longshore current reverses direction; Qp is the total longshore flow rate actively
pumped through the external recirculation system; Qr is the total longshore flow
rate that internally recirculates in the offshore region; and Qc is a secondary
offshore circulation cell limited to the length of each wave board between two
adjacent baffles. In this facility, Qc develops as a result of the baffles that extend
shoreward of the wave boards. In general, it was found that Qc decreases as Qr
decreases, because Qr drives Qc. At a transect midway along the beach, and
assuming no temporal change in mean water level within the wave basin,
Qs = Qp + Qr
(1)
Iterative 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
condition. As the series of experiments progressed, new estimates of the proper
longshore current distribution were made based on previously measured
distributions along the beach. The following hypotheses were used to guide the
tuning process: (a) the degree of uniformity of longshore current in the surf zone
should increase as the proper longshore current distribution is approached; (b) the
current distribution is approached; (c) there is a point where Qr is minimized; and
(d) internal recirculation can not be completely eliminated because of
imperfections of the lateral boundaries.
The first regular wave experiment, Test 2, was conducted with no pumping
(Qp = 0 ℓ/sec). This test was conducted to investigate the case when Qr had the
largest magnitude and to examine the signs of under-pumping. This test is
equivalent to the recirculation scheme used by Putnam, Munk, Traylor (1949),
and discussed by Visser (1991) and Hamilton et al. (1997). As expected, results
from Test 2 showed very nonuniform conditions both in the magnitude and
distribution of longshore current measured at Y19, Y27, and Y35.
A total of 14 regular wave experiments were conducted with Qp > 0 ℓ/sec.
An initial estimate of the proper longshore current distribution was made using
the numerical model NMLONG (Kraus and Larson 1991). The breaking wave
height-to-depth ratio, used as a calibration parameter in the numerical model, was
adjusted based on wave height measurements in the fixed-bed experiments.
Figure 57 shows pump settings for 7 of the 14 experiments, including the
proper distribution, Test 6N, for comparison. Pump settings are shown in terms
of the depth-averaged longshore current pumped through the lateral boundaries
of the facility. As illustrated, the proper longshore current distribution was
achieved by under-pumping across the entire surf zone and gradually increasing
the pumped flow rate through the lateral boundaries.
105
Chapter 9
Establishing Uniform Longshore Currents
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