SECTION 



Figure A8 . Polyakov pan bed-load sampler 

 (from Hubbell 1964) 



efficiencies ranging from 0.38 to 0.75. Certain versions of 

 the sampler were observed to mound bed -load material in 

 front of the ramps, thereby decreasing the sediment- 

 trapping efficiency. 



(3) Pressure -difference samplers 



Pressure -difference samplers are usually composed of a 

 rigid expanding nozzle attached to a removable mesh bag in 

 which sediment collects. The concept of these samplers is 

 to compensate for changes in flow resistance as might 

 result from the collected sediment and sampler by con- 

 structing the sampler walls such that they diverge towards 

 the rear. The larger exit area creates a pressure decrease 

 which increases the fluid/sediment speed at the entrance of 

 the sampler. A removable mesh bag is attached to the 

 nozzle to allow collection and removal of the sample. The 

 pressure-difference sampler is the only direct-measuring 

 bed-load device that is extensively used in the U.S. One 

 of the earliest pressure-difference samplers and the 

 precursory device to the Helley- Smith sampler (a popular 

 present-day pressure-difference sampler) was the Arnhem or 

 Dutch sampler (Figure A9) (Hubbell 1964). This sampler is 

 held next to the bed and directly into the flow with a 

 large frame. The sampler itself is composed of a rigid 

 rectangular expanding entrance nozzle and a 0.2- to 0.3 -mm 

 removable mesh bag. The sediment- trapping efficiency for 

 the Arnhem sampler is approximately 0.70 (Meyer-Peter 

 1937). Pressure difference samplers recently developed for 

 riverine bed- load measurements include the "VUV" sampler 

 (Novak 1957, Lee 1975, Pickrill 1986) and the Helley- Smith 



A13 



