Coastal Flooding around the Globe Could Double in Decades

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A study released in Scientific Reports indicates that the frequency and severity of coastal flooding around the globe could increase rapidly and double within a few decades, even with only moderate amounts of sea-level rise.

This article is part of the June-July 2017 issue of the Sound Waves newsletter.

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 Nature Scientific Reports (https://doi.org/10.1038/s41598-017-01362-7). 

Photo shows results of wave-driven flooding and overwash on Roi-Namur Atoll, Republic of the Marshall Islands

Photo shows results of wave-driven flooding and overwash on Roi-Namur Atoll, Republic of the Marshall Islands. Photo credit: USGS: Peter Swarzenski, USGS.

Wave-driven flooding and overwash on Roi-Namur Atoll, Marshall Islands

Photo shows results of wave-driven flooding and overwash on Roi-Namur Atoll, Republic of the Marshall Islands. Photo credit: USGS: Peter Swarzenski, USGS.

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 showing different ocean features that contribute to coastal flooding, including waves, tides, and beach morphology

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

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”

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

“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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Date published: August 31, 2017

Sound Waves Newsletter - June-July 2017

New coastal flooding study released in Scientific Reports, members and staff of the Massachusetts State Senate visited the USGS WHCMSC, Richard Signell and Cassandra Ladino honored with the 2017 CDI Leadership and Innovation Award, new data obtained from the inner continental shelf off the Delmarva Peninsula, and more in this June-July 2017 issue of Sound Waves.