PART IV: SAND-TRAPPING EFFICIENCY TESTS 



53. The hydraulic efficiency tests described in Part III were designed 

 to quantitatively determine the degree to which nozzles and streamers dis- 

 turbed the fluid flow. However, sand particles moving on or off the bed may 

 behave differently than fluid particles, possibly moving at lower speeds and 

 having different paths of movement. For example, the velocity of a sand 

 particle has a constant vertical component (fall speed). A moveable bed has a 

 different roughness than the grass mat used in the hydraulic tests, resulting 

 in a different time -varying vertical profile of flow speed as the bed surface 

 changes. In addition, an apparatus installed on the bed to collect bed load 

 may cause scour, thereby artificially increasing or decreasing the local 

 transport rate. Therefore, an experiment was conducted to measure the sand- 

 trapping efficiency of nozzles placed on the bed or completely in the flow off 

 the bed to provide an indication of how well the nozzles predict ambient sand 

 transport. 



54. Sand- trapping characteristics of selected streamer trap nozzles 

 were evaluated in the unidirectional flow tank described in Part III . Nozzles 

 found to have near-optimal hydraulic efficiencies, as well as nozzles with 

 characteristics designed to decrease scour at the bed (straight and curved 

 bottom lips), were examined in the sand-trapping efficiency tests. The H-S 

 sampler, a riverine sampler discussed in Part III and Appendix A, was also 

 evaluated for comparison. 



55. The purpose of these tests was to quantify the sand- trapping 

 efficiency of nozzles with previously determined near-optimal hydraulic 

 efficiencies over a range of flow speeds and bedforms representative of the 

 surf zone. Sand- trapping efficiency E s is defined as the ratio of a trap- 

 predicted sand transport rate q t (weight per unit width per unit time) to 

 the ambient sand transport rate q (weight per unit width per unit time) 

 that occurs for the same flow condition without the trap: 



q t 



E s (8) 



q Q 



42 



