Morphological evolution of a non-engineered managed realignment site following tidal inundation
Introduction
Coastal wetlands, such as saltmarshes and mudflats, provide a number of important ecosystem services including habitats for juvenile fish species, water quality regulation and wave attenuation (Barbier et al., 2011). However, there has been a global decline in the extent of these habitats (Adam, 2002; Barbier et al., 2011) as a result of loss and degradation caused by pollution, urbanisation, land claim, changes in sediment supply, and erosion driven by sea level rise. Subsequently, there are a number of schemes that have been implemented to restore and compensate for the loss of intertidal saltmarsh habitat (Callaway, 2005). One approach, which has become the preferred option in Europe and North America, is managed realignment (MR); where sea defences are deliberately breached allowing tidal inundation of the previously defended terrestrial land. MR is usually performed on low lying land that has previously been reclaimed for agricultural purposes (French, 2006), and is often of lower economic value than the cost of maintaining the external flood defence and protection schemes.
Following tidal inundation, saltmarsh plants and invertebrate species have been found to colonise relatively quickly (Garbutt et al., 2006; Mazik et al., 2010; Wolters et al., 2005). However, at multiple sites, the diversity of key plant species has been recognised to not be equivalent to natural saltmarsh communities (Mossman et al., 2012). This has been associated with differences in sub-surface sediment structure due to agricultural activities, such as ploughing, leading to poor drainage and anoxic conditions (Spencer et al., 2017; Tempest et al., 2015). As a result, these sites may not be delivering the targeted level of ecosystem services such as carbon storage (Moreno-Mateos et al., 2012) or wave attenuation (Moller et al., 2014; Moller and Spencer, 2002; Rupprecht et al., 2017). Site design often includes a mosaic of morphological features including channels, raised areas and lowered sections to encourage a range of intertidal habitat, utilising the distinct elevational niches of different saltmarsh species (e.g. Masselink et al., 2017; Sullivan et al., 2018). Consequently, elevation is considered to be the key physical parameter in the design of MR sites (Howe et al., 2010). However, this approach does not consider post-site breaching changes and the rate of sedimentation. For example, at Paull Holme Strays (Humber estuary, UK) rapid accretion of sediment occurred due to the high suspended sediment load in the Humber estuary (Mazik et al., 2010; Wolanski and Elliott, 2016). Furthermore, at the Medmerry Managed Realignment Site (West Sussex, UK) artificially lowered areas accreted at a faster rate than anticipated due to the internal re-distribution of sediment following site breaching (Dale et al., 2017). In both of these cases, accretion of sediment resulted in the loss of lower elevation environments targeted by the scheme, and did not result in the creation of the range of intertidal habitat to the extent intended.
Predictions of a site's morphological evolution following inundation tend to be derived from theoretical models based on observations of established saltmarshes (e.g. Allen, 2000), or post-site breaching measurements of the rate of accretion (e.g. Dale et al., 2017; Spencer et al., 2017; Spencer et al., 2008). However, empirical measurements from MR sites tend to be focused on sites that have undergone a considerable amount of landscaping and engineering works during site construction prior to site breaching, such as the creation of artificial drainage channels and changes in elevation to encourage a range of habitat types (e.g. Burgess et al., 2014; Dale et al., 2017). As a result of the changes to the elevation, morphology and drainage systems caused by the pre-breach landscaping, these sites may not provide a realistic representation of the natural patterns and rates of sedimentation following intertidal inundation. Furthermore, these studies have relied on traditional surveying techniques, such as differential GPS and LiDAR (e.g. Chirol et al., 2018; Lawrence et al., 2018). As a result, small (but important) changes in morphology such as embryonic creek formation and the availability of distinct elevation niches, which are important for topography variability and therefore plant diversity (e.g. Morzaria-Luna et al., 2004), are likely to have been missed. To address these shortcomings, Dale et al. (2020) demonstrated the benefits of using repeat high resolution digital surface models (DSMs) to examine morphological change in MR sites and intertidal wetlands more widely. These authors produced DSMs using the emerging low-cost photogrammetric method Structure-from-Motion, on images collected using a small-unmanned aerial system (sUAS), in order to identify the morphological evolution. Nonetheless, their study focused on a specific area under 2000 m2 and known to be developing morphologically, and failed to provide a holistic evaluation of morphological changes across the entire site as it developed.
Therefore, there is a need for inclusive high resolution, full-site studies of recently breached non-engineered MR sites to assess the morphological evolution without the influence of extensive (and costly) site design and engineering works. To address this requirement, this study presents a whole site analysis of a recently breached non-engineered MR site at Cwm Ivy Marsh, Gower Peninsula, Wales. Specifically, the rate and spatial variability of the morphological evolution through measurements of the morphological change, drainage network development and sediment accretion and erosion across the site are evaluated via topographic surveys collecting using a sUAS. These measurements are combined with analysis of the suspended sediment concentration (SSC) in relation to the pattern of sedimentation to evaluate the variability in the deposition and supply of sediment over a tidal cycle. In addition, the preservation of the terrestrial surface, and its influence on the physio-chemical evolution of the sediment sub-surface, is assessed through sediment core analysis.
Section snippets
Study site
Cwm Ivy Marsh is situated on the Gower Peninsula (Fig. 1), on the northern coast of the Bristol Channel; a major inlet on the south west coast of the UK which separates south Wales from southern England. The site, which is located in the Loughor estuary, is banked by sand dunes and the open coast to the north and northwest, and an expansive area of saltmarsh to the northeast. Tidal waters are fed to the site through external marsh's dendritic creek network and there is limited freshwater input
Morphological development
Analysis of the two small-Unmanned Aerial System (sUAS) topographic surveys showed that, over the four-year period, 55.24% of the 383,000 m2 new intertidal area experienced a change in elevation above the level of detection (LoD). Of this area, 22% (12% of the entire site) experienced erosion, with sediment accreting in the other 78% (43% of the entire site). Erosion was limited to the areas around the main channels running through the site, apart from in the south-western corner of the site
Surface morphological evolution
Examination of the spatial and temporal pattern of erosion and accretion at Cwm Ivy Marsh, presented via an innovative combination of sUAS derived topographic and morphological, hydrological and geochemical analysis, provides a new insight into the development of newly inundated intertidal wetlands. Evaluation of whole site topographic variability, assessed through analysis of the DSM rugosity, indicated a decrease in surface heterogeneity between the two surveys; and highlights the importance
Conclusion
High spatial resolution topographic models, innovatively combined with short-term high frequency hydrological, bed elevation and sediment core data have been examined to assess the morphological evolution of a non-engineered MR site. During the flood tide SSC decreased, and increased during the ebb, suggesting sediment is transported landwards during the incoming tide and flushed seaward during the outgoing tide. Bed elevation increased and decreased during both the flood and ebb tides, with
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank: Shari Gallop and two other anonymous reviewers for their detailed and supportive comments; Magda Grove and Matthew Leake (both University of Brighton) for their assistance during field work; Corrinne Benbow (National Trust) and James Moon (Natural Resources Wales) for their support, guidance and for providing accommodation during field campaigns; Natural Resources Wales for granting access to previous survey data carried out by Future Aerial Innovations on
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