Movable-Bed Laboratory Experiments Comparing Radiation Stress and Energy Flux Factor as Predictors of Longshore Transport Rate

Abstract

Three-dimensional movable-bed laboratory tests were conducted to compare radiaton stress and energy flux factor as predictors of the longshore sediment transport rate. The tests were performed in the U.S. Army Coastal Engineering Research Center's (CERC) Shore Processes Test Basin (SPTB). This report presents derivations of the radiaton stress and the energy flux factor, documents the experimental setup and procedure, tabulates most of the data and performs the data analyses. Many photos were taken during the tests; however, only a few were used in the report. The complete set of test photos is available from CERC's Coastal Engineering Information and Analysis Center (CEIAC). An analysis of the radiation stress and the energy flux factor shows that both predict longshore transport rate to comparable degrees. Approximately 70 percent of the variance of the transport rate about its means 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, the radiation stress has the advantage of being constant seaward of the breaker zone while the energy flux factor is not. This makes the calculation of the radiaton stress more convenient than the energy flux factor, which must be determined at the breaker line. On the other hand, the energy flux factor has the advantage of having the same units as the longshore transport rate, which means that the coefficient is dimensionless. The empirical coefficents are far from constant although they are 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 the coefficients will increase with the surf similarity parameter. The considerable scatter evident in Figure 28 can be partly explained by the relation between the empirical coefficients and the surf similarity parameter. The data in this report and past laboratory and field data are compared in Figure 28. The laboratory data generally predict lower values of the longshore transport rate for a given energy flux factor compared to the field data. Part of this trend can be explained by the differences in the surf similarity parameters, assuming the field data were collected under conditions of high surf similarity. Also, laboratory and scale effects probably contribute to the lower laboratory transport rate. The relative importance of these factors is suggested as a subject of future research.