Effect of belowground structure on coastal wetland erosion resistance using X-Ray Computed Tomography

Coastal wetlands provide multiple ecosystem services through carbon storage, rich biodiversity and provision of harvested goods. A key service is their provision of ‘free’ coastal defence by dissipating storm wave and tidal energy, and their ability to accrete vertically and provide a natural buffer against the impact of projected sea-level rise. However, under IPCC climate projections, extreme hydrodynamic events associated with storm surges are expected to increase in both frequency and magnitude, exposing the margins of salt-marshes to increased erosion stress. The resistance of coastal wetlands to erosion during these events is poorly understood, and lateral erosion rates vary dramatically between UK salt-marshes. The NERC-RESIST project is exploring why this resilience to erosion varies, with a focus on the effect of the structural properties of the marsh substrate, to develop rapid evaluation tools of salt-marsh resistance for coastal engineers and inform future conservation efforts.

 

The NERC-RESIST project explores how subsurface and surface structural characteristics of UK coastal wetlands affect their erodibility under tidal forcings, in order to provide coastal engineers with improved guidance for conservation schemes. In order to link internal sediment structure to erodibility, X-Ray CT scans were undertaken on large sediment cores recovered from two coastal wetlands (Tillingham, Essex; Warton, Lancashire) that are currently experiencing contrasting rates of lateral erosion. X-Ray CT scanning is a non-destructive imaging technique that allows a quantified analysis of 3D sediment properties, pore-space and root structure. After scanning, the cores were exposed to a variety of realistic wave energy conditions at the Grosser Wellen-Kanal (GWK) Large Flume Facility in Hannover, Germany, and high-resolution structure from motion imagery were collected to identify patterns of wave-induced erosion.

 

This talk presents a 3D characterisation and detailed mapping of the topology of both pore and root networks within cores from the two salt-marshes. Two basic hypotheses are tested: the first examines the contribution of root systems in binding saltmarsh sediments and thus strengthening them against lateral erosion, and the second examines the role of macropores in facilitating the penetration of storm-wave water and energy into the sediment, contributing to weakening and increased erosion. A distance-mapping method is applied based on these hypotheses to develop a simple index of sediment structural vulnerability to erosion. These predictions are then compared to observed rates and patterns of storm wave-induced erosion from the GWK experiments. This informs an evaluation of the relative importance of inherent sediment properties (sediment type, cohesion, strength) and sediment structural characteristics in determining the erodibility of salt-marsh sediments.