Fluid-structure interaction of European brushwood groynes in waves and tides: Simulating natural decay

Tidal mudflats and salt marshes adapt to sea level rise (SLR) through vertical growth and inland retreat. However, in areas of coastal squeeze where retreat is constrained, SLR may exceed vertical accretion due to limited sediment supply or unfavourable hydrodynamics. In Northern Europe, brushwood groyne fields, historically used for land reclamation, have been constructed to enhance sedimentation and foreshore resilience. These structures, comprising rows of wooden posts with compacted brushwood bundles, create hydrodynamically quiescent zones that facilitate suspended sediment deposition and reduce sediment remobilization by dampening waves and currents. Scholarly and practical interest in optimizing brushwood groyne arrangements continues to rise in the framework of nature-based solutions. Their biodegradable material supports ecological integration but limits structural lifespan and alters flow resistance over time. Thus, a detailed assessment of their flow-retentive properties, accounting for deterioration, is essential for optimization. This study addresses the knowledge gap regarding the evolution of flow resistance in European brushwood groynes. The study assesses current- and wave-structure interactions together with a quantification of material parameters for the confined brushwood along the vertical structure using two laboratory campaigns. Parametrizations are provided through Darcy–Forchheimer and wave transformation coefficients. A Forchheimer coefficient of β = 1 . 44 was found to best represent a range of groynes with varying porosities and compositions. Parametrized hyperbolic fits described both wave reflection and transmission behaviour. All parametrizations enable improved design and longevity of brushwood groynes, enhancing their ability to support mudflat and salt marsh resilience, thereby preserving both their ecological and coastal protection functions. • Investigation of the flow retention of brushwood groynes for waves and currents. • Darcy–Forchheimer coefficients are derived from the current structure interaction. • Wave transformation coefficients are derived from the wave structure interaction. • Flow retention is correlated with the material parameters of the constructed groynes. • First-time provision of bulk and vertical varying brushwood material parameters.