High fidelity numerical modelling of European brushwood groyne fluid-structure-interaction: Parametrization through Darcy–Forchheimer, reflection and transmission coefficients

Abstract

The shoreline retreat of salt marshes and tidal flats due to both accelerated rates of sea level rise (SLR) and altered sediment dynamics as a result of the interlinked impacts of climate change is becoming increasingly visible on a global scale. In particular, salt marsh retreat amplifies pressure on the main coastal protection facilities in areas of coastal squeeze and at the same time leads to the loss of unique biodiverse wetland ecosystems that provide a wide range of key ecosystem services. Salt marshes are generally able to dynamically adapt to SLR through vertical sediment accretion and lateral expansion under hydrodynamically calmed conditions, as long as sufficient sediment budgets are available. However, in areas of little or no foreshore growth, facilitating sufficient sediment accretion is essential to ensure optimal coastal foreshore management. In Northern Europe, brushwood groyne fields used for centuries provide such hydrodynamically calmed settlement spaces that facilitate sediment accretion, yet they are insufficiently investigated and parametrized in regard to their flow-retentive effectiveness. Hence, this study parametrizes European brushwood groynes in the framework of a Darcy–Forchheimer model through a three-dimensional numerical modelling suite within the numerical framework REEF3D:CFD to quantify the fluid–structure interaction of European brushwood groynes systematically. Through validation with an existent laboratory dataset, steady-state current as well as oscillatory wave brushwood groyne interaction is investigated, providing details on flow retention, wave transmissivity and reflectivity. For the first time, laminar and turbulent resistance coefficients of European brushwood groynes are presented that enable the representation of European brushwood groynes in phase-resolved numerical modelling approaches. Furthermore, in-depth wave transmission and reflection coefficients are derived for a vast range of hydrodynamic conditions and numerous relevant brushwood groyne construction variations relevant to coastal protection. The numerical results revealed transmission coefficients in the range of 0.15 to 0.87 and reflection coefficients in the range from 0.17 to 0.73. State of the art and novel parametrized fit-equations are derived from the wave transmission and reflection coefficients, providing readily available tools to estimate European brushwood groyne transmissivity and reflectivity. In turn, this study serves as a first primer for optimizing the design of European brushwood groyne fields and comparable coastal protection structures aimed at facilitating sediment deposition and foreshore stabilization in order to foster the protective capabilities of coastal wetlands and their ecosystem services now and in the future.