Some of the biggest waves on record took a toll on beaches
The 2015-16 El Niño may have been a bust as far as precipitation in California, but it still ended up as one of the strongest episodes of the last 145 years, according to U.S. Geological Survey scientists who studied erosion along the West Coast.
They found that, on average, erosion was 76 percent above normal, with most beaches in California eroded beyond historical extremes. If, as some research suggests, El Niños become stronger in a warming climate, then the West Coast, with its 25 million inhabitants, will become increasingly vulnerable to coastal hazards.
The study covered seasonal changes at 29 beaches from Washington to Southern California. They used 3-D maps and aerial lidar imaging, GPS topographic surveys, and direct measurements of sand levels, combined with wave and water level data to measure the changes.
Winter beach erosion or the removal and loss of sand from the beach is a normal seasonal process, but the extent of erosion can be more severe during El Niño events than in other years.
“Wave conditions and coastal response were unprecedented for many locations during the winter of 2015-16. The winter wave energy equaled or exceeded measured historical maximums along the U.S. West Coast, corresponding to extreme beach erosion across the region,” said USGS geologist and lead author of the report, Patrick Barnard.
The scientists found that not all beaches were affected equally. 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 is 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.
Barnard said the waves measured during the most recent El Niño were “exceptional and among the largest ever recorded … Further, the lack of rainfall means the coastal rivers produced very little sand to fill in what was lost from the beaches, so recovery has been slow.”
Artificial beach replenishment also helped protect some beaches from from retreating beyond landward extremes. Unlike California, many of the Pacific Northwest beaches had been gaining sediment in the years leading up to the 2015-16 El Nino, due, at least in part, to a series of mild winter storm seasons that prevented these beaches from eroding to such extremes.
Although Pacific Northwest beaches were buffered from catastrophic damage, study co-author and Oregon State University coastal hazards expert Peter Ruggiero said, “several beaches did experience significant retreat and it may take a while for those beaches to rebuild. We’re not completely recovered yet and it may take years for some beaches to build back up. After the 1997-98 El Niño, it took some beaches a decade to recover.”
The full report, “Extreme oceanographic forcing and coastal response due to the 2015-16 El Niño,” was published online in the journal “Nature Communications.”