Figures 36, 37, and 38 show the calculated and measured distributions of

the longshore current, mean water level, and wave height, respectively, for

Test 8E, which involved random waves. The same friction coefficient values

were applied as for Test 6N, and a Rayleigh pdf was assumed to describe the

random properties of the wave height in the horizontal portion of the basin

(wave period and incident wave angle were held fixed during the Monte-

Carlo simulations). The agreement between the measured and calculated

current distribution improved somewhat compared to the monochromatic test,

and functioning of the roller model is necessary for obtaining satisfactory

simulation results. At the seaward end, the calculated current tails off with a

smaller slope than what the measurements seem to indicate. Thus, the

mixing formulation appears to somewhat overestimate the lateral momentum

exchange in deeper water, which is also indicated in the simulations of Test

6N (Figure 33).

Similar to the calculations for the monochromatic test, the computed

mean water level distribution is located more seaward than the measured

distribution, and the maximum setdown is underestimated (Figure 37).

Again, the underestimation might be related to nonlinear wave shoaling, not

represented in NMLong-CW, although it is not as easily seen in the wave

height distribution as for Test 6N because the statistical wave height

computed tends to smooth the action of individual waves. Figure 38 shows

the agreement between the calculated and measured (significant) wave

heights, and some discrepancy is observed. The discrepancy is attributed

primarily to differences arising from the calculated and reported wave

heights: the calculated wave height is the significant value determined from

the mean of the one-third highest waves, whereas the measured wave height

is the energy-based significant height derived from the spectrum. The

measured height includes the long-periodic motion in shallow water, which is

often filtered before the wave height is computed.

0.4

Calc. with roller

Calc. without roller

0.3

Measured

0.2

0.1

0.0

0

5

10

15

20

25

Distance Across Shore, m

Figure 36. Calculated (with and without roller model) and measured

longshore current for Hamilton and Ebersole (2001), Test 8E

57

Chapter 6 Verification of Longshore Current Model

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