Credit: Christine Hegermiller, U.S. Geological Survey. Public domain.

Severe West Coast Erosion During El Niño

In a study released today, scientists from the U.S. Geological Survey (USGS), Scripps Institution of Oceanography at the University of California San Diego, and other research centers document how the 2015-2016 El Niño was one of the most powerful climate events of the last 145 years.

Investigating 29 beaches along the West Coast from Washington to Southern California, researchers found that winter beach erosion was 76 percent above normal, and most beaches in California eroded beyond historical extremes. If severe El Niño events become more common in the future as some studies suggest, this coastal region, home to more than 25 million people, will become increasingly vulnerable to coastal hazards, independently of projected sea-level rise.

Seasonal beach behavior was assessed for beaches along a 2,000-kilometer (1,243-mile) span of the U.S. West Coast. The survey included making 3-D surface maps and cross-shore profiles using aerial light detection and ranging (lidar), GPS topographic surveys, and direct measurements of sand quantities combined with wave and water level data at each beach from 1997-2016. Winter beach erosion or the removal and loss of sand from the beach is a normal seasonal process, but during El Niño events, the extent of erosion can be more severe than in other years.

“Oceanographic forcing and coastal response was unprecedented for many locations during the winter of 2015-16. The winter wave energy equaled or exceeded measured historical maximums across the U.S. West Coast, corresponding to anomalously large beach erosion across the region,” said USGS geologist and lead author of the report Patrick Barnard.

The 2015-16 El Niño was one of the three strongest events ever recorded, along with El Niño winters of 1982-83 and 1997-98. While most beaches in the survey eroded beyond historical extremes, some beaches fared better. The condition of the beach before the winter of 2015 strongly influenced the severity of erosion and the ability of the beach to recover afterwards through natural replenishment processes.

Rivers still supply the primary source of sand to California beaches, despite long-term reductions in the 20th century due to extensive dam construction. But as California had been in the midst of a major drought at the time of the survey, the resulting lower river flows equated to even less sand being carried to the coast to help sustain beaches. Unlike California, many of the Pacific Northwest beaches have been gaining sediment in the years leading up to the 2015-16 El Nino, due at least in part to relatively more production of sand from local watersheds, dune growth, and a series of mild winter storm seasons that prevented these beaches from eroding beyond historical landward extremes.

“Artificial beach nourishments using mechanically imported sand placed prior to the winter of 2015-16 also protected some Southern California shorelines from retreating beyond landward extremes,” said co-author Bonnie Ludka, a postdoctoral researcher at Scripps.

Funding for the research came from the U.S. Army Corps of Engineers, the California Department of Parks and Recreation Division of Boating and Waterways, the USGS, the Northwest Association of Networked Ocean Observing Systems, and the National Science Foundation.

The study, “Extreme oceanographic forcing and coastal response due to the 2015-16 El Niño,” was published online today in the journal Nature Communications. Researchers from UC Santa Barbara, the Oregon Department of Geology and Mineral Industries, the Washington State Department of Ecology, Oregon State University, and UCLA contributed to the study.

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