Morphological evolution of creek networks in 10 restored coastal wetlands in the UK

Coastal wetlands provide crucial ecosystem services including flood protection and carbon storage, but are being lost rapidly worldwide to the combined effects of sea-level rise, erosion and coastal urbanisation. Managed Realignment (MR) aims to mitigate for these losses by restoring reclaimed land to tidal influence. Data of creek evolution is critical to assess the performance of design strategies and improve design and implementation practices. This data descriptor provides a dataset of the horizontal morphological evolution of creek systems from various initial conditions in 10 MR schemes across the UK. Using a semi-automated workflow, morphological creek parameters were extracted from 52 lidar datasets at 1 m horizontal resolution spanning 2 to 20 years post-breach. This constitutes the most comprehensive systematic monitoring of MR creek morphology to date. The dataset will assist future MR design and provide baseline morphological information for ecological and biogeochemical surveying

Erodibility of salt marsh sediments under storm-surge conditions

Salt marshes are unique habitats that provide diverse ecosystem services including coastal protection during storm conditions in addition to storing carbon from the atmosphere. The loss of salt marshes is a worrying phenomenon on a global scale and little is known about their likely response, in terms of likelihood of erosion, to predicted increases in storminess. We investigated the relationships between hydrodynamic forcing and the erosion of intertidal sediments during high-magnitude events in the Large Wave Flume (GWK) facility in Hannover. A range of different intertidal sediments (sandy to clay-silt rich) were extracted from the field and exposed to a variety of true-to-scale simulation of storm conditions (inundation depth and wave height combinations). Sediment surfaces were exposed both horizontally and vertically to investigate sediment mobilisation from marsh platforms or mudflats and marsh edge scarps respectively. We use structure-from-motion and laser scanning to quantify volumetric changes of the sediment surfaces and micro-CT scanning to characterise the internal structure of the sediments. We find erosion of sediment surfaces exposed under water depths commonly found during storm surge events to be minimal despite maximum bed velocities during the highest simulated energy conditions exceeding those recorded in field studies during storm conditions. For horizontal surfaces, the introduction of micro-topographic features through sculpting of the sediment surface is shown to increase the sediment volumes eroded. For vertical faces, sediment erosion was greatest when exposed at mean water level rather than at depth. A strong contrast in behaviour is also seen between sediment types. We conclude that marsh sediments, both on the surface and at the margin, are likely to be relatively stable under storm surge conditions, despite significant hydrodynamic forcing. We find that sediment is mobilised when turbulence is introduced through the interaction between wave-driven near-bed current velocities and the bed characteristics themselves (e.g. micro-topography). These interactions are dependent on, inter alia, the elevation of the water level relative to the exposed surface. This suggests a strong feedback between landform structure and morphodynamic response for given conditions and implies that low-frequency, high-magnitude events may be less significant contributors to marsh erosion than more secular processes

Morphological Evolution of Creek Networks in Restored Coastal Wetland Schemes

The restoration of coastal wetlands in the UK and worldwide often involves the excavation of artificial creek networks to encourage hydrological and ecological processes; however the evolution of these artificial creeks post-implementation is insufficiently monitored, and their design still lacks scientific guidance. This thesis analyses the morphological evolution of creeks in restored coastal wetlands in the UK to help inform future design strategies. A new semi-automated creek parametrisation method was developed for lidar datasets, which is faster and less subjective than manual mapping. Morphological equilibrium was defined for natural saltmarshes; power-law relationships for creek dimensions and distribution at equilibrium were derived from the analysis of 13 mature natural saltmarshes to provide a range of potential end targets for MR creek design. A relationship was found between the overmarsh path length (mean distance to a creek anywhere in the marsh) and the mean site elevation within the tidal frame. This analysis also found that creek morphological equilibrium should be described as a range of potential states rather than as one quantifiable target. The evolution of 10 MR creeks in their first 2-20 years post-breach was then studied. MR creeks evolved near-linearly towards a larger, more sinuous and better distributed system. These evolution rates towards the proposed equilibrium targets were then related to the initial conditions of the sites: low-lying sites with high accretion rates and large openings had faster-evolving creeks, while high, constrained sites displayed limited creek growth and required more extensive initial creek excavation. The 10 MR schemes and two accidentally realigned sites considered behave as though they should stabilise within 100 years into an alternative equilibrium state to that of natural systems, with a lower density distribution of creeks, mainly concentrated around the breach areas, while the further reaches of the site have fewer channels. Furthermore, the channels have a lower sinuosity due to inherited drainage ditches that remain visible even after 100 years, and a flatter substrate.MR creek expansion may be hindered by the overcompacted soil and lack of small-scale topography inherited from the previous agricultural land use, which prevents creek incision and preferential deposition around topographic flow paths. Based on these findings, future studies should link the soil geotechnical properties, creek development and biodiversity of MR schemes to better understand creek-forming processes and improve the monitoring, management and design of MR sites. <br/

The influence of bed roughness on turbulence: Cabras Lagoon, Sardinia, Italy

Estimates of bed roughness used for predictions of sediment transport are usually derived either from simple scalars of the physical roughness (i.e., ripple height or grain size) or from the hydrodynamic roughness length (Zo) based upon velocity gradient estimates in the benthic boundary layer. Neither parameter accounts for irregular bed features. This study re-evaluates the relation between hydrodynamic roughness and physical bed roughness using high-resolution seabed scanning in the inlet of a shallow lagoon. The statistically-robust relationship, based on a 1D statistical analysis of the seabed elevation at different locations of the Cabras lagoon. Sardinia, has been obtained between Zo and the topographical bed roughness Ks by defining Ks = 2*STD + skin friction, with STD the standard deviation of the seabed elevation variations. This correlation between Ks and Zo demonstrates that the roughness length is directly influenced by irregular bed features, and that the Reynolds number accounts for the total drag of the bed: the data points collapse on the Law of the Wall curves with a fitting factor x = 0.5. Further testing must be done in other locations and in the fully-rough domain in order to test how widely those new parameters can be applied

Creek networks : natural evolution and design choices for intertidal habitat recreation

With the current rise in sea level jeopardising coastal biodiversity and the efficiency of traditional flood defence solutions, more sustainable coastal management options are being considered. The use of artificially restored intertidal habitats (mudflats and saltmarshes) as buffer zones for tidal and wave energy has been tested at numerous sites in the UK and around the world. Since the 1970s, the design of these habitats has significantly evolved along with our understanding of the natural processes involved. This paper reviews the state of the art in intertidal habitat recreation, focusing mainly on creek networks, for which no global systematic design method exists. The main parameters that control their initiation and development are investigated through the observation of natural creek networks and through the reproduction of this development in laboratory and numerical models. In areas like San Francisco Bay, such parameters have already been used in geometric relationships to provide guidelines for creek network implementation, but these empirical relationships are likely to be very site dependent. A series of steps is proposed to extend this implementation technique to other sites, thus making the design strategy more systematic and globally applicable. The sustainability of existing habitat recreation schemes remains a debated point, making the need for quantitatively defined objectives and better implementation guidelines all the more pressing.15 page(s

Parameterizing tidal creek morphology in mature saltmarshes using semi-automated extraction from lidar

Coastal saltmarshes provide a range of ecosystem services, such as flood protection and carbon sequestration, but face rapid global losses. Managed realignment (MR) is an increasingly popular method to artificially recreate these habitats by reinstating tidal regimes to reclaimed land. However, to improve MR design, better knowledge of the processes that control morphological evolution in natural saltmarshes is needed. In this paper, we develop tools to assist in the monitoring of creek network evolution towards dynamic morphological equilibrium, a state of landform stability under current physical forcings. Using lidar (Light Detection and Ranging) datasets, we combined a semi-automated creek extraction algorithm, based on elevation and slope thresholds, with a novel algorithm for morphometric creek analysis. A comprehensive suite of morphological creek characteristics was extracted for 13 natural British saltmarshes, including: amplitude, length, sinuosity ratio, junction angle, width, depth, cross-sectional area, creek order, bifurcation ratio, drainage density, and drainage efficiency. Results closely matched with field-validated manual digitization results, and were significantly faster and less subjective to produce. Morphological equilibrium relationships from the literature were found to be applicable to the new dataset, despite yielding high prediction errors due to the inherent variety of creek network shapes in saltmarshes. New equilibrium relationships were also defined relating the creek network drainage efficiency to the mouth cross-sectional area and the marsh elevation. To improve future scheme designs, these tools will be used in further studies to monitor rates of evolution towards equilibrium in MR sites depending on their initial conditions

Dataset: Morphological evolution of creek networks in 10 restored coastal wetlands in the UK

This dataset supports the publication: Chirol, C. et al (2022). Morphological evolution of creek networks in 10 restored coastal wetlands in the UK in Scientific Data.</span

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

Species-dependent variation in geotechnical properties and erodibility of salt marsh sediments

Salt marshes provide diverse ecosystem services including coastal protection, habitat provision and carbon sequestration. The loss of salt marshes is a phenomenon that is observable on a global scale and is of great socioeconomic concern due to the substantial benefits these environments provide. The causes of spatial variability in rates of marsh loss are inadequately understood for the purposes of predicting future ecosystem functions and distributions in the context of global environmental change. We investigate the relationship between vegetation of different genera and the mechanical properties of the substrate. We couple in-situ and laboratory tests of substrate geotechnical properties with micro-CT imaging of undisturbed root network structures to assess the contribution of three halophytes to sediment stability. We investigate the role of Puccinellia spp., Spartina spp. and Salicornia spp. in the modification of geotechnical parameters such as critical shear strength and cohesion when compared to un-vegetated sediments. We then compare these effects between clay-rich and sandy contexts on the East and West coasts of the United Kingdom respectively. We find that the three genera are characterised by different root network morphologies which, in part, explain the differences that we observe between the geotechnical properties of sediments colonised by these contrasting vegetation types. The presence of roots within the sediment structure increases the cohesion, as measured using a laboratory shear box test, when compared to bare sediment, with the magnitude of this effect varying by root morphology and sedimentology. In-situ shear vane tests reveal a localised spatial variability in sediment shear strength that is related to halophyte species distributions. This allows multispectral UAV imagery to be used to map species distributions and thereby infer a component of the sediment’s vulnerability to erosion that supports the prediction of future marsh distributions and, ultimately, ecosystem service provision