ous runs. In tracer experiments performed within the surf zone, multiple 

 injections were used with different colors injected into the beach face versus, 

 for example, the offshore bar. 



Experiment procedure 



In the offshore experiments, the tracer sand was injected in a narrow line 

 across the width of the wave channel. This was accomplished by a diver 

 using SCUBA gear so that the tracer emplacement would be in the prescribed 

 position and would have an even distribution across the channel width. Just 

 prior to injection, a large sample of the bottom sediment was collected to 

 determine the quantity of "background" tracer that might remain from earlier 

 experiments. In the experiments within the surf zone, the injection was made 

 on a line across the channel or sometimes at a point on the midline of the 

 channel, generally on the beach face or over the offshore bar. This was ac- 

 complished by wading. In all cases the injection operation was completed 

 during still-water conditions prior to wave generation. The tracer was placed 

 in plastic bags so that it could be pre-wetted prior to injection, and a small 

 quantity of liquid dish soap was added to reduce grain surface tension which 

 can cause the particles to float to the water surface or to adhere to one another 

 in clumps. 



After an interval of wave generation and sand movement, the wave genera- 

 tor was turned off and samples were obtained on a grid along the length of the 

 midline of the wave channel. This procedure ensured that all samples would 

 be synoptic with respect to the sand dispersion processes. In the offshore 

 experiments, a SCUBA-equipped diver systematically sampled the bed 

 (Figure 5-1). The diver was guided by a weighted line, to which sample vials 

 were attached, that was lowered to the beach profile (Figures 5-2 and 5-3). 

 The vials were labeled to identify the sample-grid locations. The grid varied 

 somewhat between experiments and within the series of sample runs in each 

 experiment, governed by the energy of the waves and the expected tracer 

 dispersal distances. In general, the grid samples were obtained at 0.5-m 

 increments close to the injection line (+ 3 m from the injection line) and 

 spaced at 1-m increments at greater distances. All samples were taken by the 

 same person, who attempted to sample consistently to a depth of 1 cm into the 

 bed. The inshore samples were obtained on a grid marked along the wall of 

 the wave channel. In the inshore, sampling was accomplished by wading. 



Five offshore sand-tracer experiments were completed. Table 5-1 summa- 

 rizes the dates of the offshore experiments, including the colors and quantities 

 of tracer used and the sampling times. The A/-values given in Table 5-1 are 

 time intervals after injection for the first sample run and the time intervals 

 between subsequent sample runs. Ideally, each experiment would consist of at 

 least three to four sample runs so that dispersion of the tracer with time could 

 be documented. This was accomplished in four of the five experiments, but 

 the fifth (28 August) offshore tracer experiment was terminated after two sam- 



Chapter 5 Tracer Measurements of Sand Dispersion 



95 



