Einstein 1937, Hubbell et al . 1987). 

 Therefore, short-term measurements of bed load are not likely to be represen- 

 tative of an average transport at a particular point. Development of bed-load 

 samplers lagged behind that of suspended load samplers for three reasons . 

 First, bed load is more difficult to sample than suspended load; the sampler 

 must rest on or near the bed and collect sediment without disturbing material 

 on the bed. Bed material is defined as that material which composes the river 

 bed and may have arrived there as the result of previous suspended and/or bed- 

 load movement. Second, samplers were usually developed to collect data on 

 specific rivers, and many highly utilized rivers have beds composed of finer 

 grained material, the majority of which is usually transported as suspended 

 load. Finally, designing a bed-load trap that collects only bed load (exclud- 

 ing suspended load) is difficult. 



4. Sediment samplers have traditionally been classified as either bed- 

 load or suspended load measuring devices; however, the boundary between 

 material moving as bed load and suspended load is not well defined and varies 

 with time, location, and nature of the material. Therefore, bed-load traps 

 may collect some suspended material , and suspended load samplers , if close to 

 the bed, may collect material moving along the bed. 



5. An ideal sediment sampler collects the quantity and size distribu- 

 tion of sediment flowing through a particular area during a time period 

 equivalent to the quantity and size distribution of sediment that would have 

 passed through that area in that time period had the sampler not been there. 

 An ideal sampler has the following characteristics: 



a. Shape and size that minimize flow disturbance. 



b. Flow speed at the intake equal to the ambient (undisturbed) 

 speed at that point. 



c. Proper vertical and horizontal orientation during the 

 sampling period. 



6. Two quantities commonly used in evaluation and calibration of 

 sediment samplers are the hydraulic efficiency E h and the sediment- trapping 



A2 



