Knowledge on tidal inlet sand transport patterns and morphodynamics is a prerequisite for developing sustainable planning and management schemes of these highly dynamic coastal systems. Moreover, information on the forcing mechanisms of the different tidal inlet morphological units and the different transport pathways shaping the system is needed to assess the impact of potentially changing environmental conditions, such as accelerating sea level rise, increasing storm intensities and frequencies, or shifting wind directions.
The aim of this study is to investigate the sand transport pathways and morphodynamics in a natural tidal inlet system, the Knudedyb tidal inlet in the Danish Wadden Sea, by coupling investigations in the sub-tidal inlet channel and the adjacent inter-tidal and supra-tidal areas to encompass the complete system. More specifically, the objective is to develop a conceptual model for the sand transport patterns and morphodynamics in the tidal inlet system, including an assessment of the forcing hydrodynamic drivers of the different sand transport pathways, e.g. which are driven by tidal currents during normal, calm situations and which are driven by wave-generated currents during storm events.
Successive bathymetric surveys were carried out covering the deeper inlet channel using a vessel borne high-resolution shallow-water multibeam echosounder (MBES) system. The exposed inter- and supra-tidal areas and shallow sub-tidal areas were covered by successive airborne topographic and topobathymetric surveys using high-resolution red and green Light Detection And Ranging (LiDAR), respectively. Detailed digital elevation models with a grid cell size of 1 m x 1 m were generated and analysed geomorphometrically.
The analyses reveal a main ebb-directed net sand transport in the main channel; however, due to the geometry of the main channel, displaying a confluent meander bend, confined areas in the main channel are characterised by an opposite-directed net sand transport. In the inter-tidal areas the main net sand transport is flood-directed. However, also here the analyses reveal the existence of oblique second-order sand transport pathways, transporting sand from the inter-tidal flat to the inlet channel during falling tide due to drainage of the inter-tidal areas. As opposed to this, the orientation and migration direction of isolated swash bars on the inter-tidal flat indicate that during storm events with winds from SW, sand is transported from the inlet channel to the intertidal flat.
Hence, in addition to the typical main sand transport directions with net export in the inlet channel and net import over the adjacent inter-tidal flats, these investigations suggest an exchange and possible recirculation of sand between the inlet channel and the inter-tidal flat with the direction of the exchange depending on the forcing conditions, i.e. whether calm or storm conditions.

Acknowledgements
This work is funded by the Danish Council for Independent Research | Natural Sciences under the project “Process-based understanding and prediction of morphodynamics in a natural coastal system in response to climate change” (Steno Grant no. 10-081102).