An assessment of the unintended consequences of structural coastal flood protection

Abstract

This thesis investigates the unintended socio-hydrological consequences of structural coastal flood protections (SCFPs) and assesses the implications for coastal flood risk management strategy in the UK. Climate change, and the continued urban development of flood exposed areas can exacerbate coastal flood risk, and thus flood risk management authorities often tend towards structural coastal flood protection measures to minimise losses. However, these structurally proactive measures can lead to infrastructural lock-ins, whereby the decrease in flood probability from the defence can lead to increased urban development and population, ultimately leading to higher losses due to an inundation event. This process has been referred to as the Safe Development Paradox (SDP), a cross-cutting science-practice-policy challenge that requires a systematic understanding in the context of increased uncertainty associated with climate change and the United Nations Sustainable Development Goals. However, literature of the phenomena is limited, compounded by a lack of consistent terminology, limited geographic distribution, and a skewed emphasis on fluvial flooding. Moreover, despite being an island nation, the UK, to date, has had very little research conducted into these unintended consequences of structural flood protection. This thesis developed and applied a methodology that captures these coupled human-flood processes, by integrating well-established methods from other spheres of flood risk assessment in a novel way to explore the currently poorly understood phenomena in coastal settings. The study contributes to addressing this knowledge gap based on insights from three contrasting UK case studies: Portsmouth, Weston-super-Mare, and Southport. Differential analysis of historic LiDAR Digital Surface Models (DSMs) was used to identify temporal changes in the urban landscape to create a DSM of Difference (DoD), representing elevation change between two locations over time. Geostatistical testing, specifically t-tests, were then used to infer statistical significance of changes in urban development. The results reveal a consistent pattern: following completion or improvement of large-scale SCFPs, there is subsequent, and statistically significant, increases in coastal population and urban development within/near flood-exposed areas in all case studies, contrary to the limited flood-exposed development in neighbouring settlements, with no comparable defences constructed, or upgraded, during the same period. On average, new urban development occurs approximately 2 years after the completion of coastal flood defence projects. These data were then inputted into a newly developed agent-based model (ABM) that simulates futures changes under different climate scenarios. The results demonstrate that each SCFP project led to an initial decrease in Affected Population (AfP) following implementation, confirming the intended immediate benefits of flood risk reduction. However, long-term projections revealed significant unintended consequences under the scenarios where SCFPs were exceeded by Extreme Coastal Water Levels (ECWLs). For Southport and Weston-super-Mare, the ABM output shows a dramatic increase in AfP once ECWL surpassed the SCFP crest height, affecting a larger population than those initially protected, primarily due to the increased population growth behind the defences and the larger flood extent. Portsmouth, however, exhibited a more limited increase in AfP, attributed to its high urban density and limited room for further development behind the defences. This highlights how pre-existing land-use and population density can act as brakes on the unintended consequences of SCFPs. The thesis concludes with a recommendation for future flood risk managers and policymakers to be aware of these unintended socio-hydrological consequences. SCFPs are crucial assets, and their construction and maintenance will continue to play an integral role in coastal adaptation to climate change, particularly in highly developed urban settlements. This thesis does not attempt to provide a comprehensive predictive modelling tool for planning, nor a detailed analysis of real estate markets, but instead focuses on socio-hydrological interactions of population change and SCFP. However, new SCFP design and implementation need to account for their long-term unintended consequences on communities and climate adaptation planning. In the short-term, flood risk communication provides a means of tackling these risks, improving flood memory, awareness, and preparedness. Furthermore, in the longer term a more holistic cost-benefit analysis and spatial planning strategy, internalising these factors should be utilised in order to create more sustainable and resilient coastal communities in the UK.