Resistance of salt marsh substrates to near‐instantaneous hydrodynamic forcing

Abstract

Salt marshes deliver vital ecosystem services by providing habitats, storing pollutants and atmospheric carbon, and reducing flood and erosion risk in the coastal hinterland. Net losses in salt marsh areas, both modelled globally and measured regionally, are therefore of concern. Amongst other controls, the persistence of salt marshes in any one location depends on the ability of their substrates to resist hydrodynamic forcing at the marsh front, along creek margins and on the vegetated surface. Where relative sea level is rising, marsh elevation must keep pace with sea‐level rise and landward expansion may be required to compensate for areal loss at exposed margins. This paper reviews current understanding of marsh substrate resistance to the near‐instantaneous (seconds to hours) forcing induced by hydrodynamic processes. It outlines how variability in substrate properties may affect marsh substrate stability, explores current understanding of the interactions between substrate properties and erosion processes, and how the cumulative impact of these interactions may affect marsh stability over annual to decadal timescales. Whilst important advances have been made in understanding how specific soil properties affect near‐instantaneous marsh substrate stability, less is known about how these properties interact and alter bulk substrate resistance to hydrodynamic forcing. Future research requires a more systematic approach to quantifying biological and sedimentological marsh substrate properties. These properties must then be linked to specific observable erosion processes, particularly at the marsh front and along creek banks. A better understanding of the intrinsic dynamics and processes acting on, and within, salt marsh substrates will facilitate improved prediction of marsh evolution under future hydrodynamic forcing scenarios. Notwithstanding the additional complications that arise from morphodynamic feedbacks, this would allow us to more accurately model the future potential protection from flooding and erosion afforded by marshes, while also increasing the effectiveness of salt marsh restoration and recreation schemes. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Effect of vegetation cover and sediment type on 3D subsurface structure and shear strength in saltmarshes

“…μCT has the potential to capture the whole 3D structural complexity of the saltmarsh: future studies could also incorporate shell deposits, or refine the method for smaller scales to resolve fine roots. Whilst we focused on monospecific locations to describe the root structure of common saltmarsh species, the impact of species richness should also be explored: root structure depends not only on the growth strategy specific to each plant species, but can also change as a function of nutrient availability, redox potential and competition with other species (Bardgett et al, 2014;Bouma et al, 2001;De Battisti et al, 2019). Enhanced biodiversity has been found to exacerbate competition strategies between species and lead to greater root biomass and greater sediment cohesivity (Ford et al, 2016).…”

“…While wave flume experiments can help us understand the specific effect of wave thrust on the erosion of ‘transposed’ marsh cliffs, in‐situ measurements have the advantage of preventing disturbance during sampling, transport and storage (Grabowski, 2014). At the local scale, this resistance to deformation depends on bulk sediment properties such as the grain size, cohesivity and water retention properties of the sediment (Crooks & Pye, 2000; Grabowski et al, 2011), but also on biogeomorphological factors such as the presence and morphology of vegetation, roots and pores (Brooks et al, 2020; Wang et al, 2017). While influences on surface shear strength, such as the presence of vegetation and biofilm, have been extensively studied (Feagin et al, 2009; Gedan et al, 2011), the impacts of subsurface structures and processes on shear strength remain challenging to observe and quantify (Brooks et al, 2020).…”

“…At the local scale, this resistance to deformation depends on bulk sediment properties such as the grain size, cohesivity and water retention properties of the sediment (Crooks & Pye, 2000; Grabowski et al, 2011), but also on biogeomorphological factors such as the presence and morphology of vegetation, roots and pores (Brooks et al, 2020; Wang et al, 2017). While influences on surface shear strength, such as the presence of vegetation and biofilm, have been extensively studied (Feagin et al, 2009; Gedan et al, 2011), the impacts of subsurface structures and processes on shear strength remain challenging to observe and quantify (Brooks et al, 2020). Structural pores or macropores, caused by cracks, burrows and decaying roots (as opposed to micropores or matrix pores which are formed by the space between sediment particles; Rabot et al, 2018), can create areas of structural vulnerability in the soil (Vu et al, 2017).…”

“…Roots are another important architectural component of the marsh substrate. The tensile strength provided by the roots complements the sediment, which is naturally strong in compression (Gyssels et al, 2005), and thus helps to prevent block failure (Brooks et al, 2020; Wang et al, 2017). However, the roots’ exact role in substrate stability depends on species‐ and environment‐specific structural characteristics (Gyssels et al, 2005).…”

“…However, the roots’ exact role in substrate stability depends on species‐ and environment‐specific structural characteristics (Gyssels et al, 2005). A particularly understudied aspect is how the 3D architectures of roots and pore networks interact within different types of substrates, and influence the internal shear strength of a saltmarsh (Brooks et al, 2020).…”