Qualitative characteristics of bottom nozzle behavior in the sand environment, 

 particularly the potential for scour, were also observed. Part IV describes 

 the trapping efficiency tests and presents sand- trapping rates and efficien- 

 cies for each nozzle tested. 



Experiment Design 



Measurement of ambient sand transport rates 



56. An evaluation of nozzle sand-trapping efficiency requires either a 

 measurement or reliable prediction of the ambient sand transport rate occur- 

 ring over the range of flow conditions tested. Methods considered to measure 

 this rate include: a streamer large enough to fit at the rear of the tank to 

 collect all moving sand; a syphon concentration sampler to sample suspended 

 sand at various elevations; dyed sand to serve as a tracer for sand movement; 

 and a pit sampler. Appendix A gives a description of the latter three 

 methods. 



57. A streamer of cross section 0.76 by 0.76 m constructed to fit 

 tightly inside the rear of the tank and collect moving sand was tested and 

 rejected as a method to measure the total ambient sand transport rate. This 

 giant streamer presented a large obstruction to the flow and significantly 

 altered flow conditions as compared to the situation with only a single 

 streamer trap in the tank. It would have been extremely difficult to achieve 

 the same with- and without-large streamer flow conditions, and there was 

 concern about maintaining structural integrity of the giant streamer through- 

 out the tests. The giant streamer concept was therefore abandoned. 



58. Collection of a representative sand concentration with a syphon 

 concentration sampler involves positioning the intake tube exactly in the 

 direction of the flow and withdrawing the sample at the flow speed. This type 

 of sampler was not chosen because it would have been beyond the time and cost 

 limitations of this experiment to develop, test, and use, including prepara- 

 tion of a sand sample collection facility. Also, concentration methods do not 

 provide a direct measurement of the transport rate; rather the rate must be 

 calculated as the product of concentration and flow speed. Finally, bed-load 

 sand transport rates, expected to be the dominant mode in the unidirectional 

 flow tank, could not be measured by using this method. 



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