EXPERIMENTS AND RESULTS
Total Longshore Transport Rate
The first step of each experiment was to determine the distribution of wave-induced longshore
current and to properly adjust the re-circulation pumps. Visser (1991) determined from laboratory
experiments that if the re-circulated (pumped) currents either exceeded or were less than the wave-
driven currents, an internal current would develop and re-circulate within the offshore portion of the
basin. Visser also found that as the pumped currents approached the proper (wave-driven) current,
the internally re-circulated current was minimized. Therefore, it was desired to match wave-driven
currents with pumped currents. Initial pump settings were based on results of the numerical models
NMLONG (Kraus and Larson, 1991) and NEARHYDS (Johnson, 2003). The iterative approach
described by Hamilton and Ebersole (2001) and Hamilton, et al. (2001) was used to determine the
optimum pump settings. After the beach had evolved to an equilibrium or quasi-equilibrium profile,
and the pumped currents matched measured velocities, experiments on longshore transport rate were
initiated.
Irregular incident wave conditions were designed to obtain and compare longshore transport
rates for different breaker types. Four conditions generated in the LSTF are listed in Table 1, where
Hmo is the energy-based significant wave height measured near the wave makers. Each test was
conducted with a water depth, h, of 0.9 m at the wave generators.
Table 1. Longshore Sediment Transport Experiment Wave Conditions
θb
Experiment
Breaker
Hmo
Hsb
Tp
h
Number
Type
m
γb
deg
m
m
s
mb
1
Spilling
0.25
0.26
1.5
0.9
6.5
0.57
0.035
3
Plunging
0.23
0.27
3.0
0.9
6.4
0.96
0.023
5
Spilling
0.16
0.18
1.5
0.9
6.7
0.29
0.021
6
Plunging
0.19
0.21
3.0
0.9
6.4
0.58
0.023
Determination of Hsb for irregular waves is somewhat subjective. In the present study, the main
breaker line was determined as the location landward of which significantly accelerated rate of
wave-height decay was measured. This criterion was based on the comprehension that a dramatic
wave-energy loss, and therefore, wave height decrease, should follow dominant wave breaking.
Visual observations during the experiments supported the above determination.
Wave height distribution and the average profile associated with each experiment are shown in
Figures 2 through 5. Waves broke by spilling during Test 1, and Figure 2 shows a gradual decay of
wave height through the surf zone, with incipient breaking occurring approximately 13 m from the
shoreline. The beach profile is near planar inside the surf zone. Figure 3 shows the distribution of
wave heights and profile associated with Test 3. Test 3 waves shoaled to a height of 0.27 m and
sharply decreased at approximately 13 m from the shoreline. Wave heights are similar to those of
Test 1 in the inner surf zone. Test 3 waves broke by plunging as evident by the breakpoint bar and
trough formed at approximately 11 m. Test 5 waves show a very gradual decrease across the surf
Smith et al
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