bias is when the nozzle axis is horizontal and parallel with the wave 

 crest. For the sampling equipment used in these tests, the nozzle was 

 oriented approximately perpendicular to the pier axis, which was usually 

 within 10° of the wave crest although it may have been as much as 20° to 

 25° away from parallelism with short-period wave crests. The nozzle is 

 approximately horizontal when the boom is at a standard angle of approxi- 

 mately 15° with the vertical. Deviations from the horizontal position 

 during the tests were slight, and are considered unim.portant . 



b. Measuring Nozzle Height . Two methods for measuring nozzle height 

 were devised. The first method, used in the Nags Head sam.ple collection, 

 consisted of two measurements made with each change of the nozzle height. 

 This method involved reading the sampler pipe angle, 6, and measuring 

 the displacement, R, of the sampler pipe above its minimum level when 

 the nozzle was at the ocean bottom. The value of R was determined 

 indirectly by scaling the motion of a point on the winch cable which con- 

 trolled the up or down motion of the sampler pipe. Therefore, in the 

 Nags Head samplings, the nozzle height above bottom^, E, is given by: 



E = R cos 6 . (2) 



For the Ventnor samplings, the tractor-mounted sampler was modified by 

 adding a sampler pipe stop (Fig. 6). The stop was added to prevent the 

 nozzle from clogging in the ocean bottom, and to establish a fixed initial 

 nozzle height above bottom [the initial nozzle height with 6 = 13.4° was 

 0.25 foot (7.6 centimeters)]. As an aid in obtaining nozzle elevation 

 quickly, graphs were developed to relate the initial nozzle height to 0, 

 and then to determine E from 9 and R. 



Samples were collected over a range of nozzle heights above the bottom 

 to determine the gradient of the concentration above the sediment bed. 

 Values of E ranged from the minimum of about 0.25 foot to the middepth 

 level which averaged 2.5 feet (0.76 meter) above bottom. Vertical spacing 

 between samples was approximately 0.2 foot (6 centimeters). 



3. Sand Ripple Effects . 



In addition to the vertical variation of suspended- sediment concentra- 

 tion, the concentration also varies in space and time because suspended 

 particles boil upward in clouds of particles when wave crests pass over 

 the sand ripple crests (Fairchild, 1959; Kennedy and Locher, 1972). As 

 observed in wave tanks, sources of the particle clouds appear to be ran- 

 domly spaced along the ripple crest, but whether these source locations 

 are purely random or not is not known. The following hypothesis offers 

 one explanation for these observations. Random locations of particle clouds 

 may result from flow separation and "continuity" effects imposed across the 

 flow by the upstream rippled bottom. In this way, constriction of the near- 

 bottom flow into zones of low ripple height may explain why particle clouds 

 are sometimes lifted above segments of ripple crests which appear smoother 

 than adjacent irregular segments. Those particles immediately below the 

 local maximum velocity in these constricted zones would be the first to be 



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