Exploring co-occurring UK HYDRo-meteorological extremes that exAcerbate risk (HYDRA)

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In wintertime, infrastructure in Great Britain (GB) is threatened by multiple significant meteorological hazards, and it is increasingly apparent that these interact in a complex yet poorly studied multi-hazard system (e.g. Hillier et al, 2020). For example, the impacts of the dramatic February 2022 storm sequence (Dudley, Eunice, Franklin) demonstrated the pressing need for a more sophisticated view of GB's multi-hazard risk. The high windspeeds of Eunice left over 1 million homes without power and caused major transport disruption (trains and flights cancelled, roads blocked) in southern England, with snow causing further disruption in Scotland and northern England. Persistent heavy rain then resulted in the flooding of >400 homes during Franklin. In addition, there was ~£3 billion in direct insurance damage, road closures due to landslips (e.g. A57 'snake pass', Derbyshire, closed for 1 month) and other societal impacts in GB (e.g. school and port closures, trees felled). The impacts underlined the need to properly account for severe episodes that might (i) encompass a number of storms, (ii) span many days, and (iii) present a variety of hazards that manifest in different locations and affect various critical systems of UK plc (e.g. infrastructure).This contrasts to current practice where GB's hydro-meteorological hazards (related to wind and rain) are almost always considered separately (e.g. the UK's Climate Change Risk Assessment very much places single weather risks into siloes). There is some grouping for convenience (e.g. risks to transport from high and low temperatures), and the related issue of cascading failure is discussed, but there is no integrated logic applied to risk driven by co-occurring hazards. Likewise, insurers apply sophisticated, stochastic risk models (e.g. >10,000 simulated events), yet inland flooding and extreme wind are still modelled as separate and independent.A critical advance to better understanding flooding and extreme wind was identifying that they were systematically linked (Hillier et al., 2015). Spurred on by this initial observation, various UKRI funded projects (i.e. UKCGFI, a KTP with Lloyds Bank Group, STORMY-WEATHER, ROBUST) are building a scientific evidence base for a link between these two hazards. As part of this, in 2022 the Bank of England introduced an initial (cautious) requirement for insurers to account for this link in the stress tests to which they must submit. HYDRA will take this work to a new level, extending this important line of enquiry to the wider wintertime multi-hazard system (i.e. inland flooding, landslide, extreme wind, storm surge, extreme cold, snow). HYDRA is an exploratory project that aims to identify, quantify, and provisionally explain how the co-occurrence of six key GB hydro-meteorological wintertime extremes exacerbates risk. HYDRA will use historical observations (assimilated in ERA5, GLOFAS) and UKCP18 regional climate projections to better understand how linked hazards will evolve into the future. Rail and other infrastructure providers' sensitivity to identified co-occurrences will then be mapped in workshops book-ending the scientific work. HYDRA's science is ground-breaking due to the lack of systematic, highly multi-hazard risk evidence currently available. In addition to contributing to NERC science, it will define the engineering (EPSRC) challenge by identifying drivers for risk modelling of infrastructure networks, and will feed into policy (via DEFRA and the CCRA) and industry (e.g. reinsurance) practice.