59. The use of sand tracer to predict sand transport rates is a labor- 

 intensive process. This method of measuring ambient sand transport rates was 

 rejected because of tedious data reduction, time constraints, and the expected 

 variability in repetitive tests. 



60. A pit sampler consists of an open area at the bed into which moving 

 sand falls either as contiguous bed load or descending suspended particles. 

 After some experimentation, three basins located adjacent to each other in the 

 direction of flow were implemented as a means of measuring the ambient sand 

 transport rate. The pit sampler, or catch basin, was constructed as a series 

 of three sections aligned in the direction of flow so that suspended load 

 would settle in the downflow basins and give an indication of the efficiency 

 of the pit sampler itself by the relative amounts of sand trapped in succes- 

 sive sections. An efficient sampler would presumably show a small amount of 

 material in the farthest downflow basin. The pit sampler method was chosen 

 for its simplicity and inherent nonintrusiveness to the flow. 



Test section 



61. The unidirectional flow tank used for the hydraulic tests described 

 in Part III was modified for the sand-trapping tests. A 15.2-m-long test 

 section consisting of a 3.0-m-long ramp, a 6.1-m-long sand transport testing 

 area that was 15.2 cm deep, and a 6.1-m-long pit (divided into three catch 

 basins) was installed in the tank (Figure 21). The test section was made of 

 2- by 6-in. (5.1- by 15.2-cm) wooden boards at the sides and cut plywood for 

 the ramps . The entire apparatus was tightly wedged into the tank and caulked 

 along the sides. The sand transport testing area served to contain the sand 

 used in the experiment and provided a reference volume to be filled with sand 

 and leveled prior to each run. Quartz sand with a median grain size of 



0.23 mm was used. 



62. Large sheets of monofilament sieve cloth identical to the streamer 

 cloth were secured to the bottom of individual catch basins using tacks. Sand 

 transport rates without the trap for a range of flow conditions were deter- 

 mined by the quantity of sand collected in the cloths placed as linings in 

 these basins over the sampling interval. 



44 



