Figure 5-2. 



Weighted tracer sampling line with vials attached at fixed 

 intervals 



ters from the injection position. As presented in Table 5-3, the data immedi- 

 ately provide an impression of the dispersion of the sand tracer during the 

 experiments. It is apparent that generally during an experiment there is a 

 progressive decrease in tracer concentration at the injection line, with a corre- 

 sponding increase at grid-sample locations away from the injection site. This 

 decrease in concentration is produced by the progressive diffusion of the 

 tracer, presumably as it is lifted above the bed within suspension vortices shed 

 from ripples and subsequently drifts along the length of the wave channel. 

 The measured concentrations in Table 5-3 also reveal that, in most experi- 

 ments, there is an asymmetry in the concentrations about the injection line. 

 Generally more tracer was carried onshore than offshore, probably represent- 

 ing a net tracer advection, although an along-channel variation in the rate of 

 diffusion cannot be ruled out. 



Figure 5-4 shows examples of histograms for representative sample runs of 

 the data listed in Table 5-3. The histogram for 7 August, Run 13A, plots the 

 tracer distribution after 100 min of wave action following tracer injection. 



Chapter 5 Tracer Measurements of Sand Dispersion 



97 



