Evaluating the viability of coastal wet grassland to a changing management regime through flood hazard modelling

Highlights

• Preventing breaching is key for sustainability of embanked coastal habitats.

• Reprofiling and lowering of embankments may ‘buy’ extra years of flood defence.

• 10 days inundation potentially a key threshold for wet grassland community change

Abstract

The high-risk nature of low-lying coastal areas means that many are characterised by protective dikes, sea walls, and earthen embankments. Across Europe, such structures are employed to limit landward sea flooding and have been used historically to reclaim wetlands, many of which have since gained international protected habitat status. Sea level rise and storms threaten the future viability of fixed flood defences which will be overtopped with increasing frequency and, in extreme cases, breached. This raises questions surrounding defence maintenance and the fate of the reclaimed wetlands they encircle and support. One approach is to design defences with the expectation that they will be overtopped during extreme events, but that catastrophic failure as a result of breaching will be prevented. Such an approach was recently implemented at Blakeney Freshes, North Norfolk Coast, UK, an embanked area of reedbeds and coastal wet grassland. Lowering and widening of the Freshes embankment was undertaken following extensive breaching during the 5 December 2013 storm surge. In this paper we develop and apply a numerical model chain, comprising storm surge water levels, waves, and overtopping, to explore inundation extent, depth, and duration resulting from storm surge induced flooding under several sea level rise scenarios. Modelling results revealed that 99.5% of the flood volume of the 2013 event resulted from embankment breaching. Simulating the same storm event after embankment reprofiling shows that flooding of the Freshes is reduced by 97%, largely because the lower, wider embankments preclude breaching. However, under future sea level rise scenarios, storm surge induced overtopping results in increased inundation depths and drainage times, raising questions regarding the resilience of vegetation communities within the Freshes. By 2100 under the lowest SLR scenario, and by 2050 under the mid SLR scenario, over half of the Freshes will be inundated for >10 days, a potentially critical threshold for current wet grassland survival. Our findings suggest that while effective defence redesign may increase the viability of reclaimed wetland habitats in the short term, as sea levels rise, lengthened inundation durations may render these habitats increasingly vulnerable to ecosystem change under extreme events.

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:

• 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;

• 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;

• 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

• Explore how shifts in flooding regime, as a result of sea level rise and management changes, may impact coastal wet grassland vegetation communities.