Quantifying the Mean Sea Level, Tide, and Surge Contributions to Changing Coastal High Water Levels

The likelihood of coastal inundation (high tide flooding) depends on the coincidence of mean sea level (MSL), high tide, and surge, relative to the height of local thresholds. Here we present a nonparametric Joint Probability of Maxima Method for deriving frequency-based exceedance thresholds and for quantifying the changing heights of water level maxima. Our approach builds upon the Skew Surge Joint Probability Method (Batstone et al., 2013, https://doi.org/10.1016/j.oceaneng.2013.02.003) in using predicted high water (HW) level and skew surge components, adding MSL as a third component, and accounting for seasonal variation. From the derived distributions, we obtain threshold levels exceeded at specific average recurrence intervals of 1, 5, and 10 years. Changes in exceedances are compared between two 19-year epochs for 166 tide gauges: 140 from a global data set (GESLA: 1983–2001 vs. 2002–2020), and 26 from an Australian data set (BoM: 1992–2010 vs. 2004–2022). We find that the change in exceedance levels between epochs typically exceeds the respective change in MSL and varies between the different threshold levels. We quantify the specific contribution of each component, finding that MSL contributions are influential at most sites, while tides contribute a lesser amount, and surge contributions vary substantially. Exceedance frequency more than doubles on average and increases most at those locations with the smallest variability in the height of high tide. Our approach serves to address a critical gap in offering specific and localized measures of changes in coastal inundation frequency.