higher efficiency was attributed to collection of suspended 

 material in the sampler. For material with a diameter 

 greater than 16 mm, the sampler was 70 percent efficient. 

 Emmett (1980) recommends that the sampler not be used for 

 material finer than 0.25 mm and at sites where bed-load 

 material can also be transported as suspended sediment. 



(4) Slot or pit samplers 



Slot or pit samplers consist of an excavated region in the 

 stream bed from which sediment collected during a time 

 period can be retrieved and the bed -load transport rate 

 estimated. A recent use of this sampling method was made 

 by Emmett (1980) to calibrate the Helley-Smith sampler in 

 the field. Emmett used a series of eight gates above a 

 conveyor belt built into the river bed (Figure All) and 

 therefore was able to continuously measure the quantity and 

 size distribution of the bed-load material. This sampling 

 method has been shown to have a sediment- trapping 

 efficiency of 100 percent if the slot widths are from 100 

 to 200 times the grain diameter (Einstein 1944). Other pit 

 samplers developed for riverine use include the Birkbeck 

 Bed-load Sampler (Reid, Layman, and Frostick 1980), the 

 Waslenchuk portable pit sampler (Waslenchuk 1976), the 

 vortex tube system permanently installed at Oak Creek, 

 Oregon, and the conveyor belt system permanently installed 

 at East Fork, Wyoming (Klingeman and Emmett 1982). Pit 

 samplers that use an excavated region to collect sediment 

 are considered the ideal type of sampler because they do 

 not introduce apparatus that may disturb the flow regime. 

 However, this type of sampler is usually costly to install, 

 site permanent, and requires periodic emptying. The 

 Birkbeck Bed-load Sampler (Reid, Layman, and Frostick 1980) 

 consists of a sediment-collection box inside a precast 

 concrete liner. The inner box is free to move up and down 

 inside the liner, and rests on a rubber pressure cushion 

 that responds to the continually varying masses of water 

 and collected sediment (Figure A12). River stage data can 

 be collected simultaneously with the variation of fluid 

 pressure in the cushion; therefore, instantaneous differ- 

 ences in cushion pressure head can be related proportion- 

 ally to the mass of sediment in the trap. The sampler is 

 recommended for coarse-grained alluvial channels because of 

 the tendency for finer grained materials to settle on the 

 pressure pillow and inhibit function of the device. Six of 

 the samplers were installed at Turkey Brook in England. 

 Bed-load sediment transport values corresponded well with 

 predictions from the Meyer -Peter and Muller bed -load 

 transport equation. The primary advantage of the Birkbeck 

 Bed-load sampler is that sediment transport can be contin- 

 ually recorded. The Waslenchuk portable pit sampler 



A17 



