The last few experiments (6K through 6N) focused in more detail on the
offshore tail of the distribution, which proved to be more difficult to tune.
Figure 60 shows the results from Test 6N, the fifteenth and final iteration. Peak
currents at the three primary transects were relatively uniform, and the degree of
uniformity along much of the beach increased noticeably. In the offshore tail,
agreement between pump settings and measurements was the best observed in
any prior experiment. Current measurements in the offshore tail always were
slightly greater than the pump settings, because of the internal recirculation. The
magnitude and extent of the small flow reversal region near the shoreline at the
upstream end of the beach was significantly less, and the longshore current
measured closest to the shoreline at Y35 was now directed downstream (positive
values). However, the flow reversal near the shoreline was still present farther
upstream. The flow reversal appeared to cause the higher current magnitude
measured immediately offshore of the peak at Y35. The internal recirculation, Qr
was the lowest that had been observed during all previous tests, based on the
measurements at the ADV furthest offshore. Therefore, it was concluded that the
pump settings used during Test 6N were the proper settings, and they produced
the highest degree of uniformity of longshore currents in the surf zone.
Additional measurements were made offshore at the Y27 transect to more
accurately quantify the magnitude of the internal recirculation in the offshore
region. They are discussed in more detail in the next subsection.
0.50
Measured at Y19
0.45
Measured at Y27
0.40
Measured at Y35
0.35
Pump Settings
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
0
2
4
6
8
10 12 14
16
18
20
Cross-shore Location (m)
Figure 60. Test 6N proper pumping: Measured longshore current distribution at
Y = 19, 27, and 35 m
The same iterative process used for the regular wave case was repeated for
the irregular wave case. The final longshore current distribution was achieved in
Test 8E, after five iterations. A more complete set of results that illustrates the
108
Chapter 9
Establishing Uniform Longshore Currents
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