whereas Thornton and Morris monitored a single elevation with a 

 nephelometer. Although the almometer seemed to have potential for a 

 breakthrough in the field of sediment transport measurement, it was difficult 

 to calibrate, was not responsive to the full range of sediment concentrations 

 observed in the surf zone, and has not been used in the field by others. 

 Thornton and Morris reported that peaks in the suspended sediment spectra 

 occurred at approximately twice the frequency of peaks in the velocity spectra, 

 indicating two or more maxima per wave period. They also concluded that 

 suspended sand concentration decreased exponentially with height above the 

 bed and that the rate of decrease and total concentration were related to the 

 mean bed shear stress. 



Several years later, Downing (1984); Sternberg, Shi, and Downing (1984); 

 and Jaffe, Sternberg, and Sallenger (1984) used Optical Backscatter Sensors 

 (OBS) (Downing, Sternberg, and Lister 1981) and Marsh-McBirney 

 electromagnetic current meters to observe time-dependent fluctuations of 

 suspended sediment and fluid forcing, respectively. The combinations of 

 these two types of fast-response instrumentation allowed, for the first time, the 

 calculation of suspended sediment flux. These pioneering studies indicated 

 that suspended sediment re-suspension events were episodic and exhibited 

 considerable low-frequency modulation at wave group and edge wave 

 frequencies. Calculations by Jaffe, Sternberg, and Sallenger (1984) showed 

 that net suspended sediment flux was dominated by transport associated with 

 oscillatory velocities rather than mean currents. These results indicated that 

 the onshore movement of a bar crest could be accounted for by the net 

 movement of the suspended load. These exciting results were further 

 substantiated by Huntley and Hanes (1987) who indicated that the direction 

 and magnitude of suspended sediment transport were functions of frequency; 

 that is, net transport of sediment was a result of several competing processes, 

 mean flows, and incident-band and infragravity-band forcing. 



The SUPERTANK Laboratory Data Collection Project was guided by these 

 and subsequent field studies, and utilizes the approach of combining rapid, 

 synchronous measurement of suspended sediment concentration and fluid flow 

 to understand fluid-sediment interactions and to calculate net suspended 

 sediment flux. 



Objectives 



The primary objective of this investigation was to increase understanding 

 of cross-shore sediment transport processes in response to a wide variety of 

 offshore wave conditions. Establishing the degree to which suspended load 

 transport is responsible for observed beach profile change is of fundamental 

 importance. Addressing this single objective requires accurate measurement 

 of the time-varying concentration field, fluid forcing, and beach profile 

 change. The large wave channel at Oregon State University's O. H. Hinsdale 

 Wave Research Laboratory (WRL) provided an unparalleled opportunity to 

 control sensor elevations, cross-shore locations, and offshore wave conditions 

 to a degree unattainable in the field. 



Chapter 6 Suspended Sediment Concentration 



107 



