By David Perlman, San Francisco Chronicle

Photo: Rex Sanders / USGS

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Scientists detect severe beach erosion along California coast

Erosion damaged miles of beaches along the West Coast more severely than ever during the powerful El Nio event that hit the Pacific a year ago, and scientists warn that even heavier erosion could hit shorelines in coming years as sea levels rise and threaten coastal communities.

The beaches lining the coast between Mexico and Canada form a protective barrier that keeps the turbulent ocean from eating away at seaside cliffs and flooding low-lying coastal towns and cities, scientists say.

In a study of 29 major beaches, including those in the Bay Area, a team of coastal experts found that immense quantities of sand had been lost during last years El Nio winter. Shorelines had retreated dangerously, and rivers that normally carry fresh sand downstream to the sea from surrounding hills and mountains failed to make up for the losses.

Beaches and shorelines normally lose sand every winter, but the scientists calculated the loss at 76 percent greater than normal. It was by far the worst loss of shoreline since surveys began nearly 150 years ago, said Patrick L. Barnard, a coastal geologist at the U.S. Geological Survey who led the erosion study.

The scientists went to extraordinary lengths to make their measurements. They hiked the beaches hauling GPS devices in their backpacks, used boats to measure waves and underwater sand levels, drove specialized vehicles to cross deep sand, and took to the air to measure each beach with sophisticated light detection and ranging instruments, known as LIDAR, that measured sand levels with laser beams.

The scientists covered a total of 2,000 kilometers (1,242.8 miles) to measure the trail of erosion caused by high waves from El Nio that pounded the beaches at the highest energy levels ever recorded, said Barnard and his team of experts from seven coastal science institutions.

On one December night in 2015, for example, offshore buoys measured wave heights along California beaches that ranged from 26 to 36 feet, and along the Oregon coast the waves ranged from from 39 to 62 feet more typical of the surf at Mavericks.

Record high waves were also recorded by the buoy off Fort Point beneath the Golden Gate Bridge, at Point Reyes National Seashore and along the Monterey Bay coastline, Barnard said.

The team also measured changes along San Franciscos Ocean Beach before and after the El Nio event and found that the entire stretch between the ocean and the Great Highway had narrowed by as much as 180 feet, Barnard said. Erosion had carried the lost sand far out to sea, and it may never recover, he said.

Scientific forecasts of future changes in Earths climate indicate that the frequency of severe El Nio events will double in coming years, bringing higher temperatures and lowered precipitation along the coasts. That means less runoff of water from the interior and less sand carried by that water to rebuild beaches and threaten shorelines where 25 million people now live, Barnard said.

Barnards colleagues at the USGS Pacific Coastal and Marine Science Center in Santa Cruz include oceanographers Daniel Hoover and Alex Snyder. The full teams report is published in the journal Nature Communications.

David Perlman is The San Francisco Chronicles science editor. Email: dperlman@sfchronicle.com Twitter: @daveperlman

More here:

Scientists detect severe beach erosion along California coast - SFGate - SFGate