Katherine R. Royse

Salt marshes are globally distributed, vegetated intertidal wetlands and marsh edge erosion is common on many shores. To understand how and why marsh edge erosion occurs, the response of salt marsh substrates to applied shear and vertical stress must first be quantified. This response is likely influenced by marsh substrate biological, geochemical and sedimentological composition. However, currently there is little systematic research into the between‐marsh variability in these properties and how they affect both marsh edge erosion processes and the ability of a marsh to maintain its position vertically within the tidal frame.This paper compares two marshes of contrasting sedimentology at Tillingham marsh, East England and Warton marsh, Northwest England. Soil shear strength and compressibility are determined by applying geotechnical methods to determine marsh resistance to shear and vertical effective stresses. This research was able to isolate the influence of roots on substrate shear strength in a three‐dimensional sample. In response to vertical effective stress, both the expected displacement magnitude and the vertical recovery potential of a marsh substrate are affected by past stress conditions on the marsh, particularly those resulting from desiccation. The substrate response to vertical effective stress also influences substrate shear strength through the effect of consolidation on the void ratio (or bulk density).We present evidence for the connection between marsh composition and substrate behaviour under applied stress. The results shed light on potential determinants of marsh resistance to edge erosion, which is ultimately essential for the informed implementation of both nature‐based coastal flood defences and coastal restoration schemes.

Salt marsh ecosystems are important for supporting biodiversity, sequestering carbon and providing natural coastal protection. Evidence for their existing and potential future loss through marginal erosion is therefore of concern. However, the factors governing spatial variability in the rates of erosion at salt marsh margins – including between creek banks within individual salt marsh sites – remain relatively poorly understood. Accurate prediction of changes to the marsh edge, and thus marsh areal extent, requires more complete understanding of the dynamics and mechanisms occurring at exposed marsh fronts.In this study, we present observations of the responses of vertical sections of marsh substrate exposed to tidal flat conditions, during a field experiment over a six-month period. Vertical sections were extracted from natural and restored sites at two salt marshes in the UK: Northey Island, eastern England, where sediment is fine-grained, and Hesketh Out Marsh West, north-west England, where sediment is typically sand/silt-dominated. The study specifically investigates the role of different sedimentology and downcore substrate properties, including lamination and rooting structures, on observed change in the exposed vertical sections. Images captured in the field are analysed using structure-from-motion photogrammetry and used to create 3-D models of surface change. This is coupled with laboratory testing of downcore sedimentary characteristics, such as particle size distribution and organic matter content.The study finds that within-core and between-core variability in substrate response to erosive forcing appears to be partly related to variability in sedimentology. Sediment from sand-dominated layers, such as those found in the cores extracted from Hesketh Out Marsh West, was more rapidly and consistently (i.e. across the sediment cores) removed than clay-silt rich sediment. This grain-scale sediment removal resulted in specific morphological responses, whereby ‘chunks’ of substrate were lost, creating cavity areas further exposed to hydrodynamic forcing. Intrinsic biophysical characteristics, including sediment type and the presence of vegetation structures, can impact vertical connectivity within salt marsh substrates. Observations of structural change in the vertical sections over the six-month study period suggest that reduced downcore connectivity in restored salt marsh substrates results in increased desiccation, cracking and bulk sediment removal. An improved understanding of how such intrinsic substrate properties impact marsh front dynamics will facilitate more accurate predictions of marsh evolution and potential ecosystem service provision under future conditions.

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

Salt marshes attenuate waves and currents, thus protecting landward-lying constructed defences and the hinterland from incoming waves and extreme water levels. As such, understanding the stability of the marsh sedimentary platform is important, particularly as marsh edge erosion is common on many shores. To understand why marshes are losing material from their exposed fringes, we must better understand the relations between the marsh fabric and incoming hydrodynamic energy; this is likely to be strongly influenced by marsh biological, geochemical and sedimentological/geotechnical properties. Currently there is little systematic research into the within- and between-marsh variability in these properties and how they affect both marsh edge and marsh surface erosion processes.   We compare Tillingham marsh, eastern England, where the sediment is clay/silt-dominated and the marsh canopy is species-rich, to Warton marsh, Morecambe Bay, NW England, where the sediment is sand/silt-dominated and the vegetation species-poor. We determine soil shear strength by applying geotechnical methods which, to the best of our knowledge, have not previously been applied to salt marsh environments. Shear box and ring shear tests are used to determine the natural- and residual (i.e. post-failure) shear strength of the substrate, respectively. This is expressed as the cohesion of the sediment and the angle of internal friction. We demonstrate that the ring shear test consistently returns a lower angle of internal friction for the substrate, which is expected for the residual angle of internal friction. However, we are also able to link this reduction in the angle of internal friction to substrate composition (e.g. root content, organic matter and particle size distribution). This enhanced methodological understanding will improve our comprehension of marsh resistance to edge erosion and thus our ability to predict future erosion. Ultimately, accurate measurements of the shear strength of natural foreshores are essential for the informed implementation of nature-based coastal flood defences, including ‘de-embankment’/‘managed realignment’ schemes.

Engineering works carried out in central London over many decades have revealed a number of buried hollows that exhibit curious characteristics. Some extend deep into the bedrock geology and are in-filled with disturbed superficial deposits and reworked bedrock. Others are contained within the superficial deposits. They can be up to 500 m wide and more than 60 m in depth. As the infill material often has different behavioural characteristics from the surrounding deposits failure to identify them during an initial site investigation can prove costly. This paper considers their common characteristics and describes the method used to develop a buried hollow hazard susceptibility map. This map provides planners with a broader awareness of the potential location of difficult ground conditions associated with them, thereby reducing the potential for unforeseen ground conditions through effective site investigation design. The paper continues with a discussion of some of the likely processes associated with their formation, which are attributed to cryogenic processes, and concludes with potential future research directions.

This book chapter is closed access.Global economic losses related to natural hazards are large and increasing, peaking at\ud US$380 billion in 2011 driven by earthquakes in Japan and New Zealand and flooding in Thailand.\ud Catastrophe models are stochastic event-set based computer models, first created 25 years ago, that\ud are now vital to risk assessment within the insurance and reinsurance industry. They estimate likely\ud losses from extreme events, whether natural or man-made. Most catastrophe models limit the level\ud of user interaction, stereotyped as ‘black boxes’. In this paper we investigate how model fusion\ud techniques could be used to develop ‘plug and play’ catastrophe models and discuss the impact\ud of open access modelling on the insurance industry and other stakeholders (e.g. local government)