D.G. Hamilton, B.A. Ebersole r Coastal Engineering 42 (2001) 199218
206
beach, and assuming no temporal change in mean
water level within the wave basin,
Qs s Qp q Qr
Z 1.
6.1. Iteratie examination of the longshore current
distribution
Pump settings were systematically adjusted in an
effort to establish the proper mean longshore current
distribution in the surf zone, for a given wave condi-
tion. As the series of experiments progressed, new
estimates of the proper longshore current distribution
were made based on previously measured distribu-
Fig. 3. Progression of pump settings for regular wave tests.
tions along the beach. The following hypotheses
were used to guide the tuning process: Z1. the degree
of uniformity of longshore current in the surf zone
aries of the facility. As illustrated, the proper long-
should increase as the proper longshore current dis-
shore current distribution was approached initially by
tribution is approached; Z2. the magnitude of internal
under-pumping across the entire surf zone, and then
recirculation, Qr , should decrease as the proper long-
gradually increasing the flow rate pumped through
shore current distribution is approached; Z3. there is
the lateral boundaries.
a point where Qr is minimized, and Z4. internal
Results from Test 6D, an under-pumped case that
recirculation cannot be completely eliminated due to
was the fifth experiment in the series, are shown in
imperfections of the lateral boundaries.
Fig. 4. The peak longshore current measured at each
The first regular wave experiment, Test 2, was
of the three transects was much higher than the peak
conducted with no pumping ZQp s 0 lrs.. This test
current pumped through the lateral boundaries. Off-
was conducted to investigate the case when Qr had
shore recirculation was diminished substantially in
the largest magnitude and to examine the signs of
both extent and magnitude, compared to the no-
under-pumping. This test is equivalent to the recircu-
pumping case; however, recirculation remained rather
lation scheme used by Putnam et al. Z1949., and
strong at Y27. The longshore current distribution at
discussed by Visser Z1991. and Hamilton et al.
Y35 showed a region of flow reversal near the
Z1997.. As expected, results from Test 2 showed
shoreline Znegative values.. This phenomenon was
very non-uniform conditions both in the magnitude
observed during all 15 experiments, and was found
and distribution of longshore current measured at
to be limited within a region extending from the
Y19, Y27, and Y35.
upstream boundary to y s 34 m and offshore 2 m
A total of 14 regular wave experiments were
from the still-water shoreline. This region of flow
conducted with Qp ) 0 lrs. An initial estimate of the
reversal decreased significantly in magnitude and
proper longshore current distribution was made using
spatial extent as the proper distribution was ap-
the numerical model NMLONG ZKraus and Larson,
proached with subsequent pump settings, but it was
1991.. The breaking wave height-to-depth ratio, used
never completely eliminated.
as a calibration parameter in the numerical model,
By the tenth experiment ZTest 6I. it appeared that
was adjusted based on wave height measurements
the proper longshore current distribution was being
made in the laboratory experiments.
approached at the peak and shoreward of the peak.
Fig. 3 shows pump settings for seven of the 14
However, there was concern that the offshore tail of
experiments, including what turned out to be the
the distribution was being over-pumped. To investi-
proper distribution, Test 6N, for comparison. Pump
gate the ramifications and signs of over-pumping,
settings are shown in terms of the depth-averaged
Test 6J was performed. Fig. 3 shows the longshore
longshore current pumped through the lateral bound-
current distribution that was recirculated in Test 6J,
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