Structure-induced turbulent upwelling and sediment mobilization in an anthropogenically modified tidal inlet

Coastal protection structures, such as groynes, are known to exert a lasting influence on tidal dynamics and sediment pathways of inlet systems. Yet, the complex fluid-structure interactions and system-scale implications remain insufficiently quantified. Aerial observation at the Harle inlet of the German Wadden Sea during highly energetic forcing by spring tide and intense storm conditions revealed pronounced turbidity patterns linked to the structural influence of a large-scale groyne. A unique field campaign combining aerial drone imagery, multiparameter profiling, and spatially paired ADCP measurements captured hydrodynamic, turbulence, and transport characteristics before, during, and after storm conditions. Thus, enabling isolation and assessment of the governing mechanisms, their recurrence, and their engineering implications. The groyne creates a submerged overflow, generating a near-bed roller in its wake that drives a primary upwelling process. This upwelling drives turbulent structures, transporting available sediments to the surface, rendering them visible in drone imagery. The Reynolds stress describing average fluxes of turbulent horizontal momentum by turbulent vertical motion ( ) proved to be a robust proxy for the observed spatially extensive turbulent upwelling. Consistent detection of the hydraulic mechanisms and turbulent phases throughout the campaign indicates that the observed processes are recurring, structurally mediated features rather than storm-exclusive phenomena. Estimated sand transport capacities suggest that the fluid-structure interaction contributes a considerable fraction to the inlet’s sediment budget. These findings emphasize the morphodynamic relevance of anthropogenic infrastructure in tidal inlets and highlight the critical need to account for long-term, system-wide consequences of these engineered structures in coastal protection strategies.