The calculation results for Case 3 are displayed in Figures 22, 23, and 24,

where *c*f = 0.009 was found with the roller model included, and *c*f = 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 *c*f = 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 *c*f-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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