Counting the contributions of tides and surges to changing coastal flooding

Globally, the frequency of high tide exceedances is increasing with sea level rise. However, the rate of mean sea level (MSL) alone is not enough for estimating changes in coastal flooding. Understanding the drivers of changing exceedance frequency must also factor in additional contributions from tides and surges, accounting for the importance of local variability and interactions. Our novel Joint Probability of Maxima Method represents these complex processes nonparametrically, efficiently enabling the estimation of exceedance thresholds at user defined average recurrence intervals (ARIs). We compared exceedance levels between two recent 19-year epochs for 166 widely distributed coastal tide gauge sites, at 1, 5, and 10 year ARIs. We then quantified the specific contributions of MSL, tide, and skew surge components to the net changes in exceedance levels in metric terms, relating them directly to the height of coastal protections. Our approach demonstrates that high water exceedance levels are, on average, increasing more than MSL alone, and that changing exceedance frequency can depend significantly on local characteristics of sea level variability. On average, exceedance frequency doubled over the epochs assessed.