Synthesising Unprecedented Coastal Conditions: Extreme Storm Surges (SUCCESS)

This project will engage with two project partners (Environment Agency and EDF energy) with significant assets and infrastructure at risk from extreme storm surges. For both partners this project will deliver understanding of the impacts of plausible extreme coastal surge and wave events on the function, resilience, design and standard of protection of key infrastructure. It will provide them and other Flood and Coastal Erosion Risk Management Authorities with an improved level of understanding around current and future standards of protection. For key coastal regions - determined with our partners - we will synthesise a number of "black swan" storm surges - events that have not been observed but that are physically plausible. This has never previously been done for extra-tropical weather systems. It is important to sample storminess beyond the observed range of natural variability since our record of severe storm surges is probably too short (we have only had two extreme North Sea storm surges in 60 years - in 1953 and 2013). We will do this by analysing and grouping European storm systems from reanalysis data, and then perturbing the atmospheric systems using a well tried and tested forecasting tool (made available to us by the Met Office). The modified wind and pressure fields will drive coupled storm surge and wave models to create the plausible worst cases. Our work will provide a credible alternative for worst case storm surges that complements the H++ scenarios obtained from climate models alone.

The results of the project will assist our project partners and other stakeholders in planning and mitigation, the siting and protection of coastal infrastructure, and long term investment decisions. Our deliverables will take the form of: 2-D data fields for storm surges and waves along the affected regions (determined with the partners); new calculations of extreme value statistics for those regions; site-by-site analyses of tide/surge/wave combinations that then feed into the downstream modelling of the two partners. For the Environment Agency, the new data would feed into National and / or local flood risk and forecasting models to help understand what impacts would be associated with such events to inform investment decisions and incident preparedness. The outputs would also be used both to quality control the current best-practice statistical methods for estimation of extreme sea levels and extend those. For EDF Energy, the new data will feed into their current statistical methodology for estimating return levels of extreme sea levels and provide information that could be used in strategic decisions on probable maximum extremes and lower bounds on the 10,000-year level for different natural hazards.

For the Environment Agency this project will deliver understanding of the impacts of plausible extreme coastal surge and wave events on the function, resilience, design and standard of protection of key infrastructure. It will also provide them and other Flood and Coastal Erosion Risk Management Authorities with an improved level of understanding around current and future standards of protection. The extreme storm surge and wave values would feed into National and / or local flood risk and forecasting models to help understand what impacts would be associated with such events to inform investment decisions and incident preparedness. The outputs would also be used both to quality control the current best-practice statistical methods for estimation of extreme sea levels, and also form an element of a planned update to that Coastal Boundary Conditions project. The knowledge would also feed into current analyses being undertaken as part of the National Risk Assessment (updates to scenario H19). The proposed work would provide a new dimension to EA hazard modelling exercises in support of incident management: it would facilitate a more intelligent treatment of extreme coastal scenarios rather than a simultaneous (and unrealistic) 1:1000 year return period event everywhere.

For EDF Energy the new data will feed into their current statistical methodology for estimating return levels of extreme sea level. In particular, the results from the project would be compared against current work within EDF Energy that uses purely statistical approaches to simulate wind and wave fields that are more extreme than those previously observed. The comparison would allow them to assess the benefits of an approach that suggests improvements to their statistical models. The work would also provide information that could be used in current discussions on probable maximum extremes and lower bounds on the 10,000-year level for different natural hazards. By perturbing extreme events in the observational record to produce even more extreme events, one can assess whether current expected upper bounds are realistic. If a particular upper limit is found to be realistic, this could also help to screen out results from future statistical analyses which are excessively large and therefore not physically plausible. An additional benefit of the current methodology would be to have information of direct extreme sea heights and waves at many sites around the UK, especially sites that currently are far from national "Class A' tidal gauges.