800
Qs
700
Qr = (Qs - Qp)
600
Qsu
500
Test 6N
400
300
200
Test 6N
100
Qpu
0
0
100
200
300
400
500
600
700
800
Qp (litres/sec)
Figure 61. Surf zone and internal recirculation flow rates at Y27 for 15 regular
wave experiments
Values of Qs and Qr for the five irregular wave experiments are shown in
Figure 62. The slight upward curvature in the Qr curve is evidence that Qp had
been increased sufficiently to reach the minimum value of Qr, perhaps even
slightly exceeding the proper flow rate, Qpu, in Test 8E (the largest value of Qp).
However, evidence provided in the next section suggests that Qp may have been
slightly less than Qpu. For Test 8E, Qs and Qp were calculated to be 545 and
478 ℓ/sec, respectively. Hence Qr is indirectly estimated to be 67 ℓ/sec. Based
on ADV measurements, Qr+Qc was calculated to be 135 ℓ/sec. Based on dye
measurements, Qc was estimated to be 60 to 70 ℓ/sec, flowing downstream
65 to 75 ℓ/sec, and the value of Qr and Qc are comparable. Hence, both the direct
and indirect measurements of the internal recirculation, Qr, were in relatively
good agreement. For Test 8E the ratio of (Qs-Qp)/Qs was approximately
12 percent. More internal recirculation was generated in the irregular wave case
because 2 to 3 percent of the waves broke slightly offshore of x=18 m, the
offshore limit of the external recirculation system.
Results for both the regular and irregular wave test series suggest that the
recirculation criteria proposed by Visser (1991) is valid for the LSTF. However,
for the LSTF, the gradient in the Qs-Qp curve tends to increase more gradually as
Qp is increased, compared with the results presented by Visser (1991).
Figures 61 and 62 both suggest that Qp could vary by as much as +20 percent,
relative to Qpu, without a significant increase in Qr.
110
Chapter 9
Establishing Uniform Longshore Currents
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