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 *c*f 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 *c*f-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 *c*f = 0.010 was obtained with the roller model to be compared with

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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