D.G. Hamilton, B.A. Ebersole r Coastal Engineering 42 (2001) 199218
208
of results that illustrates the degree of longshore
uniformity that was achieved, are presented and dis-
cussed later in Section 7 and provided in tabular
form in Appendix A.
6.2. Verification using minimum Qr concept
The values of Qp used during Tests 6N and 8E,
assumed to be the optimum values, were verified
using the two criteria proposed by Visser Z1991.. As
mentioned previously, these two criteria consider
only the total longshore flow rate being recirculated,
Qp , not the cross-shore distribution Zi.e., not the 20
Fig. 7. Qs and Qr at Y27 for 15 regular wave experiments.
individual pump flow rates..
In the present experiments, Qr , which is influ-
enced by Qc Zsee Fig. 2., was estimated directly
during Tests 6N and 8E, by making additional mea-
Qp were 505 and 465 lrs, respectively. Hence, Qr is
surements in the offshore region at transect Y27. For
indirectly estimated to be 40 lrs. Based on ADV
all other experiments in the Tests 6 and 8 series, Qr
measurements, the internal recirculation flowing in
was estimated indirectly assuming Qr s Qs y Qp ZEq.
the upstream direction, Qr q Qc , was calculated to
be 48 lrs. The secondary circulation cell, Qc , flow-
and it could only be roughly estimated using dye.
ing downstream directly in front of each wave gener-
As shown in Appendix B, the mean longshore
ator was estimated to be 10 lrs, using dye. There-
currents were rather invariant with depth. Therefore,
fore, the inferred value of Qr was 38 lrs. Hence,
the flow rate in the surf zone, Qs , could be calcu-
direct measurement of the internal recirculation, Qr ,
lated using the longshore current measured one-third
and the indirect estimate, Qs y Qp , gave good agree-
of the water depth above the bottom, the local mean
ment. The ability to minimize internal recirculation
water surface elevation, and an estimate of the
relative to the flow rate in the surf zone, can be
quantified as ZQs y Qp .rQs , which for this test was
cross-sectional area represented by each ADV. The
pumped flow rate, Qp , was calculated using data
approximately 8%.
Values of Qs and Qr for the five irregular wave
from the in-line flow sensors in each pump-and-pip-
ing system.
experiments are shown in Fig. 8. The slight upward
curvature in the Qr curve is evidence that Qp had
The first criterion proposed by Visser Z1991. will
be illustrated conceptually, using the Qs and Qr
been increased sufficiently to reach the minimum
value of Qr , perhaps even slightly exceeding the
values obtained during the 15 regular wave experi-
ments, see Fig. 7. Qpu is the value of Qp associated
proper flow rate, Qpu , in Test 8E Zthe largest value
of Qp .. However, evidence provided in the next
with the proper and nearly uniform longshore flow
rate in the surf zone, Qsu . This method is based on
section suggests that Qp may have been slightly less
the premise that Qpu can be determined by minimiz-
than Qpu . For Test 8E, Qs and Qp were calculated to
be 545 and 478 lrs, respectively. Hence, Qr is
ing Qr as a function of Qp. Visser Z1991. postulated
the following: Za. if Qp - Qpu , then the flow rate Qs
indirectly estimated to be 67 lrs. Based on ADV
measurements Qr q Qc was calculated to be 135 lrs.
will increase in the downstream direction and the
Based on dye measurements Qc was estimated to be
surplus, Qs y Qp , will return offshore and increase
Qr , or Zb. if Qp ) Qpu , then the increased flow rate
6070 lrs, flowing downstream directly in front of
each wave generator. Therefore, the inferred value of
Qs will increase Qr offshore due to increased advec-
Qr was 6575 lrs, and the value of Qr and Qc are
tion and lateral friction.
comparable. Hence, both the direct and indirect mea-
Although there is some scatter in the data, Test
surements of the internal recirculation, Qr , were in
6N had the lowest value of Qr . For Test 6N, Qs and
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