the current was zero), an extrapolation had to be performed to derive realistic
current values in this region. After an ebb jet passes through an inlet gap, it
experiences a reduction in mean velocity because of lateral spreading
(entrainment of ambient fluid) and the increase in water depth. The effects of
these two mechanisms were estimated separately and in a simple manner. By
applying the continuity equation, the decrease in the velocity was obtained
from the increase in water depth. Furthermore, through an analogy with a
plane jet the lateral spread of the ebb jet and associated decrease in velocity
were estimated (Fischer et al. 1979). The net effect was obtained by
multiplying the reduction from each of these two mechanisms. The
sensitivity of the calculated cross-shore wave height distribution to the
estimated decrease in current velocity with distance offshore was not strong.
In the region where marked energy dissipation due to wave breaking was
observed, there was almost no influencce from the extrapolated current
distribution. However, the calculated waves in the region of shoaling (prior
to breaking) displayed some sensitivity to the selected current distribution at
the seaward end of the grid.
Figures 4, 5, 6, and 7 display the calculated significant wave height for
Runs 5, 7, 9, and 11, respectively, together with the measured wave height
(note that the x-axis originates at the first measurement point and is defined
as positive going offshore). The calculation result for the situation of
neglecting the current are also included for each of the runs (dashed line; the
only difference in these calculations was that the cross-shore current was set
to zero). Overall, the agreement is satisfactory, with much improved results
if the wave-current interaction is taken into account, although Figure 4
showing the run with the longest period in combination with the weakest
current displays little improvement with the current taken into account. For
the runs with the stronger current (Runs 9 and 11), neglect of the current on
the waves produces simulation results that significantly deviate from the
measurements. Use of linear wave theory yielded good results, in agreement
with many other studies on wave transformation in the surf zone, where the
interaction between currents and waves was not taken into account. The
generalization of the Dally (1980) model to arbitrary water depths appears to
work well also in combination with a criterion for incipient breaking that
includes wave steepness at greater water depths.
Chawla and Kirby (C&K) Data
Chawla and Kirby (C&K; 1998, 1999 and 2002) also carried out
experiments on wave transformation on an opposing current, but employed
conditions corresponding to intermediate and deep water (emphasis was on
the deeper water to avoid complicating influences from the bottom profile).
The main objective of their study was to investigate the energy dissipation
due to wave breaking on the opposing current. Wave and current conditions
were initially selected so that blocking would occur in some of the tests.
However, in comparing their measurements with predictions of the blocking
conditions based on linear theory, in several tests blocking did not occur
although Equation 19 indicated that this should be case. A larger current
speed was needed to block a specific wave, which was attributed to
nonlinearities where the amplitude dispersion became a significant factor
controlling the wave propagation speed.
27
Chapter 4 Verification of Wave Model
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