if the data are averaged below and above a midflow speed equal to 60 cm/sec 

 (approximate starting flow speed for sheet flow) . The quantity of sand 

 collected in the second basin increases from 1.3 percent for midflow speeds 

 below 60 cm/sec to 2.0 percent for flows greater than 60 cm/sec. Similarly, 

 the quantity of sand collected in the third basin also increases from 0.6 to 

 0.7 percent as the data are averaged below and above 60 cm/sec, respectively. 



79. The expected tendency for grain size to decrease in the downflow 

 basin was observed in the analyzed basin sand samples. For samples taken 

 after a test with midflow speed equal to 68.7 cm/sec, median grain size 

 decreased from 0.32 mm in Basin 1 to 0.22 and 0.21 mm in Basins 2 and 3, 

 respectively. Since the basins used in this experiment program closely repli- 

 cate all characteristics of an ideal basin, it is concluded that the basins 

 were efficient. 



Method of data analysis 



80. The method for obtaining the sand transport rate for a particular 

 trap deployed in the surf zone is to integrate the nozzle predicted fluxes 

 (weight per unit area per unit time) through the sampling depth. The flux 

 between nozzles is estimated by linearly interpolating from adjacent nozzle 

 fluxes (see Part II). Because of the existence of eddies in the surf zone 

 caused by injection of turbulence from the water surface, suspended sediment 

 is typically more homogeneous through the water column than was observed in 

 the laboratory. Linear interpolation of fluxes between nozzles does not 

 appear reasonable for the laboratory measurements, since sand fluxes for 

 adjacent streamers varied by more than an order of magnitude. The total sand 

 flux at an elevation above approximately 5 cm from the bed in the laboratory 

 tests was, at the most, an average of 4.3 percent of the sand flux at the 

 bottom nozzle (Table 5). Therefore, linear interpolation for fluxes between 

 streamers would introduce significant error into the calculations. 



81. The 4.3 percent of sand flux occurring above 5 cm is probably lower 

 in value, based on qualitative observations of nozzle performance. The per- 

 centage was calculated by totaling the upper nozzle sand flux, dividing that 

 quantity by the bottom nozzle flux, and multiplying by 100 to obtain a per- 

 centage. Because of scour that occurred at the bottom edge of SUPERDUCK 

 bottom nozzles during some flow conditions, the bottom nozzle flux may be too 



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