ment flux. Published results of such analyses include Sternberg, Shi, and 

 Downing (1984) and Beach and Sternberg (1987). Chapters 6 and 7 of this 

 report document the OBS approach and measurements undertaken during the 

 SUPERTANK Laboratory Data Collection Project. 



There is a similar dichotomy of sediment transport measurements in the 

 offshore, that is seaward of the surf zone but at depths shallow enough for 

 active sediment movement by waves. Examples of the use of sand tracers in 

 this offshore zone include the field studies of Inman and Chamberlain (1959) 

 and Murray (1967), and the combined field and laboratory wave-channel 

 investigation of Miller and Komar (1979). In this offshore zone the sediment 

 movements are produced by the oscillatory wave-orbital motions over a bed 

 covered by ripples, and the sediment is placed in suspension by the vortices 

 that develop adjacent to the ripples and is periodically ejected above the bed. 

 A net advection of this suspended sediment can be produced by unidirectional 

 currents superimposed on the oscillatory wave motions or by asymmetries in 

 the onshore versus offshore components of the wave orbits. 



Sand tracers and OBS measurements provide somewhat different views of 

 the sediment transport processes as well as measurements of quantities of 

 sediment movement. Although direct comparisons need to be made between 

 the measured quantities of sediment transport from these two techniques, such 

 simple comparisons are not entirely valid and not particularly informative. A 

 more enlightened approach is to recognize that the two techniques compliment 

 one another in providing fuller documentation of sediment-transport processes, 

 and that simultaneous measurements using both sand tracers and arrays of 

 OBS will yield more complete understanding of sediment movements. It was 

 with this view that fluorescent sand tracers were used during SUPERTANK to 

 investigate the movement of sand in both the offshore and within the surf 

 zone. The results of the experiments document the dispersion of large 

 numbers of sand grains, results that could be important to the development of 

 models for the movement of nearshore sediments. 



Objectives of sand tracer measurements 



The objective of this component of SUPERTANK was to obtain mea- 

 surements of sand-tracer dispersal patterns in both the offshore and within the 

 surf zone, patterns that can be analyzed to determine cross-shore (along-chan- 

 nel) rates of sand advection and diffusion. Some experiments were designed 

 specifically to document the movement of sand between the bars and the beach 

 berm as part of measured profile response to changing wave conditions. 

 These latter experiments were primarily intended to provide qualitative infor- 

 mation, to be evaluated both by visually observing the tracer movement, for 

 example, as seen during overwash, and by tracer counting. 



Chapter 5 Tracer Measurements of Sand Dispersion 



93 



