The second observation is that most of the data fall beneath the SPM curve 
connoting low values of « Since the SPM curve is based on field data, 
mostly from Komar and Inman (1970), a possible explanation is that the field 
data were collected under conditions of higher values of &€ than those for 
the laboratory data. Kamphuis and Readshaw (1978) suggest that Komar and 
Inman's data were indeed collected under conditions of high Epe It seems 
reasonable to assume that the &€ values were also high. 
VII. SUMMARY AND CONCLUSIONS 
An analysis of the radiation stress, S,y» and the energy flux factor, 
Pop» shows that both predict longshore tranSport rate, Iz, to comparable 
degrees. Approximately 70 percent of the variance of Ip about its mean is 
explained by each term. There appears to be no major advantage in choosing 
one over the other to predict the longshore transport rate. However, Sy. 
has the advantage of being constant seaward of the breaker zone while Pop 
is note This makes the calculation of S§& more convenient than P»), 
which must be determined at the breaker lines On the other hand, Pop has 
the advantage of having the same units as Ip, which means that K is 
dimensionless. 
The empirical coefficients, K, and » are far from constant although 
is commonly assumed to be so in practice. Part of the variation of the 
coefficients can be related to the variation of the surf similarity parameter, 
—&, as shown in Figures 26 and 27. These figures show that K, and will 
increase with §. The considerable scatter evident in Figure 28 can be partly 
explained by the relation between the empirical coefficients and &. The data 
in this report and past laboratory and field data are compared in Figure 28. 
The laboratory data generally predict lower values of I for a given P 
compared to the field datas Part of this trend can be explained by the dif- 
ferences in the surf similarity parameters, assuming the field data were 
collected under conditions of high €.- Also, laboratory and scale effects 
probably contribute to the lower laboratory transport rates. The relative 
importance of these factors is suggested as a subject of future research. 
45 
