The 2015-16 El Niño was one of the strongest climate events in recent history with extraordinary winter wave energy, a new study shows, though its impact on beaches was greater in California than in Oregon and Washington.
The reason, researchers say, is that the Pacific Northwest had experienced comparatively mild wave conditions in the years prior to the onset of the El Niño, while California was experiencing a severe drought and “sediment starvation.”
Results of the study are being published this week in Nature Communications.
“Rivers still supply the primary source of sand to California beaches, despite long-term reductions due to extensive dam construction,” said Patrick Barnard, a geologist with the U.S. Geological Survey and lead author on the study. “But as California was in the midst of a major drought, the resulting lower river flows equated to even less sand being carried to the coast to help sustain beaches.
“Therefore, many of the beaches in California were in a depleted state prior to the El Niño winters, and thereby were subjected to extreme and unprecedented landward erosion due to the highly energetic winter storm season of 2015-16.”
The West Coast, on average, experienced a “shoreline retreat” – or degree of beach erosion – that was 76 percent above normal and 27 percent higher than any other winter on record, eclipsing the El Niño events of 2009-10 and 1997-98. Coastal erosion was greatest in California, where 11 of the 18 beaches surveyed experienced historical levels of erosion.
Peter Ruggiero, an Oregon State University coastal hazards expert and co-author on the study, said Oregon and Washington were not affected to the same extent.
“You would have thought that there would be massive damage associated with erosion in Oregon and Washington with the strength of this El Niño,” said Ruggiero, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “But the previous three years had mild winters and therefore the sand buildup was much higher than in California. It helped the Northwest offset the potential erosion from the El Niño.”
Oregon and Washington also have broader beaches than in California, Ruggiero pointed out, which also eases the erosion of sand dunes and impacts to development.
The 2015-16 El Niño, in some ways, was stronger than the 1982-83 event, which caused an estimated $11.5 billion in damages, the researchers say in the study. Only a portion of the damage was directly related to coastal erosion, with damage to houses and roads, they note. Most of the impact was from related storms, flooding and other damage that occurred inland.
The Nature Communications study is important, the authors say, because it is one of the first attempts to document the oceanographic “forcing” directly related to beach impacts created by El Niño. The study documents the amount of power created by winter storm waves, using height and “period” – or the length of time between waves. It is the level of forcing, along with relative beach health, that dictates the amount of erosion that occurs and the associated impacts from that erosion.
“During an El Niño, the nearshore experiences higher water levels because of the storms and the fact that the water is warmer and expands,” Ruggiero said. “In Oregon, the water was about 15-17 centimeters (roughly 6-7 inches) higher than average, which led to higher storm tides.”
Although Northwest beaches were buffered from catastrophic damage, Ruggiero said, they did experience significant retreat. And it may take a while for the beaches to rebuild.
“We’re not completely recovered yet, and it may take years for some beaches to build back up,” he said. “After the 1997-98 El Niño, it took some beaches a decade to recover.”
Ruggiero, his students and colleagues have been monitoring Northwest beaches since 1997, and in 2015, they received a National Science Foundation rapid response grant to study the impact of El Niño on beaches. Ruggiero also receives support from the Northwest Association of Networked Ocean Observing Systems (NANOOS) and the U.S. Army Corps of Engineers for additional monitoring.
Source: Oregon State University