’ 
Reference. 
~~ 
Figure 4. Photo of sand trap. 
two walls; this produces some complicated wave variability problems (eceg., 
see Fairchild, 1970). With no downdrift training walls, the reflected wave 
energy moves away from the beach area into the outer parts of the test basin 
where most of it is eventually dissipated by the rubble slope along the edge 
of the basin (Fig. 2). This, however, creates a problem with wave diffrac- 
tion. The energy of the wave leaving the generator spreads laterally into 
still water and gradually decreases the wave height toward the updrift end of 
the wave crest. 
To minimize the decrease in wave height over the test beach, it was 
designed using the diffraction diagram for a wave traveling past a semi- 
infinite breakwater from Figure 2-33 of the SPM. The period and angle used in 
the diffraction analysis were 3 seconds and 10°, respectively, since these 
values produced the maximum diffraction closest to the beach. The spreading 
of wave energy into the shadow of a breakwater is analogous to the spreading 
of wave energy into the area of the test basin downdrift of the generators. 
The diagram (Fig. 5) indicated that the alongshore length of the beach should 
be 7.62 meters. Most of the diffraction-caused decrease in wave height occurs 
over the downdrift concrete apron. 
Rubble, ranging in size from 7.62 to 15.24 centimeters, was placed at 
several locations in the basin to absorb wave energy and provide gradual 
slopes between the concrete aprons and the basin floor. The beach, sand 
traps, concrete aprons, and adjacent rubble were all built to the same shore- 
normal profile (Fig. 6). This profile was based on Chesnutt's (1978) long- 
term two-dimensional tests in which waves were run onto a sand beach to 
determine profile response. After superposing several of Chesnutt's (1978) 
18 
