Evaluation of Extreme Sea Levels and Flood Return Period Using Tidal-Day Maxima at Coastal Locations in the United Kingdom

Tidal storm surges can result in significant damage and inundation if sea defences are insufficiently robust. Calculation of flood risk enables coastal planners to site sea defences and coastal developments appropriately. Since the original work by Gumbel on extreme value statistics, several modifications and new methods have been proposed for the analysis of tidal inundation, with the Skew Surge Joint Probability Method (SSJPM) recently gaining popularity. However, SSJPM is complex, often requiring manual intervention, and is difficult to automate. Guided by the search for a method specifically applicable to tides that is amenable to automation, this paper proposes several modifications to Gumbel's original approach, including reducing the time unit for ranked maxima. This novel technique, here called TMAX, uses a time unit of one tidal-day, rather than the usual annual maxima (AMAX), to carry out an extreme-level analysis of recent tidal data at 35 United Kingdom coastal locations. The TMAX method gives a significantly better internal fit and reduced variance than the AMAX method and compared to a recent study using the SSJPM method at identical coastal locations, shows broad agreement, yet offers a considerably simpler implementation, amenable to automation. This new approach better informs strategies for coastal management and resilience.