The storm surge event that affected the coastal margins of the southern North Sea on 5-6 December 2013 produced the highest still water levels on record at several tide gauges on the UK east coast. On east-facing coasts south of the Humber estuary and north-facing Norfolk, water levels were higher than in the twentieth century benchmark surge event of 31 January-1 February 1953. Maximum significant wave heights were highest off the North Norfolk coast (peak H s = 3.8 m offshore, 2.9 m inshore) and lowest off the Suffolk coast (H s = 1.5-1.8 m inshore); comparable offshore wave heights in 1953 were 7-8 m and ca. 3 m. The lower wave heights, and their short duration, in 2013 explain both localised breaching, overtopping, and back-barrier flooding associated with gravel ridges and relatively low earthen banks as well as the lack of failure in more highly-engineered coastal defences. On barrier coasts and within estuaries, the signal of maximum runup was highly variable, reflecting the modification of the tide-surge-wave signal by inshore bathymetry and the presence of a range of coastal ecosystems. The landscape impacts of the December 2013 surge included the notching of soft rock cliffs and cliffline retreat; erosion of coastal dunes; and the augmentation or re-activation of barrier island washover deposits. Whilst surge event-related cliff retreat on the rapidly eroding cliffs of the Suffolk coast lay within the natural variability in inter-annual rates of retreat, the impact of the surge on upper beach/sand dune margins produced a pulse of shoreline translation landwards equivalent to about 10 years of 'normal' shoreline retreat. The study of east coast surges over the last 60 years, and the identification of significant phases of landscape change -such as periods of rapid soft rock cliff retreat and the formation of new gravel washovers on barrier islands -points to the importance of high water levels being accompanied by high wave activity. Future developments in early warning systems and evacuation planning require information on the variable impacts of such extreme events.
Abstract:Recreation or restoration of salt marsh through the deliberate removal of flood defences (managed realignment or deembankment) is a common practice across Europe and the USA, with potential to enhance delivery of ecosystem services. However, recent research suggests that physical, chemical and ecological processes may be impaired in recreated sites as a result of the modified morphology, sediment structure and hydrology associated with both the restoration process and historic land use. This paper compares physical sediment properties and subsurface water levels recorded in paired natural and de-embanked (recreated) salt marshes in SE England. Using a combination of statistical and time-series modelling, significant differences between the natural and recreated marshes are identified. Sediment properties (bulk density, moisture content and organic content) within each marsh were statistically different and imply that de-embanked sediments are compacted, which may affect subsurface water movement. Analysis of hydrological time series reveals that the de-embanked salt marsh is characterized by a damped response to tidal flooding with elevated and less variable water levels. This, combined with analysis of hydrographs and hysteresis patterns over individual tidal cycles, suggests that fast, horizontal near-surface flows enhanced by the relict land surface may play a greater role in drainage of the de-embanked salt marsh. The importance of hydrological functioning in governing many important physical and biogeochemical processes in salt marshes suggests any modifications would have significant implications for the delivery of ecosystem services.
Saltmarshes are being lost or degraded as a result of human activity resulting in loss of critical ecosystem services including the provision of wild species diversity, water quality regulation and flood regulation. To compensate, saltmarshes are being restored or re-created, usually driven by legislative requirements for increased habitat diversity, flood regulation and sustainable coastal defense. Yet, there is increasing evidence that restoration may not deliver anticipated ecosystem services; this is frequently attributed to poor drainage and sediment anoxia. However, physical sediment characteristics, hydrology and the sediment geochemical environment are rarely examined in restoration schemes, despite such factors being critical for plant succession. This study presents the novel integration of 3D-computed X-ray microtomography to quantify sediment structure and porosity, with water level and geochemical data to understand the impact of pre-restoration land use and disturbance on the structure and functioning of restored saltmarshes. The study combines a broad-scale investigation of physical sediment characteristics in nine de-embanked saltmarshes across SE England, with an intensive study at one site examining water levels, sediment structure and the sediment geochemical environment. De-embankment does not restore the hydrological regime, or the physical/chemical framework in the saltmarshes and evidence of disturbance includes a reduction in microporosity, pore connectivity and water storage capacity, a lack of connectivity between the sub-surface environment and overlying floodwaters, and impeded sub-surface water flow and drainage. This has significant consequences for the sediment geochemical environment. This disturbance is evident for at least two decades following restoration and is likely to be irreversible. It has important implications for plant establishment in particular, ecosystem services including flood regulation, nutrient cycling and wild species diversity and for future restoration design.
Observations of plant-flow interactions on salt marshes have revealed a highly complex process dominated by the tightly coupled effects of plant characteristics and hydrodynamic conditions. This paper highlights the importance of vegetation structures such as plant density and height, as well as their spatial variability and mechanical properties including flexibility, upon energy dissipation and flow modification. Many field, laboratory and modeling studies which attempt to predict flow dissipation or improve our understanding of plant-flow interactions use simplified structural measures of salt marsh vegetation or artificial representations. These simplifications neglect important plant and canopy elements and are unlikely to be truly representative of their natural counterparts. Such approaches limit our understanding of plant-flow interactions and potentially compromise the predictive accuracy and application of numerical flow models. It is important therefore that improved techniques to measure vegetation structure are adopted in order to better define the key relationships between measurable plant characteristics and dragrelevant plant properties.
Salt marshes can play a vital role in mitigating the effects of global environmental change by dissipating incident storm wave energy and, through accretion, tracking increasing water depths consequent upon sea level rise. Atmospheric CO concentrations and nutrient availability are two key variables that can affect the biological processes that contribute to marsh surface elevation gain. We measured the effects of CO concentrations and nutrient availability on surface elevation change in intact mixed-species blocks of UK salt marsh using six open-top chambers receiving CO -enriched (800 ppm) or ambient (400 ppm) air. We found more rapid surface elevation gain in elevated CO conditions: an average increase of 3.4 mm over the growing season relative to ambient CO . Boosted regression analysis to determine the relative influence of different parameters on elevation change identified that a 10% reduction in microbial activity in elevated CO -grown blocks had a positive influence on elevation. The biomass of Puccinellia maritima also had a positive influence on elevation, while other salt marsh species (e.g. Suaeda maritima) had no influence or a negative impact on elevation. Reduced rates of water use by the vegetation in the high CO treatment could be contributing to elevation gain, either directly through reduced soil shrinkage or indirectly by decreasing microbial respiration rates due to lower redox levels in the soil. Eutrophication did not influence elevation change in either CO treatment despite doubling aboveground biomass. The role of belowground processes (transpiration, root growth and decomposition) in the vertical adjustment of European salt marshes, which are primarily minerogenic in composition, could increase as atmospheric CO concentrations rise and should be considered in future wetland models for the region. Elevated CO conditions could enhance resilience in vulnerable systems such as those with low mineral sediment supply or where migration upwards within the tidal frame is constrained.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.