600
Qs
Test 8E
Qr = (Qs - Qp)
500
400
300
200
100
Test 8E
0
0
100
200
300
400
500
600
Qp (litres/sec)
Figure 62. Surf zone and internal recirculation flow rates at Y27 for five irregular
wave experiments
Verification using gradient in Qs concept
The second method proposed by Visser (1991) is based on the following
hypotheses: (a) if Qp < Qpu, Qs increases in the downstream direction; (b) if
Qp = Qpu, Qs is essentially uniform in the longshore direction; and (c) if Qp > Qpu,
Qs decreases in the downstream direction.
Figure 63 shows results from 3 of the 15 regular wave experiments. In Test
6D, Qp was approximately 40 percent less than the value of Qp in Test 6N (the
proper value), and Qs increases in the downstream direction. Conversely, in Test
6J, Qp was approximately 47 percent greater than Qp in Test 6N, and as shown,
Qs decreases in the downstream direction. In Test 6N, Qs is essentially uniform
in the alongshore direction. These results give credence to the conclusion made
previously that Test 6N represents the proper longshore current distribution for
the regular wave case. However, in the case of under- or over-pumping, the
longshore gradient in Qs is relatively small. Therefore, significant care should be
taken during the iterative process of selecting and converging on the proper
longshore current distribution.
Results from two of the five irregular wave experiments are shown in
Figure 64. In Test 8A, Qp was approximately 28 percent less than the value of Qp
in Test 8E, and Qs increases in the downstream direction. Results for Test 8E
(judged to be the proper value of Qp) show that Qs increases slightly in the
downstream direction, which suggests that Qp may have been slightly smaller
than Qpu. However, this contradicts evidence shown in Figure 62, which
111
Chapter 9
Establishing Uniform Longshore Currents
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