Tidal marsh restoration for flood risk mitigation : The effectiveness of managed realignment at Freiston Shore, Lincolnshire, UK
The ecosystem services delivered by coastal wetlands are among the most valuable onthe planet, including the mitigation of climate related risks by sequestering carbon at rates several orders of magnitudes faster than tropical rainforests as well as the provision of natural coastal protection. Despite their ecological and socio-economic importance, coastal wetlands have been lost on a large scale in the past centuries, mostly due to human induced stress factors. The projected acceleration of Sea-Level Rise (SLR) may exacerbate the vulnerability of wetlands in the coming centuries, particularly in case of limited accommodation space due to the presence of anthropogenic infrastructure. Increasing flood risk for low lying coasts and the continued reliance on traditional engineered solutions that have become economically and ecologically unsustainable in many locations require the development of new measures to mitigate these risks. In the last decades, Managed Realignment (MR) has been implemented with the aim to provide a cost effective and ecologically sustainable alternative to conventional coastal defence schemes. MR constitutes one of several Nature-Based Solutions (NBS) making use of the natural wave and surge attenuating capacity of coastal wetlands and their ability to build up vertically at rates often higher than historical SLR. MR involves the realignment of river, estuary or coastal defences to (re-)establish tidal exchange, supporting the formation of coastal wetlands such as mudflats and saltmarshes. Yet, an important knowledge gap constitutes lacking evidence on the true protective value of MR, which fosters political and societal opposition, ultimately counteracting large scale coastal restoration efforts. This thesis tackles the above knowledge gap in a combined approach, including field measurements and hydrodynamic modelling, to study the effectiveness of High Water Level (HWL) attenuation across one of the earliest and, at time of establishment, largest coastal MR schemes of the United Kingdom: Freiston Shore, located in Lincolnshire on the east coast of England. Between August and October 2017, a series of 16 water level loggers was deployed across the MR site and the adjacent natural saltmarsh to measure the reduction of peak water levels during the highest tides of the year. Subsequently, these data were used to calibrate and validate a hydrodynamic model of the study area, which enabled studying the effects of MR scheme design on the site’s HWL attenuation capacity during these tides. In a last step, the model was implemented to investigate HWL attenuation rates inside the MR site under the influence of very high storm surge levels, by additionally identifying MR width thresholds for HWL attenuation in relation to surge height and vegetation cover. The main findings of this thesis are: 1) The MR site of Freiston Shore does not provide effective HWL attenuation under all measured conditions. 2) At the open coast of Freiston Shore, only large and wide MR sites can effectively attenuate very high tides, and the reduction of storm surges with return periods of more than ten years requires MR widths of >1148 m (measured perpendicular to the coastline). 3) Increased vegetation cover and larger MR widths enable the attenuation of even higher surges. 4) Breaching dikes should be preferred over complete dike removal when coastal protection is the target of MR implementation. Three priority areas for future research are recommended: 1) Generating more in situ data on MR internal water level dynamics and HWL attenuations, particularly under storm surge conditions. 2) Freiston Shore’s HWL attenuation function is particularly effective when tidal exchange is restricted through narrow dike breaches, which could also be achieved by applying sluices or culverts (i.e. Regulated Tidal Exchange (RTE)). However, this should be balanced against reduced sedimentation rates, limited MR drainage and vegetation establishment, and the potential erosion and widening of dike breaches, which may all result from breaches being designed too narrow or RTE. 3) Investigating the effectiveness and applicability of several MR schemes and other NBS to mitigate flood risks on a regional scale.