sample most nearly representative of the true suspension. Consequently 

 certain preliminary samples were collected using different nozzle 

 orientations (Figure 10) in order to determine the most suitable 

 orientation. It was believed that the inherent nature of the forces 

 accompanying the wave action and the resulting complexity of flow 

 disturbance with any type of nozzle or orientation precluded any clear- 

 cut selection of the best nozzle orientation (or shape). Prom close 

 observations of the various criteria presumably influencing the accuracy 

 of the several nozzle orientations and the analyses of preliminary 

 sampling tests it was decided to use in the tests a nozzle intake 

 orientation directed normal to the oscillatory (stream) flow. 



ANALYSIS OF RESULTS 



Sediment Concentration . The suspended sediment concentration is 

 generally defined as the percent by weight of sediment in a given volume 

 of fluid- sediment mixture, flowing through some specified cross-section 



of area. If we assume a uniform concentration for successive cross- 

 sections along and perpendicular to the flow, then the average and the 

 instantaneous concentrations would everywhere be equal. While this 

 may occur under carefully controlled ideal conditions for unidirectional 

 flow, it does not for pulsating or reversing flow — as with waves. 

 Hence any concentration figure represents a certain average value. 



The sediment concentration by weight as reflected by an individual 

 sample was determined from the net weight of the sample and the measured 

 weight of the water and sediment collected. Figures 11 and 12' present 

 experimental curves showing the relation of average suspension concen- 

 trations to such parameters as wave height, depth, velocity, etc. 



The value of a sediment sampling process depends largely on its 

 reproducibility under identical sampling conditions. The curves of 

 Figures 11 and 12 showing the variability of sediment concentration 

 versus elevation above bottom show relatively poor consistency for the 

 same wave conditions. Care was exercised to maintain constant con- 

 ditions as to wave parameters and pump-sampling procedure, but even 

 with close controls, the cloudlike formations exhibited by the suspended 

 particles varied considerably with time. 



Sediment Concentration and Local Bottom Irregularities . Visual 

 observations of the clouds of suspended particles with the interspersed 

 voids or areas of minimum suspension provided some explanation for the 

 wide scatter of the sampling data. The observations revealed that the 

 clouds and voids (areas of minimum suspension) were constantly shifting 

 about. The shifting was gradual and in two directions; one was along 

 the direction of wave travel concurrent with the shoreward movement of 

 the sand ripple; the other was along the length of the sand ripple and 

 coincided rather closely with local changes of ripple height and curvature, 

 The sand ripple movement along the direction of wave travel was estimated 

 to range from 1 to 1^ inches for the sampling period which averaged 100 

 seconds* 



52 



