From Representative Period Selection to Process‐Based Sediment Dynamics : A study of residual and storm-driven sediment transport in a mixed-energy system (North Frisian Wadden Sea)

The North Frisian Wadden Sea (NFWS) is a mixed-energy tidal system of interconnected tidal flats, channels, ebb-tidal deltas and barrier islands, in which tides, waves and wind jointly govern sediment transport. Despite its ecological and morphological significance, the region remains comparatively understudied, and its typical residual and storm-driven sediment pathways are poorly understood. This thesis develops an integrated, process-based modelling framework to address these gaps.

A high-resolution Delft3D-4 model of the NFWS was developed and validated to resolve coupled hydrodynamics and sediment transport. To reduce computational cost, a wind-based method was introduced to identify a representative forcing period from long-term records, thereby avoiding multi-year simulations; it was implemented in the open-source tool RepD3D.

Under representative forcing, residual circulation and sediment-transport pathways were mapped across the system. Tidal forcing dominates residual flow in the back-barrier, whereas waves control sediment transport on the intertidal flats, and coastline orientation promotes southward longshore transport along the barrier islands. To characterise the seabed where samples are sparse, the accumulated sediment flux (ASF) approach was developed to infer grain-size distributions from modelled sediment fluxes; it reproduces observed patterns in the dynamic channels and flats, but skill decreases offshore, where relict sediments dominate.

Finally, the influence of storm directionality was assessed. Storms of comparable intensity but differing approach angles reorganise transport pathways and alter sediment exchange between adjacent tidal basins in distinct ways.

Collectively, the model, tools, and methods provide a transferable framework for studying sediment dynamics on comparable mixed-energy coasts, with applications in coastal management, nourishment design, and habitat mapping.