Non-linear tide-surge interactions around the coast of the UK through the lens of tidal level, phase, and skew surge

Coastal flooding, driven by extreme sea levels, is a significant threat to the coastline of the United Kingdom. The primary contribution to extreme sea levels is the combination of tide and surge and understanding how these components interact is critical to assessing extreme sea levels at the coast. Here, we analyse the interactions of skew surge and tidal high water, non-tidal residual and tidal phase, and non-tidal residual and tidal level using the entire observational tide gauge network of the UK, a near 500-year model, and a model run of 2013/14 with an artificially adjusted forcing to examine how storm arrival time impacts these interactions. We show that the levels of tide-surge interaction at most sites are relatively insensitive to the magnitude of the extreme value threshold and the declustering window size. Measured data show greater levels of interaction than modelled data and although there is little interaction between skew surge and tidal high water, there are sizeable tide-surge interactions between the non-tidal residual and the astronomical tide, the largest being for tidal phase. Around the UK, extreme non-tidal residuals generally occur favourably between 1 and 5 h before tidal high water and at tidal levels that are at, or below, the average tidal level. When storm arrival time is artificially shifted, the overall change in interaction around the UK is relatively small, with skew surge and non-tidal residual maxima occurring at similar respective tidal high waters, tidal phases, and tidal levels, although variation is seen on smaller spatial scales. Our findings advance the understanding of non-linear tide-surge interactions around the UK, which is essential for the accurate estimation of extreme sea level probabilities and thus the defence of the coastline against coastal flooding.