Combination Hazard of Extreme rainfall, storm Surge & high Tide on estuarine infrastructure (CHEST)

Background and challenges: UK estuaries are at risk from combination flooding. Sea-level rise and predicted changes to UK storm patterns (affecting both surge and river flows) will alter the joint probability of multiple hazard events, making previous understanding of risk, and mitigation measures, potentially obsolete. Existing probabilistic methods for assessing combination hazard (e.g. FD2308) provide only water level hazards, with limited detail on where and when issues may occur, and vitally cannot readily accommodate revised estimates of event distributions due to sea-level rise and climate change.

Combination hazards, therefore, present a clear risk to project partner's infrastructure in estuaries (e.g. Flood defences, railways, roads, water treatment works, nuclear power stations), both under present conditions - and more with greater uncertainty in the future. The ability to better forecast the specific locations and timings of such combination hazards will enable effective planning and timely warnings to industry operators, partners and the public.

All of our project partners have considerable investments in areas prone to estuarine flooding including flood defences (EA), rail networks (Network Rail), water supplies (Welsh Water) and nuclear power stations (EDF Energy). The scale of national exposure to combination hazard is phenomenal. Taking the Humber alone, the projected economic costs of the 2015 surge tide occurring with 0.3m of sea level rise would be £12.5bn in direct flood damage and £10.8bn in consequential losses. Quite simply, our project partners cannot ignore how combination hazard may increase with climate change.

Aims & Objectives: CHEST will assess if the management of combined river-surge-wave-tide flooding in UK estuaries (and surrounding low-lying areas) needs to be amended, especially in the light of sea-level rise and changing climates. This leads to three objectives:
1. Establish if the interaction between the combination hazards of rivers, surge, waves and tides is important to resolve for estuarine flood risk.
2. Determine which of these factors and in what combination pose the greatest risk. i.e. What is the sensitivity to the different hazards and does it differ in different locations (a) within an estuary (b) between different estuaries?
3. With sea-level rise and climate change - ascertain whether the relative importance of the combination hazards change or shift. i.e. with SLR do surges become more dominant?

To achieve these objectives, CHEST will use novel, fast numerical models (developed in previous NERC and EA funded projects) to simulate the combination hazard for two contrasting pilot studies (Humber and Dyfi estuaries) where the primary project partners have particular concerns. In a second phase the CHEST modelling framework will be documented, packaged and where necessary tailored to a wider group of additional project partners allowing the application to estuaries around the UK.

Deliverables will be predictions in pilot basins of 'worst case' scenarios, system sensitivity to different combination hazards and for the impacts of climate change (Milestones 1, 3 & 4). As requested by PP's these will be in the form of digital maps/data of flood inundation areas, max depth, max velocities, water surface (AOD), flood duration, and a matrix/look up table for combination hazard sensitivity. For the model roll out phase, deliverables will be the software (source and exe) along with documentation and tutorials (Milestone 5). This may also involve tutorials/webinars - as requested by PP's at meetings C and D.

Project duration is 10 months involving researchers from the University of Hull and Bangor University. Cost is £104 700 (80%) including 10 months PDRA (Hull); CoI and PI costs and travel for meetings. Lewis' contribution is in kind (20% for 10 months), as a Ser Cymru NRN-LCEE Research Fellow on the QUOTIENT project.

The main expected benefits to all project partners will be an increased understanding of the magnitude and likelihood of flooding from a combination hazard impacting on their infrastructure and its operation. This includes developing a method to determine how this hazard may shift due to climate change with the availability of the next generation of climate projections (e.g. UKCP18); as well as the how the sensitivity of estuaries (of different sizes and shapes) to hazards may alter (e.g. which combination of hazards is most potent in what context). For ALL project partners this will influence day to day management, incident response as well as planning for future investment decisions.

For our primary project partners, the Environment Agency, the findings from CHEST will feed directly into the Humber Strategic Review presently being developed, answering questions for future flood defence investment. Furthermore, this proposal directly addresses some of the questions raised by the National Flood Resilience Review, including using integrated modelling for flood forecasting and the potential to adjust fluvial flows to represent changing land use policies (e.g. NFM). Due to the ability of combinations of hazards to shift loci of flooding from typical vulnerable spots - this will provide important insight upon which to base investment decisions (e.g. defence weaknesses or where to locate flood storage/managed retreat sites). A key feature of CHEST for the EA is the sensitivity analysis enabling the EA to understand the role the different factors play, and which are most important in determining the picture of risk. This will help the EA more effectively manage risk during an actual incident. It will help inform the future EA Long- term Investment Strategy (LTIS) program as CHEST may highlight the potential "currently "unquantified" risk in these areas, especially if it is readily transferable to other key locations, nationally.

For primary project partners, Network Rail, we will overlay their infrastructure maps with model results of flood inundation locations and depths - enabling them - for the first time to establish how vulnerable their assets are to existing flood hazard as well as combination hazards. We will also integrate the geotechnical properties of their assets (e.g. railway embankments) enabling them to make rapid decisions on asset viability based on inundation durations as well as depths. For both Dyfi and Humber estuaries this represents a significant improvement allowing very high resolution identification of likely sites of vulnerability to flooding and combination hazards.

Following successful piloting of this project on the Humber and Dyfi estuaries, we will involve our secondary project partners to help guide a roll out to other locations around the UK. The aim is to provide secondary project partners with the modelling tools to readily assess other estuaries for combination hazard risk. These include several nuclear power stations (EDF) and assets held by clients of partners ARUP, Atkins and Welsh Water. EDF are interested in the vulnerability of their nuclear installations especially to 1 in 10000 year extreme events as well as to climate change. Welsh Water in particular are concerned about asset damage (waste water treatment works) from saline intrusions and flooding extent from potentially untreated sewerage - two processes that they know little about. Their asset concerns extend throughout Wales and they will roll out the tools we develop for water quality studies.