The Influence of Future Changes in Tidal Range, Storm Surge, and Mean Sea Level on the Emergence of Chronic Flooding

Sea-level rise is leading to increasingly frequent coastal floods globally. Recent research shows that changes in tidal properties and storm surge magnitudes can further exacerbate sea-level rise-related increases in flood frequencies. However, such non-stationarity in tide and storm surge statistics are largely neglected in existing coastal flood projection methodologies. Here we develop a framework to explore the effect that different realizations of various sources of uncertainty have on projections of coastal flood frequencies, including changes in tidal range and storminess. Our projection methodology captures how observed flood rates depend on how storm surges coincide with tidal extremes. We show that higher flood rates and earlier emergence of chronic flooding are associated with larger sea-level rise rates, lower flood thresholds, and increases in tidal range and skew surge magnitudes. Smaller sea-level rise rates, higher flood thresholds and decreases in sea level variability lead to commensurately lower flood rates. Percentagewise, changes in tidal amplitudes generally have a much larger impact on flood frequencies than equivalent percentagewise changes in storm surge magnitudes. We explore several implications of these findings. Firstly, understanding future local changes in storm surges and tides is required to fully quantify future flood hazards. Secondly, existing hazard assessments may underestimate future flood rates as changes in tides are not considered. Finally, identifying the flood frequencies and severities relevant to local coastal managers is imperative to develop useable and policy-relevant projections for decisionmakers. It is not just increases in mean sea level that cause coastal floods to become more frequent. Tidal range is the typical difference between the daily high and daily low tides that arise due to the gravity of the moon and sun acting on the Earth and how the resultant water level changes interact with coastlines. Many studies have shown that past changes in tidal range have led to observed changes in flood frequency that could not be explained by sea-level rise. Despite tidal ranges being expected to change in future, the implications of this for future flood frequencies has never been systematically assessed. We develop a new projection methodology that can consider such changes in flood risk. Our method also allows us to assess how much of an effect the way that weather and ocean patterns coincide with high tides influence how often flooding occurs. We show that for the considered Australian locations, future changes in tidal amplitude are generally expected to have a greater effect on future coastal flood rates than comparable changes in weather and ocean patterns. Understanding and accounting for potential future tidal range changes is important when formulating sea-level rise adaptation plans. We develop a new coastal flood projection method that considers how tides and storm surges coincide, better replicating observations The method facilitates the first ever Australian national assessment of how flood rates may respond to future changes in tidal range and skew surges Percentagewise, changes in tidal range generally have a much larger impact on flood frequencies than equivalent changes in storm surge