Sonoma County’s coastal cliffs no match for rising seas

Sonoma County’s coastal cliffs, softened by rain and pounded by ocean waves, are receding by as much as a foot a year and will surrender an area the size of Sebastopol by the end of the century, experts say, as climate change prompts sea levels to continue rising.

The scenic cliffs, made of soft rock formed millions of years ago on the ocean floor, are no match for nature’s ceaseless forces. Related property loss in the county over that period could total as much as $700 million.

Statewide, eroding coastal cliffs threaten billions of dollars worth of homes, highways, railways, businesses, military bases, universities, power plants and parks, and the North Bay has already seen the destructive and deadly consequences of the diminishing coastline.

At Gleason Beach, 4 miles north of Bodega Bay on Highway 1, the rapidly eroding cliff irreparably damaged 10 blufftop homes that were demolished by the owners, the last one in November.

One other home was relocated, and two of the 10 remaining homes are uninhabitable or unstable.

“Gleason Beach is a bellwether of things to come,” said Sonoma County Supervisor Lynda Hopkins, whose district covers the county’s entire coast. “It’s one of the fastest eroding places in California.”

Caltrans is currently planning a $26 million realignment of the coastal highway at Gleason Beach, moving nearly a mile of the roadway, and building a new 850-foot bridge, about 400 feet farther away from the restive ocean. Construction is expected to start in 2019.

It’s an effort fraught with uncertainty, given the difficulty of predicting future erosion rates in a complex natural system that includes rising sea levels, a symptom of climate change.

“We’re hoping we’ve moved the road back far enough,” said Lesley Ewing, a senior coastal engineer with the California Coastal Commission.

County planners, meanwhile, are revising coastal development standards that will require new structures to be set back from the bluffs a distance based on the conditions at each site.

The policy proposal is “intended to avoid development which would result in unacceptable risks to the residents, visitors, private property and public facilities” on the coast, according to the county.

A new study featuring high-tech measurements of the state’s retreating coastal cliffs said it’s going to be difficult to stay out of nature’s relentless path.

“These problems will worsen as sea levels and coastal populations increase, and effectively managing California’s changing coast will become increasingly challenging,” said the study by Adam Young, a project scientist at the Scripps Institution of Oceanography in La Jolla.

His study, published this month in the journal Geomorphology, used images from airborne laser scanning — known as Lidar — to measure changes in segments of the coastline from the Mexican border to Bodega Head over an 11-year period ending in 2009.

Significant erosion was found on 44 percent of the shoreline evaluated, with cliff tops retreating as much as 14 feet a year.

A 3,300-foot stretch of cliff on the Sonoma coast south of Bodega Head receded by one foot a year, three times the average rate, but Young said it was too small a sample to apply to the entire county.

His study stopped at Bodega Head, a rocky mass resistant to erosion, because the data going north were “pretty spotty,” Young said.

The oceanographer devised a novel rating system to identify the coast’s most hazardous cliffs, where the face has retreated faster than the cliff top, resulting in a steeper, more unstable slope.

His calculations pinpointed eight erosion “hot spots,” including Big Sur, Daly City in San Mateo County, Martins Beach in Half Moon Bay and Point Reyes National Seashore in Marin County.

At Point Reyes, a 58-year-old preschool teacher died when part of a promontory called Arch Rock collapsed in March 2015, hurling her to the beach 75 feet below amid tons of broken sandstone.

In the wake of the tragedy, the National Park Service put up fencing at the end of the popular Bear Valley Trail, along with signs warning hikers of the cliff-top hazard.

But the fencing “seemed to encourage or entice people to go around it,” said John Dell’Osso, a seashore official, and was replaced with bramble and brush.

A year ago, a 50-foot section of the Tomales Point Trail collapsed with no one around at a point beyond the park-maintained stretch of the trail, he said.

Young’s study identified a hot spot at Double Point on the Coast Trail at the southern end of the seashore, which draws 2.5 million visitors a year.

Dell’Osso said he did not know why it was singled out.

Picturesque but problematic, California’s coastal cliffs are inherently susceptible to erosion, triggered mainly by heavy rains and relentless surf.

The cliffs are relatively soft sedimentary rock, such as sandstone and mudstone, formed on the sea bottom millions of years ago and pushed upward by tectonic forces.

What’s going on now is “a battle between the rising rock and the ocean nibbling away at it,” said Doug George, a post-doctoral researcher at the Bodega Marine Laboratory in Bodega Bay.

Cliffs can fail in big pieces, years apart, and often in connection with soggy winters, experts say.

Bluffs in Pacifica, for example, have had dramatic collapses, including one during the 1997-98 El Niño that left several homes hanging over the cliff edge.

California’s world-famous 1,270-mile coast is made up, in roughly equal proportions, of rocky cliffs that plunge to the ocean, cliffs that end on small beaches and large sandy beaches without cliffs, Ewing said.

With sea-level rise, storm waves will slam against the cliffs more often and hit them higher up, she said.

Nearly half of the Sonoma coast is rock cliffs.

A report by the Pacific Institute, an Oakland-based think tank, estimated that cliff erosion from a 55-inch sea-level rise by 2100 would take out 27 square miles of coast in 11 coastal counties from Santa Barbara to Del Norte.

Sonoma’s coast would lose 1.6 square miles, an area about equal to Sebastopol.

Cliffs would retreat an average of 216 feet in the 11 counties and 134 feet in Sonoma County, where cliffs would recede as much as 623 feet.

“To us, the big story was the number of properties in jeopardy,” said Matthew Heberger, an author of the 2009 study that is still considered valid.

The study said 10,000 properties were at risk from erosion, and two-thirds of them would be destroyed. Sonoma County had 500 coastal properties at risk, with an estimated value of $1.4 million adding up to a $700 million loss.

Learn more at The Press Democrat