During extremely high tides, some locations along coasts experience shallow coastal flooding: the sea literally spills onto land, indundating low-lying areas with seawater until high tide has passed. Because these floods causes public inconveniences such as road closures, overwhelmed storm drains, and deterioration of roads and infrastructure from exposure to salt water, the events are referred to as nuisance flooding.

The extent of nuisance flooding depends on multiple factors, including the shape of the seafloor and land, height of the tide, and land cover. When coastal storms coincide with very high tides, the depth and extent of nuisance flooding can increase dramatically. Even relatively weak winds blowing toward land during these events can push additional water inland, and rainfall can add a substantial volume of water to the flood. Coastal storms aren't necessary for flooding to occur, but rising sea levels mean that storms that did not cause severe impacts in the past are capable of doing so now. Sea level rise is expected to further intensify nuisance flooding impacts over time, and reduce the time between flood events.1

Perigean spring tides

The King Tide Photo Initiative encourages the public to visually document the impact of rising seas, as exemplified during current king tide events. Photos show water levels along the Embarcadero in San Francisco, California, during relatively normal tides (top), and during an extreme high tide or “king tide” (bottom).

Extreme high tides occur a few times per year during new and/or full moon. These perigean spring tides—also known as king tides—are astronomical in origin: they depend on the locations of Earth, Moon, and Sun. These higher-than-average high tides occur when the Moon's regular orbit brings it to its closest distance to Earth (called perigee), and the Sun, Earth, and Moon align (resulting in a new or full moon).  The combined gravitational force of the Moon and Sun on Earth's ocean results in the higher-than-usual tide level. Looking ahead on local tide tables to know when perigean spring tides will occur, and raising public awareness of potential flooding, is a step toward resilience.

Shallow coastal flooding will become more frequent and severe as sea level rises. By the year 2100, king tides will happen on top of another 8 inches to 6.6 feet of sea level rise.2^,3 In other words, today’s flood will become tomorrow’s high tide, as sea level rise will cause shallow coastal flooding to occur more frequently and for longer durations of time.

Sea level rise is not just a problem of the future: it is already causing flooding in coastal communities such as Charleston, South Carolina, and Olympia, Washington, during extreme high tides.

A NOAA Technical Report1 documents that nuisance flooding now occurs with high tides in many locations due to climate-related sea level rise, land subsidence, and the loss of natural barriers. Eight of the top 10 U.S. cities that have seen an increase in “nuisance flooding” are on the East Coast. Annapolis and Baltimore, Maryland, lead the list with an increase in number of flood days of more than 920 percent since 1960. Port Isabel, Texas, along the Gulf coast, showed an increase of 547 percent, and nuisance flood days in San Francisco, California, increased 364 percent.

^{*More than one flood on average between 1957–1963, and for nuisance levels above 0.25 meters.}

• 1. a. b. Sweet, W., J. Park, J. Marra, C. Zervas, and S. Gill, 2014: Sea Level Rise and Nuisance Flood Frequency Changes around the United States. NOAA Technical Report NOS CO-OPS 073. 58 pp.
• 2. Parris, Adam, P. Bromirski, V. Burkett, D. Cayan, M. Culver, J. Hall, R. Horton, K. Knuuti, R. Moss, J. Obeysekera, A. Sallenger, and J. Weiss, 2012: Global Sea Level Rise Scenarios for the United States National Climate Assessment. NOAA Tech Memo OAR CPO-1. 37 pp.
• 3. Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, S. Doney, R. Feely, P. Hennon, V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Ch. 2: Our Changing Climate. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 19-67. doi:10.7930/J0KW5CXT.