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

Abstract. Tidal storm surges can result in significant inundation and damage if sea defences are insufficiently robust. Coastal planners need to know the risk of flooding so that sea defences and coastal developments can be specified and located appropriately. Since the original work on extreme value statistics by Gumbel (1954), several alternatives have been proposed for evaluating the risk of tidal inundation, with the Skew Surge Joint Probability Method (SSJPM) gaining popularity. However, SSJPM is complex and cannot always be applied generally. Guided by the search for a general method having wide application and amenable to automation, this paper re-examines the original approach of Gumbel and proposes a simple modification for combined peak selection and declustering; it is termed here TMAX, since it selects one maximum per tidal day. In comparison with the method of Gumbel (1954) using annual maxima (later termed AMAX), the TMAX method offers more efficient use of extreme data events and in addition, simpler handling of missing data. The results of the TMAX method are compared with those of a recent UK study using the SSJPM method at the same United Kingdom coastal locations. The broadly applicable TMAX method has the potential to offer more widespread calculation of flood return period, thereby improving strategies regarding coastal management and resilience.