degree of longshore uniformity that was achieved, are presented and discussed in
a subsequent section and provided in tabular form in Appendix A.
Verification using minimum Qr concept
The values of Qp used during Test 6N and Test 8E, assumed to be the
optimum values, were verified using the two criteria proposed by Visser (1991).
As mentioned previously, these two criteria consider only the total longshore
flow rate being recirculated, Qp, not the cross-shore distribution (i.e., not the 20
individual pump flow rates).
In the present experiments, Qr, which is influenced by Qc (Figure 56), was
estimated directly during Tests 6N and Test 8E, by making additional
measurements in the offshore region at transect Y27. For all other experiments
in the Test 6 and Test 8 series, Qr was estimated indirectly assuming Qr = Qs - Qp
estimated using dye.
As shown in Appendix A, the mean longshore currents were rather invariant
with depth. Therefore, the flow rate in the surf zone, Qs, could be calculated
using the longshore current measured one-third of the water depth above the
bottom, the local mean water surface elevation, and an estimate of the cross-
sectional area represented by each ADV. The pumped flow rate, Qp, was
calculated using data from the in-line flow sensors in each pump-and-piping
system.
The first criterion proposed by Visser (1991) will be illustrated conceptually,
using the Qs and Qr values obtained during the 15 regular wave experiments
(Figure 61). Qpu is the value of Qp associated with the proper and nearly uniform
longshore flow rate in the surf zone, Qsu. This method is based on the premise
that Qpu can be determined by minimizing Qr as a function of Qp. Visser (1991)
postulated the following: (a) if Qp<Qpu, the flow rate Qs will increase in the
downstream direction and the surplus, Qs-Qp, will return offshore and increase
Qr; or (b) if Qp>Qpu, the increased flow rate Qs will increase Qr offshore because
of the increased advection and lateral friction.
Although there is some scatter in the data, Test 6N had the lowest value of
Qr. For Test 6N, Qs and Qp were 505 and 465 ℓ/sec, respectively. Hence Qr is
indirectly estimated to be 40 ℓ/sec. Based on ADV measurements, the internal
recirculation flowing in the upstream direction, Qr+Qc, was calculated to be
48 ℓ/sec. The secondary circulation cell, Qc, flowing downstream directly in
front of each wave generator was estimated to be 10 ℓ/sec, using dye. Therefore,
the inferred value of Qr was 38 ℓ/sec. Hence, direct measurement of the internal
recirculation, Qr, and the indirect estimate, Qs-Qp, gave good agreement. The
ability to minimize internal recirculation relative to the flow rate in the surf zone,
can be quantified as (Qs-Qp)/Qs, which for this test was approximately 8 percent.
109
Chapter 9
Establishing Uniform Longshore Currents
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