Adaptation thresholds and pathways for tidal flood risk management in London
Climate Risk Management
A B S T R A C T Protecting the UK's capital city from global mean sea level rise that the IPCC considers plausible over the next centuries would require a combination of a new tidal barrier, high volume pumping and incremental raising of the system of flood walls and embankments. Using a risk and decision analysis methodology that is transferable to other vulnerable coastal cities of high strategic economic and political importance, we quantify sequences of adaptations that would be needed to
... ould be needed to protect London from flooding by the sea to the year 2300. Two critical adaptation thresholds are identified: (i) when mechanical pumping has to be provided alongside the moveable tidal barrier in order to drain the River Thames and (ii) when a permanently closed barrier with pumping to remove all of the river flow becomes the only viable means of avoiding flooding. We test the sensitivity of the costs and benefits of alternative adaptation pathways to a wide range of sea level rise trajectories. The adaptation pathway that most cost-effectively and robustly maintains risk at a tolerable level involves moving the Thames Barrier 17 km towards the sea if mean sea level rises 2 m above present levels. Our methodology provides a quantitative riskbased implementation of an adaptation pathway. T identified five adaptation thresholds in the Thames Estuary. Here we focus upon Thresholds 2 (raising the crest level of the defences), 3 (moving to an outer barrier) and 4 (modification of the barrier so that it operates as a barrage). Threshold 1 (extreme surge events) is implicit in our analysis, whilst our analysis explores engineering options for all rates of sea level rise considered so never reaches an overall engineering limit to adaptation (Threshold 5). Reporting on the TE2100 study, Penning-Rowsell et al. (2013) indicate that "both sensitivity and scenario analysis have little effect on option choice", which is a conclusion that we explore thoroughly in this paper in order to expose critical sensitivities to sea level rise. By doing so we aim to (i) present in-depth quantified analysis of risk, scenario and decisions for adaptation to sea level rise in the Thames Estuary and (ii) advance methodology for quantification of adaptation tipping points, which can be used for analysis of other coastal adaptation challenges. To address these questions we adopt a multi-layered simulation framework (Harvey et al., 2012) , at the heart of which is a model of water levels in the Thames Estuary and flood damage in the tidal Thames floodplain where London is located. The boundary conditions for this model (surge tide water levels in the outer estuary and fluvial flows in the River Thames) are sampled statistically from a joint extreme value distribution. Integration over that distribution provides an estimate of risk. The next layer simulates long term change in the factors that influence risk, e.g. sea level rise. Adaptation actions (e.g. raising of flood dikes or moving the Thames Barrier) are triggered in response to changing risk. The simulation framework calculates the costs and residual risk associated with all possible adaptation pathways. Finally, the sensitivity of those estimates to exogenous uncertainties (e.g. the rate of sea level rise) is systematically explored. We apply this framework to quantify the effectiveness of alternative adaptation strategies, in order to identify adaptation thresholds i.e. critical thresholds of sea level rise when the approach to management of tidal flood risk has to fundamentally change, by some combination of relocating the Thames Barrier, installing pumps to assist the gravity drainage of the River Thames, and finally abandoning gravity drainage with opening sluice gates altogether. We conclude with reflections on what this work implies for the concepts of 'adaptation thresholds' and 'adaptation tipping points'.