0.10
0.08
0.06
0.04
Calculated
0.02
Measured
0.00
-2
0
2
4
6
8
10
Distance Across Shore, m
Figure 29. Calculated and measured wave height for Visser (1982) Case 7
somewhat different than what NMLong produced because of the wave-
current interaction (discussed in the following paragraphs). For each case,
together with the measurements the calculations are shown with and without
the roller model, except for the wave height because the effect is negligible.
For NMLong-CW with the roller model turned on, the friction coefficient had
to be adjusted as compared to the results found by Kraus and Larson (1991)
obtained without the roller model. Typically, the friction coefficient had to
be increased 15-20 percent if the roller model was employed to obtain an
acceptable fit. In this fitting procedure, the eddy viscosity coefficient was
kept constant at Λ = 0.5, and cf was varied until the agreement between
measurements and calculations was visually satisfactory. Overall, including
the roller model created less sensitivity to Λ, or in other words, there is less
possibility of improving agreement by small changes in the value of Λ. In the
simulations without the roller model, the same Λ- and cf-values were
specified as determined by Kraus and Larson (1991) (nonlinear friction
model).
Figures 19, 20, and 21 illustrate comparisons for longshore current, mean
water level, and wave height, respectively, for Case 1. A friction coefficient
value of cf = 0.010 was obtained with the roller model to be compared with
cf = 0.009 obtained by Kraus and Larson (1991). Overall, Case 1 was the
case where previously the largest discrepancy was observed between
measurements and calculations. However, by including the roller model, this
discrepancy was effectively eliminated, and the agreement is much improved,
both for the current and mean water level (Figures 19 and 20, respectively).
Concerning the wave height (Figure 21), NMLong-CW is still unable to
describe the steep increase in height before incipient breaking, which is
attributed to nonlinear shoaling.
51
Chapter 6 Verification of Longshore Current Model
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