Erodibility of vertically exposed salt marsh sediments

Abstract

<p>Salt marshes are valuable habitats, providing natural coastal protection. However, change in the extent of salt marsh habitats is occurring globally; regional hotspots include widespread losses in Northwest Europe. These lateral losses are occurring despite relative stability in the vertical dimension (i.e. surface elevation and its relation to rising sea levels). Whilst there are an increasing number of studies reporting and quantifying salt marsh losses, the understanding of what controls lateral marsh dynamics remains weak.</p><p>Numerical models and large-scale experimentation (e.g. in wave flumes) have, to a degree, improved understanding of the mechanisms by which salt marshes can change in the lateral dimension. However, empirical field evidence exploring the role of specific marsh properties and exposure characteristics is lacking. What biophysical factors (i.e. vegetation and sediment characteristics) control internal marsh substrate stability, and how do these factors influence the vulnerability of lateral marsh margins to external forcing?</p><p>The three-dimensional biophysical response of salt marsh substrates to external forcing representative of tidal flat conditions has been investigated. Intertidal sediment sections were extracted from two contrasting UK salt marsh sites: clay-silt rich Tillingham Marsh, Essex, Southeast England, and sand-dominated Warton Marsh, Morecambe Bay, Northwest England. Vertical sections of sediment were exposed to in-situ external forcing conditions on the fronting tidal flat at Tillingham Marsh. Structure-from-motion digital photogrammetry was used to quantify volumetric and structural changes on the vertical faces of the exposed sedimentary cores at approximately 14-day intervals. Three-dimensional structure-from-motion models were analysed alongside empirical water level measurements and meteorological data. Greater loss of material, typically around root structures, characterised the upper section of the sediment core from Warton Marsh. The Tillingham Marsh sediments were more resistant to erosion, including within the upper section. This indicates possible variability in the mechanical role of rooting structures (as also found in previous work (e.g. Feagin et al. 2009; Ford et al. 2016)), under a different marsh sedimentology.</p><p>Small-scale marsh stability is thus strongly influenced by physical sedimentology, biological root structures, hydrodynamic sequencing, and the interactions between these factors. A combination of inundation history, bulk sediment strength and belowground vegetation structure is likely to influence salt marsh lateral stability, at least at the cm to m scale. Understanding under which conditions (e.g. location, wave regime) these factors become more or less important, and how these small scale controls scale up to larger scales is crucial towards modelling and predicting future salt marsh change.</p><p>