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Cover Story

Coastal Flooding around the Globe Could Double in Decades



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The frequency and severity of coastal flooding around the globe will increase rapidly and double within a few decades, even with only moderate amounts of sea-level rise, according to a study released May 18, 2017, in Scientific Reports (https://doi.org/10.1038/s41598-017-01362-7).

Wave-driven flooding and overwash on Roi-Namur Atoll, Republic of the Marshall Islands. Wave-driven flooding and overwash on Roi-Namur Atoll, Republic of the Marshall Islands.
Above: Wave-driven flooding and overwash on Roi-Namur Atoll, Republic of the Marshall Islands. Photo credit: Peter Swarzenski, USGS. Click on the images for larger versions.

Coastal flood frequency will increase rapidly along North America’s west coast and Europe’s Atlantic coast. But the greatest and most damaging effects will be in tropical regions, where the increase in flooding will threaten the livability of low-lying Pacific islands and the economies of coastal cities. Many densely populated, low-elevation river deltas are in or near tropical areas, such as the Ganges, Indus, Yangtze, Mekong, and Irrawaddy.

“In the tropics, with just 10 centimeters (4 inches) of sea-level rise, today’s 50-year flood (which has a 2 percent chance of happening in any year) will occur every 5 years,” said USGS geologist and study coauthor Patrick Barnard.

The new report from scientists at the USGS, the University of Illinois, and the University of Hawaiʻi shows that just 10 to 20 centimeters (4 to 8 inches) of sea-level rise, expected between 2030 and 2050, will more than double coastal flooding for most regions. This dramatic increase results from rising sea levels combined with storm-driven flooding, including the effects of waves and storm surge.

Diagram of different ocean features that contribute to coastal flooding.
Above: Diagram of different ocean features that contribute to coastal flooding. Image credit: Sean Vitousek, University of Illinois at Chicago. [larger version]

Global sea level is currently rising about 3 to 4 millimeters per year (1/8 to 1/6 of an inch per year) and will rise faster as global warming increases the volume of the ocean (known as thermal expansion) and mass is added through the melting of  land-based ice. Until now, global estimates of increased coastal flooding due to sea-level rise have not considered the effects of waves and thus have underestimated the potential impact. The new study—the first to include waves in its analysis—finds that even gradual sea-level rise can rapidly increase the frequency and severity of coastal flooding.

The researchers combined sea-level projections with wave, tide, and storm-surge models to estimate increases in coastal flooding around the globe. They found that regions with smaller water-level variations—due to small tidal ranges (for example, the tropics) and limited ranges in storm water levels (including North America’s west coast)—would experience the largest increases in flooding frequency. In these locations, sea-level rise represents a large proportion of the projected flood levels.

Besides including waves in their analysis, the authors used other techniques that distinguish their work from previous global projections.

Map showing the amount of sea-level rise that will double the chances of today’s “50-year floods”
Above: Map showing the amount of sea-level rise that will double the chances of today’s “50-year floods,” which have a 2 percent chance of happening in any year. Warmer colors indicate areas at greater risk. Along Africa’s equatorial coast, for example, flood risk will double with a sea-level rise of just 2 to 2.5 centimeters (3/4 to 1 inch.) Figure 5 from the new paper at https://doi.org/10.1038/s41598-017-01362-7. [larger version]

“Most previous scientific work has focused on analyzing tide gauges,” said Sean Vitousek, lead author of the study and a professor at the University of Illinois at Chicago. “Tide gauges are reasonably dense in the United States and Europe, but are sparsely located throughout the rest of the world. Using models rather than individual tide gauges provides a comprehensive picture.” Vitousek was a postdoctoral fellow at the USGS when he began this study.

Additionally, prior research started with sea-level rise scenarios and forecasted the flooding frequency. In this study, the scientists took the opposite approach, finding the amount of sea-level rise needed to double the frequency of flooding, while accounting for the uncertainty and year-to-year variability of storm patterns.

One of the surprising findings was that it doesn’t take much sea-level rise to double flooding frequency. Vitousek and his coauthors demonstrate that 10 centimeters (4 inches) or less of sea-level rise can more than double the frequency of coastal flooding for many locations across the globe, particularly in the tropics.

 “Most of the world’s tropical atoll islands are on average only 1 to 2 meters (3 to 6 feet) above present sea level,” said USGS geologist and coauthor Curt Storlazzi. “Even in the high tropical islands such as Hawaiʻi, Guam, American Samoa, U.S. Virgin Islands, Indonesia, and others, the majority of the population and critical infrastructure is located on a narrow coastal fringe at low elevations and thus susceptible to this increased flood frequency.” Low-lying tropical islands are particularly vulnerable to ocean flooding from storms today. A significant increase in flooding frequency with climate change will further challenge the sustainability of communities on the islands.

 “These important findings will inform our climate adaptation efforts at all levels of government in Hawaiʻi and other U.S. affiliated Pacific islands,” said coauthor Chip Fletcher, associate dean and professor at the University of Hawaiʻi.

The full report, “Doubling of coastal flooding frequency within decades due to sea-level rise,” is available online in Nature Science Reports.

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Related Websites
Doubling of coastal flooding frequency within decades due to sea-level rise
Nature Science Reports
A model integrating longshore and cross-shore processes for predicting long-term shoreline response to climate change
Journal of Geophysical Research–Earth Surface
Coastal Storm Modeling System (CoSMoS)
USGS
Coastal Change Hazards Portal
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Disappearing Beaches: Modeling Shoreline Change in Southern California
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Coastal Flooding Could Double in Decades

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