A new tool to help decision makers and others assess how sea-level rise and other factors will affect the frequency of high-tide flooding in U.S. coastal locations in the next 50–100 years has been developed by University of Hawaii Sea Level Center Director Phil Thompson with funding from NASA’s Earth Science Division.

High-tide flooding, also known as “sunny day” or nuisance flooding, is an increasingly frequent occurrence in coastal areas around the United States. The Flooding Days Projection Tool is an online dashboard that projects the number of high-tide flooding days per year for 97 U.S. cities, based on National Oceanic and Atmospheric Administration (NOAA) impact thresholds. These thresholds provide a safety gap between regular high-tide water levels and conditions that result in flooding. Coastal communities are built at a certain elevation above sea-level with these natural fluctuations in mind.

The tool is based on projections of sea-level rise and the height of the highest astronomical tides, which vary on a predictable 18.6-year cycle that’s determined by the moon’s orbit around Earth. Over multiple decades, changes in the moon’s orbit cause cyclical variations in the height of high and low tides in certain regions. These changes occur slowly.

“Tides aren’t as constant as people think they are,” says Thompson, who is also an assistant professor in the UH Manoa School of Ocean and Earth Science and Technology Department of Oceanography. “They change on long time scales.”

When high tides get lower, the net effect of sea-level rise on flooding is reduced. The tool predicts this will happen in many U.S. locations from the mid-2020s until the mid-2030s, when high tides will once again get higher. When increases in high tides sync up with increases in global or regional sea-level rise and other factors that cause sea levels to vary, there’s a potential for rapid increases in coastal water levels and associated impacts. For regions where the rate of sea-level rise is already accelerating, such as along the U.S. West Coast, the cycle will exacerbate those impacts.

“We’ll observe a rapid increase in high-tide flooding days for regions around the globe,” Thompson says. “For a place like California, the height of high tides will increase 3 to 5 centimeters over 10 years, on top of a similar increase from sea-level rise that’s driven by climate change.”