MOVABLE-BED LABORATORY EXPERIMENTS COMPARING 
RADIATION STRESS AND ENERGY FLUX FACTOR AS PREDICTORS 
OF LONGSHORE TRANSPORT RATE 
by 
Phtltp Vitale 
I. INTRODUCTION 
Three-dimensional movable-bed laboratory tests were conducted to compare 
radiation stress and energy flux factor as predictors of the longshore sedi- 
ment transport rate. The tests were performed in the U.S. Army Coastal Engi- 
neering Research Center's (CERC) Shore Processes Test Basin (SPTB). This 
report presents derivations ‘of the radiation 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). 
II. EMPIRICAL RELATIONS 
The longshore transport data are related empirically to the two expres- 
sions representing wave conditions. One, radiation stress, is based on momen- 
tum flux, the other on energy flux. An important concept which is also used 
in the data analyses is the surf similarity parameter. 
1. Momentum Flux. 
The dependent variable studied here is the longshore transport rate caused 
by waves approaching the beach; therefore, the consequential momentum term is 
the onshore flux of alongshore momentum. The derivation of the term follows 
Longuet-Higgins (1970) which applies the concept of wave momentum flux to the 
generation of longshore currents. 
The coordinate system used is shown in Figure 1. The y-axis is along the 
shoreline, the x-axis is normal to the shoreline and positive shoreward, and 
the z-axis originates at the stillwater level and is positive upward. Using 
this system, the onshore flux of alongshore momentum is the flux of y-momentum 
in the x-direction, S,.. This term is one component of what is commonly 
called the radiation stress tensor. 
x Shoreline 
Figure 1. Coordinate system for momentum 
flux derivation. 
ll 
