Coastal wetlands, including salt marshes, cover around 756 thousand km2 globally. With medium sea level rise scenarios (50 cm), a loss of 46-59 % of these habitats is likely by 2100 (Spencer et al., 2016). While coastal populations are increasingly at risk from flooding and erosion, this is particularly the case on coasts fronted by wetlands (Dodd and Ong, 2008). If salt marsh provides effective natural coastal protection, we need to understand how it provides protection from flooding and erosion such that we can safeguard this protective element where possible.
This study outlines some key recent advances in the science that has enabled the quantification of coastal protection provided by salt marsh habitats. Such protection is provided through both, (i) the effect of salt marsh surface characteristics on waves and tidal/meteorologically-induced currents and (ii) the provision of a resistant structure with a given elevation, such that water depths in the intertidal zone are reduced.
Results/Conclusions
Evidence for both these coastal protection functions has been provided through empirical field observations and true-to-scale experiments in extreme storm surge conditions. While such evidence is reproducible through numerical modelling approaches, these still require a significant degree of ‘tuning’ against observations of modelled parameters by way of model calibration. Neither the hydrodynamic effect of salt marshes on waves and water levels, nor the likely persistence of salt marsh landforms under a given regime of climatic, hydrodynamic, and sedimentary forcing is currently possible without such in situ calibration. We argue that this is due, in part, to a lack of attention paid to the appropriate scale of representation of critical habitat/landform features within numerical models, alongside a lack of empirical studies that quantify the behaviour of plants and the soil substrate during extreme hydrodynamic forcing conditions and for a wide range of salt marsh vegetation types and spatial configurations. Given technological advances in field and remote sensing methods, the capture of information relating to the coastal protection function of vegetated foreshores is now becoming much easier. Alongside further efforts to capture the interaction between the vegetation canopy, water, and sediment movement, these developments point a way forward for incorporating salt marshes more explicitly as key elements within a broader suite of coastal risk reduction strategies.