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Biophysical interactions and stability at salt marsh margins



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Shears, Olivia 


There is considerable alongshore and regional variability in rates of erosion at salt marsh margins, and, with widespread loss of tidal wetlands, there is a need to better understand the factors which make salt marshes more or less likely to persist over time. The resistance of salt marsh sediments to erosion is influenced by physicochemical and biological properties. So far, research on salt marsh margin erosion has largely focused on external forcing and margin response at scales much larger than those on which these marsh properties vary, resulting in an underestimation of the potentially important role of internal substrate characteristics. In order to investigate the possible internal controls on substrate stability at this sub-metre scale, resistive properties and erosion response of sediments were compared between two contrasting UK salt marshes: coarse-grain dominated Hesketh Out Marsh West (northwest England) and clay-silt dominated Northey Island (southeast England). Both sites have adjacent ‘natural’ and restored salt marsh areas, and were selected to allow additional comparisons between these two wetland types. The thesis begins by establishing the context for understanding marsh margin erosion and the growing policy ambitions for salt marsh protection and creation. Current understanding of salt marsh erosion and the balance between external forcing and internal resistance is then reviewed, specifically linking this to the properties of sediment from restored salt marsh habitat and identifying a series of research opportunities.

This study investigates the grain-to-bulk scale erosion response and physicochemical and biological sediment properties of vertical salt marsh sections from the two locations. A field experiment was designed to monitor the millimetre-to-centimetre scale response of exposed sediment sections to tidal flat conditions, with morphological change being measured using a Structure-from-Motion methodology. Morphological change is shown to be linked to both the grain-scale physicochemical properties of sediments and the bulk scale three-dimensional internal structural properties of the sediment section. There are site-specific and depth-specific (i.e. within-site) internal characteristics, resulting from longer-term processes of accumulation and colonisation (i.e. sediment supply, vegetation type), which can influence salt marsh morphological response to hydrodynamic forcing. The properties and erosion responses of restored marshes are distinct from their natural analogues. The onset of centimetre-scale erosion is shown to impact core-scale morphology in tight cumulative feedbacks over an experiment monitoring period of eight months. These results are compared to in-situ field measurements of sediment resistance (vane-measured shear strength), to assess whether such point-scale observations can be related to centimetre-scale erosion mechanisms. Results show that shear strength surveys measure different aspects of internal resistance to those observed in the field experiment. The value of such approaches is considered with reference to better predictions of salt marsh evolution, and management of restored sites. The vertical and three-dimensional internal resistive properties of sediment significantly influence erosion at exposed marsh margins. The research highlights ‘weaknesses’ at the centimetre-scale as possible drivers of larger-scale (e.g. metres-scale) mass failure and phases of lateral retreat, but suggests that this cascading effect is strongly affected by biophysical feedbacks. Thus, the identification of locations of structural ‘weakness’ at the between-site and within-site scales could provide a conceptual framework for better understanding and prediction of erosion at near-vertical salt marsh margins.





Spencer, Thomas
Moeller, Iris


Erosion, Intertidal, Managed realignment, Morphodynamics, Nature-based solutions, Salt marsh


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Natural Environment Research Council (NE/L002507/1). Case Partnership with British Geological Survey.