from an array of wave sensors. Sand transport volumes were usually determined 

 over a fraction of a tidal cycle as thB product of the width of the surf zone, 

 the longshore displacement of the center of gravity of the tracer , and the 

 thickness of tracer movement. The latter quantity was based on observations 

 of the depth to which a cylindrical "plug" of tracer had eroded over the obser- 

 vational period and depth of tracer in cores; depth values ranged between 2 

 and 10.5 centimeters. These measurements yielded a total of 14 data points, 

 and the average and standard deviation values of K for Silver Strand (4 data 

 points) are 0.77 and 0.18, respectively; the average sediment size was 0.18 

 millimeter. These values for El Moreno (10 data points) are 0.82 and 0.27, 

 respectively; the average sediment size was 0.60 millimeter. 



e. Channel Islands Harbor, California (Bruno and Gable, 1976). Data in 

 this report were based on the same general field program that is the subject 

 of the present report. Sediment transport rates were inferred from volumetric 

 accumulations behind the Channel Islands Harbor offshore breakwater, and the 

 longshore energy flux values were based on Littoral Environmental Observations 

 (LEO) . A total of 13 data points resulted with average K and standard 

 deviation values of 1.61 and 1.19, respectively. Sediment size was approxi- 

 mately 0.2 millimeter. These data were not used in the SPM sand transport 

 relationship. 



5. Discussion . 



The results presented above comprise a total of 37 data points which, 

 excluding Bruno and Gable's (1976) data, are summarized in Table 1 and 

 presented in Figure 3. Figure 3 represents the SPM sand transport rela- 

 tionship in the dimensionless form of the equation. Figure 4 attempts to 

 discern any effect of sediment size D on the quantity K. There is a 

 fairly reasonable relationship of increasing K with decreasing sediment 

 size, although there is only one data point for a sediment size exceeding 

 0.6 millimeter. 



In general, it appears that none of the data sets provide the confidence 

 that should be associated with both the wave and sediment volume data. In those 

 studies with well-established volumetric data, the wave data usually included 

 one or more visually estimated wave parameters (height and direction) . In the 

 reported tracer studies, it is believed that the inferred sediment transport 

 rates overestimate the actual transport. This expected bias is probably due 

 to the estimate of the shore-parallel displacement of the center of gravity of 

 tracer displacement being obtained from the upper 5 centimeters of the sediment 

 column, since shear transport must exist within the sand bed with the upper 

 layers moving most rapidly. Additionally, the use of a single value of trans- 

 port thickness based on the erosion depth of a tracer plug should increase bias 

 as the maximum erosion depth is expected to increase with time. Finally, the 

 longshore sediment transport in the surf zone is expected to be both spatially 

 and temporally variable although these scales are currently unknown. Since the 

 tracer studies represent longshore sediment transport over only a part of a 

 tidal cycle, it is surprising that the studies do not exhibit greater scatter, 

 i.e., standard deviations of 23 and 33 percent for the Silver Strand and El 

 Moreno K values, respectively. One advantage of a relatively long-term, com- 

 plete sediment trap is that it integrates the temporal and spatial variability, 

 thereby providing a good basis for investigating the mean structure of the trans- 

 port phenomenon before attempting to measure and understand the fine structure. 



