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Abstract

In this talk, we borrow a trigonometric concept used in architecture—the sagitta—to explore how tidal and storm surge properties influence the duration of coastal flooding, and therefore impact. We find analytically that 4 factors are important: the effective tide-wave period, the maximum water-level above a datum, the tidal amplitude, and the interaction of diurnal and semi-diurnal tides (or tides and surge). Some regions—such as the US Gulf Coast—have a combination of tidal period and tidal amplitude that leads to long flood durations. Regions with mixed tides—such as the US West coast—have longer duration floods than strongly semidiurnal regions, such as the Gulf of Maine. Through an analysis of >1000 tide gauge data sets in the USA, we show that the potential duration of flooding varies greatly within individual estuaries (e.g., San Francisco Bay, Chesapeake Bay, Saint Johns River) and on regional and basin scales. Flooding from storm surge and river flood waves are strongly modulated by tidal properties. Moreover, the future evolution of flooding duration under sea-level rise depends strongly on tidal properties, and spatially variable differences may lead to large differences in outcome. Some regions will cross into persistent flooding—defined here as being flooded 2% of the time, or 10,512 hours a year—many decades before other regions exhibit a problem.