Evaluating the viability of coastal wet grassland to a changing management regime through flood hazard modelling
Introduction
Coastal flood risks are the product of hazard and vulnerability, and are expected to increase over the coming decades. Climate change drives hazards of accelerating rates of sea level rise and potential increasing storminess (Nerem et al., 2018; Hartmann et al., 2013), which accompanies increased vulnerability/exposure of the world's low-lying coastal zones arising from greater human occupancy (Hinkel et al., 2014).
The traditional response to the risk of coastal flooding, over many centuries, has been to build defences such as dikes, sea walls and earthen embankments. However, such fixed defences bring with them continued and costly maintenance regimes, exacerbated by the need to repeatedly heighten and widen such structures in response to changes in mean water level resulting from sea level rise. Thus, for example, it has been calculated that the mean increase in coastal flood defence height required in Europe to keep current risk constant will be 0.5 m by 2050 and 1 m by 2100 (Vousdoukas et al., 2018). Whilst in some locations the protection of people and assets means that hard defences are the only option, in other locations rising costs, and the dis-benefits resulting from changing flood and erosion regimes from interference with natural coastal dynamics, has forced more attention to be directed towards non-structural responses to coastal change (e.g. Temmerman et al., 2013).
Socio-political landscapes have also been re-configured over the last fifty years around a much greater concern for the maintenance of coastal biodiversity and coastal ecosystem services (MEA, 2005). European legislation, in the form of the EU Habitats Directive, Birds Directive and Water Framework Directive, has designated large areas of reclaimed, often grazed, wet grasslands as Special Areas of Conservation. Designation has been on the basis of their unique assemblages of plants, invertebrates and birds, in part related to a hydrological regime that allows drainage of freshwater through sea walls via networks of drainage ditches, culverts and tidal sluices. Such designations, however, effectively block the restoration of full tidal exchange (Pethick, 2002). Nevertheless, even if embankments between seaward salt marshes and landward wet grasslands cannot be dismantled, a coastal defence function can be provided by allowing the overtopping of defences by storm waves and tidal surges during extreme events and hence the temporary storage of floodwaters over wet grassland surfaces. Thus, for example, in the UK east coast storm surge of 5 December 2013, Spencer et al. (2015) and Skinner et al. (2015) document the flooding of 1000 ha of coastal habitats and agricultural land on the Norfolk and Suffolk coasts and 7000 ha of urban, industrial and agricultural areas in the Humber estuary respectively. It is clear, therefore, that significant volumes of floodwater may be stored in this way under extreme conditions making a real difference to event-related coastal safety.
Designing for such storage is challenging and location-specific; many questions arise. In the case of earthen embankments, how can defences be designed to allow for overtopping but not risk defence breaching (where repair costs are considerable and access routes along defences can be legally significant)? What is the most appropriate trade-off (i.e. bank height) between allowance of more frequent inundation of freshwater wetlands and the long-term maintenance of grazing wet grassland biodiversity and ecosystem services? How might the nature of this trade-off change with rising sea levels and increased storminess? Rather than waiting for such a changed flooding regime to occur, and reacting to it, environmental modelling offers the possibility of scenario testing for future conditions not yet realised by the ecosystem.
In this paper we address these issues and approaches through a modelling study of the Blakeney Freshes, a site of nationally and internationally recognised wet grassland on the barrier coastline of North Norfolk, UK east coast. Specifically, in this paper we:
- 1)
Build and calibrate a model train framework to evaluate the impact of a major storm surge (5 December 2013) on an embanked wet grassland and reedbed area, comparing model outputs with known patterns of seawater flooding and drainage;
- 2)
Evaluate the management response to this storm surge flooding event – the repair and re-profiling of earthen embankment defences – and compare the impacts from breaching of a traditional high and narrow defence line to that of overtopping of a reconfigured defence of lower crest height and broader cross-sectional profile;
- 3)
Model future flood depths, extents and durations from a combination of 5 future sea level rise scenarios, variously to 2050 and 2100, in combination with a 2013-type storm, under this reconfigured defence; and
- 4)
Explore how shifts in flooding regime, as a result of sea level rise and management changes, may impact coastal wet grassland vegetation communities.
Section snippets
Location
The 45 km long North Norfolk coast is a barrier island coastline, lying between the chalk headland at Hunstanton and 20 m high cliffs in glacial deposits at Weybourne (Fig. 1a, b). The 2 km wide low-lying coast is characterised by extensive subtidal and intertidal mudflats and sandflats; gravel and sand barriers separated by tidal channels and ebb tide deltas; and back-barrier channels (or ‘creeks’) and saltmarshes (Andrews et al., 2000). Landward margins are characterised by sand dunes (some
Tidal levels and Extreme Water Levels
The North Norfolk coast has a macro-tidal regime, with a mean spring tidal range of 6.5 m in the west at Hunstanton, reducing eastwards to 4.4 m at Cromer. Mean High Water Springs at Blakeney is reported as 2.60 m ODN. The comparable figure for the Cromer Tide Gauge, 25 km to the east (Fig. 1b), is 2.15 m ODN with a Highest Astronomical Tide of 2.79 m ODN. Highest Astronomical Tide (HAT) at Blakeney is not known but is probably ca. 3.3 m ODN (EACG, 2010). In a UK-wide assessment of coastal
Modelling 2013 storm surge flooding of the Blakeney Freshes
The approach to modelling the 2013 flood extents involved a nested, four-stage approach (Fig. 3a) which models the transformation of waves and tides from offshore to nearshore, calculates wave overtopping and flow discharge into a flood inundation model at Blakeney Freshes (methodology adapted from Jäger et al., 2018).
Bathymetry and topography
Bathymetric and topographic data were obtained from the UKHO (UK Hydrographic Office) MEDIN bathymetry dataset, UK Environment Agency (EA) and EDINA Digimap Ordnance Survey
Modelling the performance of the post-2013 repaired and re-profiled embankment at Blakeney Freshes
The maximum flood depths and extents, as applied to the new defence configuration, for the 2013 surge, and for each of the five sea level rise scenarios are presented in Fig. 7. Significant flooding within the Freshes occurs in all scenarios tested, with RCP4.5 (50th percentile) for 2100 having the highest maximum volume within the Freshes (5.58 × 106 m3), whilst the 2013 storm surge scenario has the lowest volume (1.92 × 105 m3).
In all scenarios, flood water depth is greatest on the western
Discussion
In the face of increasing flood inundation due to climate change and extreme events, there is a fine balance between the viability of fixed flood defences and the expectation for total flood protection, particularly in low-lying uninhabited areas. The difference between the water volume, and flooding extent, within the Blakeney Freshes between the modelling of the actual 2013 event (i.e. with the old embankment configuration) and the modelled impact of the same event under the repaired and
Conclusions
As sea level rise accelerates, habitats formed through reclamation of low-lying coastal areas are increasingly under threat. Reclaimed wetland environments are now highly valued, as attested by nationally and regionally recognised protected status. Arising from extensive reclamation during the seventeenth century, Blakeney Freshes epitomises the challenge of maintaining the viability of coastal wet grassland with uncertain climate change and within the bounds of financial feasibility and
Declaration of Competing Interest
None.
Acknowledgements
This work was supported by the EU FP7 collaborative grant, Resilience-Increasing Strategies for Coasts – toolkit (RISC-KIT) (grant no. 603458), Isaac Newton Trust: Environmental modelling for better management of important coastal habitats (1507 (h)), the NERC BLUECoast project (NE/N015878/1; NE/N015924/1), and the NERC/ESRC Data, Risk, and Environmental Analytical Methods (DREAM) Centre (NE/M009009/1). Arnas Palaima was supported by funding from the European Union's Horizon 2020 research and
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