Wind-driven currents

In order to assess the contribution of wind-driven currents to the predicted sediment transport rates, a 0.5 nm DCSM model run was performed for the year of 2017 without including the effect of wind. In the Figure 75 and Figure 76 a comparison between the measurements, model currents with wind and model currents without wind is presented for the longshore and cross-shore direction respectively. From these figures it can be seen that when both wave-driven and wind-driven currents are not taken into account, only tidal currents remain and the residual currents do not change in time, increasing towards shallow water from zero to about 0.05 m/s to the east for the longshore direction and giving a small offshore-directed current in the cross-shore direction. In the Figure 77 comparison between the transport rates, calculated using modelled currents from the basic 0.5 nm and from the same model without wind for the period of the field campaign, is presented. From this figure it can be seen that wind-driven currents also play a relatively large role in the sediment transport on the lower shoreface. As it was shown in the Chapter 4, over the period of the KG2 campaign western wind was observed most frequently and the longshore sediment transport rates with the effect of wind excluded are reduced over the entire lower shoreface. The effect of wind is particularly important at the deepest frame F1, where the effects of wave-driven currents in the longshore direction are almost absent as the DCSM model with win shows good agreement with the data, which means that most of the longshore transport at this location occurs as a result of wind action. In the cross-shore direction we can see that sediment transport becomes slightly more onshore when wind is not taken into account. This probably is related to the strong south-southwestern winds that were also observed during field campaign, which to a certain degree reduces flow of water and sediment towards the tidal inlet. Also, it can be seen that the effect of wind on total transport in both longshore and cross-shore direction slightly increases towards shallower water. From the Figure 78, on which difference between sediment transport calculated with and without wind-driven currents for the entire year of 2017 is presented, we can see the similar patterns as for the period of the field campaign.

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Figure 75 Measured and modelled (DCSM 0.5 nm with and without wind) depth-average longshore residual currents at the lower shoreface frames

Figure 76 Measured and modelled (DCSM 0.5 nm with and without wind) depth-average cross-shore residual currents the lower shoreface frames

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Figure 77 Integrated total longshore and cross-shore yearly sediment transport, computed at the KG2 frames locations for the period of the field campaign from the regular 0.5 nm model and from 0.5 nm model without wind

Figure 78 Net annual longshore and cross-shore yearly sediment transport, computed at the KG2 frames locations for the year of 2017 from the regular 0.5 nm model and from 0.5 nm model without wind

6.5 DISCUSSION AND CONCLUSIONS

Sediment transport modelling results can be summarized in the following points:

 Comparison of the transport rates calculated using the measured and the modelled currents with and without different sediment transport mechanisms, particularly Longuet-Higgins streaming, Stokes drift and wave velocity asymmetry, has shown that for shallower parts of the lower shoreface at frames F3 (16 m water depth) and F4 (11 m water depth) the absence of wave-driven currents in the DCSM model results in underestimation of the mechanisms contributions for the net annual sediment transport. For the water depth of 20 m the results for the measured and the modelled currents were comparable and it was possible to quantitatively estimate contribution of different mechanisms to the longshore and cross-

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shore net annual transport. For more shallow water their contribution could be assessed only in a qualitative way and in order to the proper analysis the wave-driven currents should be taken into account.  Excluding the effects of wave-induced Longuet-Higgins streaming and wave velocity asymmetry both

results in decreased eastward longshore and onshore net sediment transport. At 20 m water depth their contribution to the net annual transport is almost zero, which is in line with the results of Van Rijn (1997). At shallower water their role increases, but compared to other mechanisms it is still relatively small.  Stokes drift can lead to a certain amount of additional eastward and onshore sediment transport and its

role is particularly important for the cross-shore sediment transport increasing the net transport for the field campaign nearly 1.5 times at all three locations. At 20 m water depth its contribution to the net yearly transport was on average 5 m3/m in both cross-shore and longshore direction and it increases towards the shallow water. Variation of the Stokes drift contribution from year to year is also very high significantly increasing for the years characterised with storm events of low frequency of occurrence.  Comparison of the transport rates calculated using currents from the DCSM model runs with and without

wind for the year of 2017 has shown that wind-driven currents contribute to the net eastward sediment transport in longshore direction and reduce the onshore transport. This is in line with the observed wind conditions which showed most frequently observed wind during stormy periods coming from the west and the strongest winds coming from the south and southwest. Comparing the transport rates for the field campaign from measured and modelled currents with and without wind it can be concluded that the relative role of wind-driven currents is the highest at deeper water and decreases towards the shallower water, where the mismatch in the transport rates due to wave-driven currents becomes more important.

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