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Saltmarshes are considered as natural coastal defences. However, owing to the large context dependency, there is much discussion over their effectiveness in providing coastal protection and the necessity of additional coastal defence interventions. The macro-tidal Taf Estuary in south-west Wales was chosen as the case study in this paper to investigate the effects of anthropogenic coastal defence interventions such as construction of hard defences, managed realignment, and altering land use of the saltmarshes on the complex hydrodynamics of the estuary. A coupled flow–wave–vegetation model, developed using the Delft3D coastal modelling software, was used. The wave and current attenuation role of saltmarshes during two contrasting storm conditions was modelled, with and without saltmarsh management interventions. The study reveals that certain saltmarsh management interventions can have widespread impacts on the hydrodynamics of the estuary. Altering the land use by allowing extensive grazing of saltmarsh by livestock was found to have the largest impact on wave attenuation, where wave heights on the marsh almost doubled when compared with the no-intervention scenario. On the other hand, managed realignment has a significant impact on tidal currents, where tidal currents reached 0.5 m/s at certain locations. Changes in estuarine hydrodynamics can lead to undesired impacts on flooding and erosion, which stresses the importance of understanding the effects of localized anthropogenic coastal management interventions on the entire estuarine system.

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Tidal sandbanks are large-scale dynamic bed forms observed in shallow shelf seas. Their plan view evolution may display a single bank breaking into two or more banks, for which two mechanisms have been proposed in the literature. However, as both were based on interpretation of observations, generic support from a processbased modelling perspective is lacking so far. Here we present a new idealised process-based model study into the transient evolution of tidal sandbanks. Key elements are the inclusion of nonlinear dynamics for topographies that vary in both horizontal directions, and the focus on long-term evolution (centuries and longer). As a further novelty, the hydrodynamic solution, satisfying the nonlinear shallow water equations including bottom friction and the Coriolis effect, is obtained from a truncated expansion in the ratio of maximum bank elevation (w.r.t. mean depth) and mean water depth. Bed evolution follows from the tidally averaged bed load sediment transport, enhanced by depth-dependent wind-wave stirring. From our model results, we identify two paths of evolution, leading to either bank-breaking or an S-shape. Which of these paths occurs depends on initial topography, with bank orientation and bank length as major control parameters. The breaking and S-shape obtained in our model results show resemblance with banks observed in the North Sea.

Salt marshes are vegetated tidal wetlands, which can typically be found at sheltered coastal areas in moderate climate zones. Their potential as natural coastal protection by wave attenuation (Möller et al, 2014), reduction of flood-surge propagation (Stark et al., 2016) and shoreline stabilization (Bouma et al, 2014) has been increasingly recognized among scientists and engineers, but it comes with risks. Our understanding of the biogeomorphological dynamics between salt marsh vegetation, hydrodynamics and sediment is limited, while these are essential to identify the protective value of marshes to coastal protection (Wu et al., 2017). In this study, we present a predictive process-based model with a newly validated vegetation module to study the potential of salt marshes to contribute to coastal protection for a case study site in West Wales, United Kingdom.