In the past, urban flooding was an occasional event, caused by major storms or hurricanes. Now, sunny-day flooding due to high tides is increasingly common, and it’s this phenomenon that is called nuisance flooding, he explained.

Sweet described the monitoring of floods through a proxy network of more than 200 tide gauges along America’s coasts, including the Great Lakes. “The infrastructure becomes vulnerable to coastal flooding somewhere between one and two feet [30 and 60 cm] above high tide, [the] average high tide today,” Sweet said, adding that Bob Dylan might now consider singing about “the tides, they are a-changin”.

Steady sea level rise has accelerated the impact of nuisance flooding along the East and Gulf coasts, according to a study by Sweet and co-author Joseph Park, which was published in Earth’s Future. West Coast sea level rise is not as steady, varying with the El Niño/La Niña phenomenon, but nuisance flooding is increasing there as well.

In Norfolk, Virginia, home of the largest US Navy base, nuisance flooding has risen from less than once per year in the 1950s to more than eight times a year at present. One place of worship in the city, according to the Washington Post, posts the daily tide chart on its Web page so, as the pastor puts it, “people know whether they can get to church”. Many other major US coastal cities are registering increases in nuisance flooding of 325 to 925 percent compared with 1960.

What does a nuisance flood look like? Sweet suggested several criteria: overtopping of a seawall, water in the streets, storm water removal systems overwhelmed by the volume of water, roads closed and warning signs, all of which may be more frequent than in the past in many cities.

Sweet speculated that communities would be moved to take action when such events occur 30 days per year or more, but when might that be? For Norfolk, which also experiences subsidence, projections place the 30-days per year mark between 2020 and 2040; by 2100, nuisance flooding could be a daily event. The experience of other coastal cities will vary but, according to NOAA, every one will reach the 30-day/year tipping point by 2050.

Jayantha Obeysekera of the South Florida Water Management District described his area as “ground zero for sea level rise impacts”. As the mean sea level increases, so does the extreme sea level, he noted, especially since the mean is expected to rise not in linear fashion but quadratically. Nuisance flooding of streets is already occurring annually in Miami Beach and elsewhere along the flat south Florida coast. It’s beginning to overwhelm the existing infrastructure for flood control and affect aquifers, Obeysekera said. His agency’s challenge is knowing what to design for today to have the level of protection needed for the next 30 to 40 years.

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