| dc.description.abstract | Coastal zones across the entire globe are threatened by coastal retreat and increased flood risk due to rising sea levels. A large fraction of the world’s population lives in coastal regions and roughly 20% of the world’s coastlines are sandy beaches which are highly dynamic. Sandy beaches are found along 20% of the worlds coast and are highly dynamic. Beaches generally erode during high energy conditions, while they accrete during the subsequent low energy conditions. The processes resulting in offshore transport are well understood, but this is not the case for the onshore transport during low energy conditions. Onshore transport is the result of non-linearities in the wave-velocity and acceleration, but the relative contribution of the acceleration-driven transport and the transport related to wave velocity is unknown. The relative contribution of acceleration-driven transport and the transport due to waves to the onshore sand transport during low to moderate energy conditions was investigated in a combined data-model study. Measurements of flow velocities, sand concentrations and the morphology in the intertidal area were performed over a period of 4 weeks at Vejers beach, a sandy uninterrupted coast along the Danish North sea coast. The hydrodynamical SWASH model was used to simulate the water motion and wave conditions measured during the field campaign. Comparison of the SWASH model output with the field data showed that the SWASH model is able to simulate the significant wave height in good agreement with the field data. However, the simulation of the current and the non-linearities in the sea surface elevation and wave velocity need to be further improved. The total normalised root-mean-square error between the SWASH model output and field data is 0.27 for the best fit parameters. A sand transport model was build based on the formulas of Fern ́andez-Mora et al. (2015) including four transport mechanisms: a) transport by the waves, b) joint action of stirring of sand by waves and transport by currents, c) the acceleration-driven transport and d) the diffusive transport. The bed level change over 25.5 hours was computed using the modelled total cross-shore sand transport and compared to the observed bed level change in order to calibrate the sand transport model. For the best fit parameters of the sand transport model, the root-mean-square of the observed and modelled bed level change was 0.0595 m. The relative contributions of the four transport mechanisms to the total cross-shore transport were evaluated as a function of time and distance. The results of the modelling exercise revealed the relative contribution of the four transport mechanisms to the total cross-shore transport and showed that the velocity skewness transport mechanism dominates over the velocity asymmetry mechanism. The results also show that the total transport is dominated by the onshore transport due to waves in the shoaling zone. In the surf zone, the total transport is offshore directed and dominated by the transport due to currents. Additionally, the offshore transport by currents dominates at the edge of the shoaling zone during low water, while during high water the onshore transport by the waves dominates. | |
