Predictable Changes in Extreme Sea Levels and Coastal Flood Risk Due To Long-Term Tidal Cycles

We demonstrate that long-term tidally induced changes in extreme sea levels affect estimates of major flood hazard in a predictable way. Long-term variations in tides due to the 4.4 and 18.6-year cycles influence extreme sea levels at 380 global tide gauges out of a total of 581 analyzed. Results show coherent regions where the amplitudes of the modulations are particularly relevant in the 100-year return sea level, reaching more than 20 cm in some regions (western Europe, north Australia, and Singapore). We identify locations that are currently in a positive phase of the modulation and therefore at a higher risk of flooding, as well as when (year) the next peak of the long-term tidal modulations is expected to occur. The timing of the peak of the modulation is spatially coherent and influenced by the relative importance of each cycle (4.4 or 18.6-year) over the total amplitude. An evaluation of four locations suggests that the potentially flooded area in a 100-year event can vary up to similar to 45% (in Boston) as a result of the long-term tidal cycles; however, the flooded area varies due to local topography and tidal characteristics (6%-13%). We conclude that tidally modulated changes in extreme sea levels can alter the potentially inundated area in a 100-year event and that the traditional, fixed 100-year floodplain is inadequate for describing coastal flood risk, even without considering sea-level rise.