Living on precipice of disaster
High-tech shoreline survey shows areas most at risk from cliff erosion
California’s coastal communities have long battled the roiling sea over claim to the shoreline, fighting to keep pristine beaches, towering bluffs and million-dollar homes from washing away as if they never existed.
A new report on cliff erosion suggests not only that the ocean is winning the conflict, but that parts of the Bay Area stand a lot to lose.
The coast of Daly City and Martins Beach south of Half Moon Bay were identified along with Big Sur and Southern California’s San Onofre State Beach as the state’s riskiest sites for collapsing cliffs. Two stretches of bluffs in Point Reyes National Seashore were also flagged as among the most likely to crumble — Double Point north of Bolinas and the park’s namesake peninsula, Point Reyes.
The report, released this month by UC San Diego’s Scripps Institution of Oceanography, employed a high-tech surveying method called lidar to evaluate erosion along 680 miles of the California coast.
The first-time endeavor showed that 44 percent of the cliff faces between Bodega Bay in Sonoma County and the Mexico border are experiencing some degree of slipping.
While erosion rates vary by place, and have varied widely over time, scientists agree that rising seas and more extreme weather — the results of climate change — will likely increase the pace of shoreline erosion.
“It is critical we study current and historical cliff retreat so we can better plan for the future,” said report author Adam Young.
The California coast is home to more than 21 million people. As much as $100 billion worth of property near the ocean, including highways, mil-
“Our consultant told us that we’d have to build a structure that would be the equivalent of a Hoover Dam.” City Manager Patricia Martel on Daly City’s efforts to deal with erosion of coastal bluffs
itary bases and power plants, is at risk, according to estimates by the California Coastal Commission.
Daly City, the most populous of the high-risk sites identified in Young’s study, has taken measures to counter the pounding waves, wind and runoff that are eating at the bluffs. Unfortunately, as in many areas, success has been limited.
Much of the city’s western edge, laced with homes, roads and sewer lines, continues to slip into the sea even after officials rerouted utilities and built seawalls. In 2000, 30 homes above Mussel Rock Beach were red-tagged because of a failing hillside, while the past two wet winters have brought similar, albeit less significant, losses.
City Manager Patricia Martel acknowledged that there’s only so much that can be done to secure the bluffs.
“Our consultant told us that we’d have to build a structure that would be the equivalent of a Hoover Dam,” she said. “Even then there was question of whether or not it would hold up.”
The story is similar in other parts of California. Last winter, near-record storms hastened coastal erosion up and down the Bay Area and beyond. The situation was perhaps most dire in Big Sur, where roads and bridges slipped down mountainsides and shut off access to the already remote region.
Highway 1 remains closed on Big Sur’s southern edge. The state has embarked upon a $40 million effort there to rebuild the road where the Mud Creek Slide washed out a quarter mile of coastline — one of the biggest slides in the state’s modern history. Crews expect to finish next summer.
Gary Griggs, a professor of Earth and planetary sciences at UC Santa Cruz and one of the state’s foremost experts on coastal erosion, said California’s erosion rates have gone up and down over the past century, largely in tandem with natural fluctuations in the ocean and atmosphere. Since the late 1970s, he said, ocean and atmospheric conditions have favored stronger waves and storms, resulting in more wear of the coast.
Griggs said he wouldn’t necessarily expect the trend to go on if it weren’t for rising seas.
“What that means for cliffs is that the water is going to get closer more often,” he said. “It’s a sign that erosion will continue.”
The aerial surveys used to evaluate coastal erosion were conducted in 1998, 2009 and 2010, with Young comparing them to historical records from 1932 and 1934 to identify longterm changes.
He found that areas with past slippage weren’t necessarily ones to worry about in the future. The steepness of the coastline, revealed through elevation maps created with lidar, which utilizes pulsed laser light, was the best indicator of problems, he said.
The study was published in the journal Geomorphology with funding from the National Oceanic and Atmospheric Administration and the state parks department.