Last years El Nino may have produced weak rainfall, but it triggered powerful waves that took a bite out of West Coast beaches, according to a study published this month by researchers with Scripps Institution of Oceanography and the U.S. Geological Survey.

The study, published in the journal Nature Communications last week, found that winter beach erosion was 76 percent higher than normal at 29 beaches from Washington to Southern California.

In San Diego County, four beaches were included in the reportand three of them Imperial, Cardiff and Solana Beach fared relatively well amid the pounding waves. Torrey Pines Beach, however, crumbled under the oceans onslaught. Waves swept away the sand, cutting intothe shoulder of Highway 101,andleaving a rockyshoreline flanked by boulders.

The difference is related to the type and timing of sand replenishment projects at those beaches, said Scripps post-doctoral researcher Bonnie Ludka, a co-author on the study.

We did see that extreme erosion at Torrey Pines, but we saw such different results for the beaches that were nourished with replacement sand, Ludka said.

The Nature Communications study pooledresults from numerous West Coast scientists who were studying beach conditions in their areas, to create a regional record of sand loss.

We combined our data sets to look at this phenomenon, Ludka said. I think its really important to understand how these El Ninos affect the coast.

The authors analyzed those figures to reveal the unseen impact of El Nino in 2015-16. West-Coast residents might have concluded that El Nino fizzled when it failed to produce predicted rains, but record heat was roiling in the water, fueling waves that crashed ashore and dragged beach sand out to sea.

The public perception was that nothing happened, but the waves were among the largest ever recorded so the beaches responded accordingly, said lead author Patrick Barnard, a coastal geologist with the USGS.

As a result, most California beaches eroded beyond historical extremes, the study stated, warning that this could become the new normal.

The exceptions were San Diego beaches that received sand shipments, as well as some Pacific Northwest shorelines that had built up naturally during previous mild winters.

The lesson there, is that these wide beaches, whether theyre wide naturally or wide artificially, are the first line of protection against storm erosion and flooding, Barnard said. And for the most part they did their job.

Beaches depend on creeks and rivers that wash sediment downstream, renewing the sand supply each year. That process faces a triple threat; the watersheds are blocked by dams, and parched by drought, leaving beaches exposed to increasingly intense winter storms.

If severe El Nio 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, the study stated.

An earlier study published by Ludka and other Scripps researchers in a journal of the American Geophysical Union last year suggested that strategic use of sand replacement could forestall those effects.

Torrey Pines Beach was one of a dozen San Diego beaches shored up in 2001, with grains similar in size to its natural sand. The entire reconstructed beach pad, however, washed away in a single, ordinary storm that winter. During last years El Nino, heavy waves further whittled the coastline, carving chunks of sand from the beach and strewing cobbles along the strand.

By contrast, coarse grain sand was added to Imperial, Cardiff and Solana Beach in 2012. That replenished sand lasted several years, and weathered last years El Nino better than expected, with wide stretches of sand left intact.

The coarser grains are harder for waves and currents to move around, Ludkasaid. Thats true because after the sand is lifted up into the water column, those grains fall more quickly to the bed than finer grains.

The three beaches, on average remained 30 feet wider last year than they did during the previous El Nino year in 2009-10, Ludka said.

Those were subject to a barrage of really big waves, and were more resilient to erosion, she said.

Although the findings suggest that beach replenishment could be a long-term strategy for fighting erosion, they also illustratethe challenge ahead for California beaches, researchers said.

Weve modified these watersheds with damming, Barnard said. Were in a drought, and droughts of this magnitude are expected to occur more frequently. And on top of that, youve got sea level rising. Were going to have an even more limited sand supply, and were going to need a lot more of it.

deborah.brennan@sduniontribune.com Twitter@deborahsbrennan

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Sand replacement helped shore up beaches against El Nino - The San Diego Union-Tribune