degree of longshore uniformity that was achieved, are presented and discussed in

a subsequent section and provided in tabular form in Appendix A.

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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