The calculation results for Case 3 are displayed in Figures 22, 23, and 24,
where cf = 0.009 was found with the roller model included, and cf = 0.007
without the roller model. Again, the shift in the peak of the longshore current
is well captured, and overall the agreement improves by including the roller
model. However, the width of the measured current distribution is somewhat
underestimated, which might be remedied with an improved formulation for
the lateral mixing. The present formulation for the mixing is related to the
local wave height and bottom orbital velocity. This typically yields a
satisfactory description of the mixing outside the surf zone, but inside the surf
zone the mixing could be underestimated because both the wave height and
bottom orbital velocity decrease. In reality, the mixing should increase
because of the breaking and associated strong turbulence. Some alternative
formulations of the lateral mixing are discussed in the following paragraphs.
Figure 23 displays the calculated and measured mean water elevation,
and it is clearly seen that including the roller model yields significantly
improved results, although the setup seems to be overestimated close to shore
in very shallow water. The wave height is also well predicted as shown in
Figure 24.
The results for Case 4 (see Figures 25, 26, and 27) exhibit the same basic
characteristics as the calculations for Cases 1 and 3. The peak in the
longshore current distribution agrees well with the measurements if the roller
model is employed, but the width of the current distribution is somewhat
obtained with the roller model, and cf = 0.005 without. Figure 26 shows that
the setup is well predicted in shallow water, although the area around the
maximum setdown is not as pronounced in the measurements as in the
calculations. Cases 1, 3, and 4 had the same roughness properties in the
experiments (smooth bottom), whereas Case 7 had a higher roughness (rough
bottom). However, the optimal cf-value consistently decreases for the three
cases (both with and without roller model), probably indicating some kind of
Reynolds number dependence for the friction coefficient.
Figures 28 and 29 compare measurements and calculations for the
longshore current and wave height, respectively, for Case 7 (no mean water
level measurements were available for Case 7). The greater bottom
roughness caused the magnitude of the longshore current to be significantly
smaller than in the other cases studied here. Thus, the optimal values for the
model, respectively.
Simulations were performed to assess the functioning of the wave-current
interaction, that is, iterating between the wave and current computations in
the manner previously described until convergence was achieved. As an
example, Figures 30, 31, and 32 compare measurements and calculations for
the longshore current, mean water level, and wave height, respectively, for
Visser Case 1, where the interaction between the waves and the current was
either taken into account or neglected. The difference between full
interaction and no wave-current interaction is not that pronounced (and even
less in the other Visser cases that had lower current speeds), but taking into
account the interaction tends to increase the current peak and decrease the
mean water level and wave height.
52
Chapter 6 Verification of Longshore Current Model
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