Price, DC; (2016) Influence of vegetation on sediment accumulation in tidal saltmarshes: An integrated field and modelling study. Doctoral thesis , UCL (University College London).

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Abstract

Tidal saltmarshes in the UK, and especially in the estuaries of southeast England, have been subject to degradation and erosion over the last few decades, primarily caused by sea-level rise and coastal squeeze. This is of great concern as saltmarshes play a key coastal defence role and function as important habitats for a variety of plant and animal species. Sediment accumulation is critical for the maintenance of marsh elevation within the tidal frame and for delivering the aforementioned functions and services. Key questions still remain, however, regarding the processes that govern deposition and the role of vegetation in enhancing sedimentation. The research presented in this thesis focuses on a case study at the managed realignment site and adjacent natural marshes at Tollesbury, in the Blackwater Estuary, Essex, UK. An innovative combination of secondary data analysis, intensive field campaigns and numerical modelling is used to advance our understanding of the processes controlling sedimentation. Results from a series of hierarchical deployments of sediment traps indicate the role of vegetation in marsh development is less clear-cut than previously thought. Gross sedimentation rates were statistically higher in non-vegetated areas, and vegetation has no overall influence over trapping efficiency. However, sediment retention was higher at the vegetated sampling points. This implies that vegetation acts primarily to inhibit resuspension by waves rather than by facilitating deposition from tidal flows. The performance of the realignment site in terms of vegetation development has been poor compared to other schemes, with only half of the site having been colonised after 18 years of tidal inundation. No evidence was found for erosion of the natural marshes surrounding the site, and it is shown that previous estimates of marsh erosion are erroneously high. It is thus speculated whether the realignment of large stretches of coastal defences are actually necessary or worthwhile.

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