Flooding caused by extremes of rainfall, river flow, tides or waves has the potential to devastate towns, cities, communities and our natural environment. Managing this natural phenomenon is essential, and the recent launch of a scientifically robust dataset to predict coastal and estuarine extremes around England, Wales and Scotland is a vital step in future flood and coastal erosion risk management.

The aim of the £485,000 project was to produce consistent, up-to-date data on design sea levels, swell wave conditions (waves generated by storms over a large distance), and tidal surges influenced by weather variations. User-friendly guidance was then developed for specialist organisations to plan successful risk-based flood and coastal erosion management.

Royal HaskoningDHV was commissioned in 2007 by the Environment Agency, in partnership with Defra (Department for Environment, Food and Rural Affairs). Royal HaskoningDHV’s multi-disciplinary team used its extensive national and regional experience of flood and coastal management to collect relevant scientific data. The wider Project Team, engaging specialists in coastal extremes from a number of leading organisations, then derived a bespoke statistical analysis for this project. Finally, Royal HaskoningDHV’s GIS (Geographic Information System) specialists mapped and presented the data spatially in standard GIS database format for use by licensed organisations.

Alastair McMillan, Project Manager, Royal HaskoningDHV, said: “The improved data on coastal extremes means that organisations will essentially be able to extract data, derived using the best available methods, to predict extreme sea levels and wave conditions from a 1 to 10,000 year return period. The project also provides a quantification of uncertainty associated with these estimates.

“We ensured that the datasets and guidance is userfriendly as well as scientifically sound. This will enable it to be used widely by specialist organisations to plan everything from coastal flood modelling, flood mapping work, design of coastal defences, flood risk assessments and strategic planning for development purposes.”

A challenging aspect of the project was the sheer volume of data required to formulate an accurate dataset capable of forecasting extreme coastal activity. Coastal locations experience around 705 high tides per year, and each high tide for each location was analysed. The maximum record length was Newlyn with over 90 years of data and the minimum was for Bournemouth, with 12 years of data. This information was used to identify skew surges (tidal surges caused by the weather) for each location.

The comprehensive data on skew surges was then combined with information gathered for predicted astronomical high tides – tides caused by gravitational forces exerted by the Moon, Sun and the rotation of the Earth – to give overall extreme sea level probabilities. This analysis, named the Skew Surge Joint Probability Method was developed specifically for the coastal extremes dataset.

To ensure the information gathered was consistent, Royal HaskoningDHV’s team analysed tidal gauge records from a number of organisations, including the Environment Agency, SEPA, UK Met Office and UK Coastal Monitoring and Forecasting Service, as well as a number of regional and local datasets. The team also used wave buoy data, and notable historic events such as the tidal flooding of 1953 along the east coast, to build a complete picture of extreme events.

A number of checks were undertaken to ensure the suitability of the data, which was routinely processed and quality controlled. Innovative statistical techniques and mathematical analyses were applied to all data to search for flaws such as missing data ‘spikes’, where gauges had applied erroneously high levels.

A further product from the work was identification of realistic tidal surge shapes. These correspond to the increase and decline of the additional rising of sea levels induced by storms. For the first time, definitive information has been produced on surge shapes likely at different points around the UK coastline. This will greatly benefit the quality and consistency of tidal flood risk appraisals and design of tidal defences.

Launched in early 2011 via a series of presentations and road shows, the improved information on coastal flood boundary conditions and guidance is already being used by Government, planners, engineers and specialist environmental organisations.

A powerful weapon in effectively assessing coastal flood risk under present day conditions from storms capable of causing large damages, the new datasets are proving to be an important tool in coastal planning, both for now and in the future.