researchers have found values of the Shields parameter equal to 1.5 for 

 inception of sheet flow in a large wave tank (Watanabe 1988). Therefore, the 

 laboratory tests indeed replicated field sheet flow conditions. 

 Development of testing procedure 



72. The development of a sand transport testing procedure that would 

 simulate longshore transport in the surf zone (although with the absence of 

 wave motion) and accurately test nozzle designs was more difficult than 

 anticipated. The first procedure attempted for both the trap-predicted sand 

 transport tests (nozzle tests) and the ambient sand transport tests (basin 

 tests) began by establishing an equilibrium flow condition with equilibrated 

 bedforms. The flow was then stopped and either the cloths tacked into the 

 basins or the trap placed at the test section. The flow was reestablished and 

 a test conducted for a specific time period. Finally, either the trap or the 

 basin cloths were removed from the tank and the collected sand weighed. 



73. However, a problem was encountered with this procedure. Sand 

 transport rates measured with the traps and basins were found to depend on the 

 relative location of the measurement device to transverse bedforms that 

 migrated along the test section. When a trap nozzle was placed just downflow 

 of a large sand ripple, the determined transport rate was much larger than 

 when the nozzle was placed at the crest of a ripple. At lower flow speeds, 

 ripples 10 cm in length migrated with a speed of approximately 0.6 cm/min; 

 therefore, a 17-min-long testing period would be required to measure sand 

 moving along a full ripple length. To observe the time variation of scour 

 near the bed- load nozzle, it was desirable to have a 5- to 10-min testing 

 interval on the order of the field tests. In addition, such bedforms moving 

 in the direction of flow are not characteristic of surf zone conditions. The 

 initial testing procedure did not result in reproducible measurements of both 

 trap-predicted and basin sand transport rates within the 5- to 10-min time 

 interval . 



74. A transport regime similar to the surf zone (flat-bed transport) 

 was found to occur for short time periods (5 min) if testing was begun from a 

 smooth, flat-bed condition. A second testing procedure was therefore 



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