vertical and horizontal location without the sampler present V (in units of 

 distance per time). The hydraulic efficiency is commonly expressed as a 

 percentage: 



v t 



Ej, = tj— 100 (%) (5) 



38. Hydraulic efficiencies for all nozzles were first measured for two 

 flow conditions (midflow speeds V mid equal to 43 and 74 cm/sec); nozzles 

 found to have hydraulic efficiencies near unity were then tested in an 

 additional two flow conditions (V mid equal to 22 and 59 cm/sec). Water depth 

 during all tests was approximately 30 cm (see step c below). The minimum test 

 flow speed was chosen to be on the order of the threshold velocity for 0.25-mm 

 sand movement as estimated from the Shield's diagram (Vanoni 1977, p. 96). 



The maximum test flow was chosen as representing an upper longshore current 

 speed in the field in which traps and their operators could be expected to 

 function. 



Vertical profiles of flow speed 



39. Representative vertical distributions of flow speed for the four 

 flow conditions are presented in Figure 14. Because of the size of the flow 

 propeller (1 cm in diameter), the first flow speed measurement from the bed 

 was located at z = 0.5 cm, where z is the elevation measured from the bed. 

 Values for the shear flow speed U* and the representative bed roughness z 

 were calculated using the logarithmic law as follows: 



u(z) t— In 



(6) 



where 



u(z) = flow speed at elevation z 



k = Von Karman constant, taken to be 0.4 

 c - constant, taken to be zero 



28 



