Environmental Engineering Reference
In-Depth Information
6
Measured
Forecast
4
2
0
-2
-4
-6
Fig. 16.10
Measured and predicted
profiles with Rayleigh wave
distribution at the Duck site
(date 27/04/2004).
-8
-10
0
100
200
300
400
500
600
700
800
900
1000
Cross-shore distance (m)
The selected test case is defined in conformity
with a gravel beach studied in their GrosseWellen
Kanal (GWK) experiments. That is, we employ a
plane beachwith a slope of 1:8 with a grain size set
to 0.021m, and the selected waves are defined by a
JONSWAP spectrum with H
s
¼
0.6m, T
p
¼
3.22 s
and H/L
the action of 100 wave periods. Here the model
output treated as a stable solution is shown as a
dash-dot line. This figure illustrates the reliability
of the model over the selected range of morpho-
dynamical time steps.
In contrast, Figure 16.12 presents two numeri-
cal experiments on the same plane slope that show
divergence from the stable solution given by 6 s.
These morphodynamical time steps correspond
to 0.076 s and 100 s. This result illustrates the
importance of careful selection of this time step
in the model; if these sensitivities are not studied,
the uncertainties in the predictions will not be
appreciated, with the danger that unreliable out-
put is used subsequently in scheme development.
This case highlights the importance of special-
ist knowledge in running, and interpreting the
output from, numerical models used in the design
¼
0.129m is the same in all the numerical experi-
ments carried out in this test. The hydrodynamic
time step is given by
D
¼
0.05. The spatial step defined by
D
x
¼
0.00076 s (i.e. instanta-
neous) and the selected morphodynamical time
steps are given by 3 s, 6 s and 12 s. To ensure that
the selected morphodynamical time step (
D
t)is
reliable in the calculation, the variability of the
model output with regards to changes in the mor-
phological time step must be small.
Figure 16.11 presents the results for these three
experiments, showing the evolved profiles after
t
0.3
0.2
0.1
SWL
0
-0.1
-0.2
-0.3
-0.4
Fig. 16.11
Sensitivity test of induced
beach profile changes to selected
morphodynamical time step:
convergence (-
-
) 3 s; (——) 6 s;
(
)12s.
-0.5
260
261
262
263
264
265
266
267
Chainage (m)
