yielded a peak in the longshore current (and the mean water level) that was
located more seaward than in the measurements. For cases with more
complex profile shapes (e.g., longshore bars), the peak in the longshore
current was in several cases located in the trough seaward of the bar, whereas
NMLong-CW predicted a peak that was located more seaward, also after
including the roller model. However, the magnitude of the peak was
typically well reproduced.
Accounting for the full interaction between the current and the waves
only modified the current and mean water level distribution across shore
marginally for the cases investigated here. Also, a more sophisticated
formulation for the lateral mixing did not improve agreement significantly
over the formulation originally included in NMLong. In most cases,
employing standard values on the coefficients in the wave model together
with a mixing coefficient of L = 0.5 produced satisfactory agreement, leaving
cf as the only parameter to be assigned based on the bottom roughness. The
cf-values obtained in the experimental cases studied indicated higher values
in the laboratory as compared to the field.
In summary, applying NMLong-CW to predict the cross-shore
distribution of the longshore current will yield reliable estimates if the
friction coefficient is assigned a realistic value. For a beach with longshore
bars, calculations might yield peaks in the current distribution that are located
too far seaward. It is not likely that alternative formulations for the friction
or mixing coefficient are going to remedy this discrepancy. Also
modifications of the roller parameters will not produce a sufficiently large
horizontal shift, as shown in trial simulations. The presence of the peak of
the longshore current in the trough is most likely a function of a strong
interaction between the forcing and the topography in the longshore
dimension, where the current develops similarly to a river flow taking its
course in the trough as guided by the profile geometry.
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Chapter 6 Verification of Longshore Current Model
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