Short wave periods are expected to maintain a higher mean suspended- 

 sediment concentration in the surf zone, because there is less time between 

 waves for the sand to settle out. However, the data suggest that a detailed 

 analysis beyond the level of this report is needed, either to prove or 

 disprove that hypothesis. This is due to the relatively small variation 

 in wave periods observed and the confusing effects of the other variables. 

 In some cases where the expected relation between C and T is apparently 

 present, variations in other variables occur which could equally explain 

 the relation. 



Breaker type was classed as spilling (sp) , spilling-plunging (spp), and 

 plunging (p); these data are tabulated in Appendix A. Although breaker 

 type is dependent, at least in part, on wave height and period (Galvin, 

 1968), it is clear from this study that breaker type is important in clas- 

 sifying suspended-sediment data. 



Figure 26 shows suspended-sediment concentrations, one set of data for 

 each of 3 days, as a function of breaker type and distance from the breaker 

 point. For these data, the highest concentration occurs with the plunging 

 waves near the breaker line. The lowest concentrations occur for spilling 

 breakers inshore of the breaker. 



5. Sources of Scatter . 



The figures in this report show that there are few clear trends between 

 variables. In part this may be due to errors in measuring and processing 

 the data; much of the scatter is due to the effects of other measured but 

 uncontrolled variables. However, it is the author's opinion that much of 

 the scatter is due to the difficulty of maintaining constant conditions 

 during sampling. In particular, scatter can be due to: 



(a) The unknown position of the nozzle with respect to the 

 points along ripples where significant sediment is being entrained. 

 Based on laboratory experience (Fairchild, 1956), it is believed 

 that variation in actual sediment suspension along a ripple crest 

 can cause C to vary by a factor of 5 or even 10. 



(b) The unknown position of the nozzle (vertical or hori- 

 zontal) with respect to ripple crests. Concentration is inversely 

 proportional to nozzle elevation above the bottom, with concen- 

 tration increasing rapidly near the bed (Fig. 19). Concentration 

 above a ripple crest may be four or five times greater than in 

 ripple troughs (Fairchild, 1959). Figures 18, 19, and 20 show 

 that most samples at both piers were taken within 1 foot of the 

 sand bottom. 



Some error occurs in identifying breaker position to establish the 

 point where S = 0. Errors also occur in measuring wave height and period, 

 but these are thought to be less important sources of scatter than those 

 due to the position of the nozzle relative to the suspended-sediment 

 sources . 



44 



