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

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.