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)

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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