Wave splashing against wall in Waikiki
High sea level in Waikīkī. Photo credit: Hawaiʻi Sea Grant King Tides Project.

Extreme flooding events in some U.S. coastal areas could double every five years if sea levels continue to rise as expected, according to a study published in Scientific Reports. Today’s “once-in-a-lifetime” extreme water levels—which are currently reached once every 50 years—may be exceeded daily along most of the U.S. coastline before the end of the 21st century.

Researchers in the University of Hawaiʻi at Mānoa’s School of Ocean and Earth Science and Technology’s Department of Earth Sciences co-authored the study along with others at the U.S. Geological Survey and the University of Illinois at Chicago. The team investigated the frequency of extreme water levels measured by 202 tide gauges along the U.S. coastline and combined the data with sea-level rise scenarios to model the rate at which flooding events may increase in the future.

The data suggests that present-day extreme water levels will become commonplace within the next few decades. Low-latitude areas will be the most susceptible, with their rate of coastal flooding predicted to double every five years. At the most susceptible sites, along the Hawaiian and Caribbean coast, the rate at which extreme water levels occur may double with every centimeter of sea-level rise.

For 73 percent of the tide gauges used in the study, the difference in water level between the 50-year extreme water level and the daily average highest tide was found to be less than one meter, and most sea-level rise projections exceed one meter by 2100. The authors’ model predicted that before 2050, current extreme water levels transitioned from 50-year, once-in-a-lifetime flooding events to annual events in 70 percent of U.S. coastal regions. Before the end of 2100, once-in-a-lifetime extremes were predicted to be exceeded almost daily for 93 percent of the sites measured.

For more information see SOEST’s website.