A cliff collapse. Three deaths. More bluff failures expected with rising seas
LOS ANGELES – When we go hiking in California’s rugged mountains, we know to look out for bears and lions.
When we set off into the vast, bone-dry high desert of Joshua Tree, who doesn’t bring extra water?
When we stand too close to the edge of a coastal bluff, everyone yells to step back.
But resting under the shade of these cliffs in view of the ocean, it’s easy to forget what could come crashing down from above.
People often think about the beach as a place to swim, to read, to relax. In reality, it’s the tip of a wild, dynamic system that is constantly moving and succumbing to the forces of nature. These sweeping cliffs that make California’s coast so iconic were themselves formed by tectonic shifts and landslides over the centuries. And from the rubble of every collapse, more sand is made for beaches.
On a popular surf beach north of San Diego on Aug. 1, tons upon tons of sandstone crashed down on Anne Clave, her mother, Julie Davis, and aunt Elizabeth Charles. A beautiful beach day – meant to celebrate Charles beating cancer – in seconds turned into a frantic, fatal scene of beach chairs strewn aside, cadaver dogs, teenage lifeguards digging people out from the crush of heavy rocks. From the water, swimmers looked on in horror.
Cliff collapses are one of California’s many hidden dangers. Like earthquakes, we know what areas they’ll most likely happen – but nailing down when, and how big, is not an exact science.
Compounding all of this is the rising sea, as waves hammer away at the vanishing coastline with every tide and storm. The sea is rising higher and faster in California – a reality more officials are now confronting. Gov. Gavin Newsom has signed a bill that amended the state’s Coastal Act to say that sea level rise is no longer a question but a fact.
“With sea level rise, there’s no doubt that we’ll see more cliff failures along the coast,” said Patrick Barnard, research director of the U.S. Geological Survey’s Climate Impacts and Coastal Processes Team. “Cliffs erode, that’s what they do ... and we certainly expect more events like this the more the bottom of those cliffs are being hammered away by the ocean.”
Cliffs are particularly difficult to study because they tend to erode slowly over time, punctuated with sudden collapse – often without warning – from landslides or during a storm. The strength of the rock, cliff height, sediment composition, wave action, the slope of the beach and the slope of the seafloor all factor into their stability.
How rainfall and groundwater seep in can also build up pressure and lead to cracks and collapses. The human urge to develop right to the edge – building blufftop homes, landscaping, carving out coastal roads – further affects erosion by adding weight to clifftops and altering water drainage.
Altogether, cliffs in Southern California alone could erode more than 130 feet by the end of the century if the sea keeps rising, Barnard and a team of scientists found in a recent study.
The science is there on cliff retreat over a long expanse of time, he said, but much more work could be done on a more decadeby-decade, perhaps even seasonby-season basis.
“We know that California cliffs on average erode about a foot a year, but that’s a long-term average,” he said. “In many cases, they won’t do anything for 30 years, but then there’s a 30-foot failure.”
Scientists often think in these averages, he added, which can be a challenge when talking about cliff erosion and other climate change issues to the public. Stretched over a long period of time, an average – however extreme the changes may be – might not sound like much. But just two or three large collapses could encompass basically the entire expected erosion of a cliff in one century.
Adam Young, a project scientist at UC San Diego’s Scripps Institution of Oceanography who has spent years studying and building a comprehensive database of cliff erosion in California, hopes that patterns and lessons from historic collapses could help the state better plan for the future.
His studies so far have identified areas such as Big Sur, San Onofre State Beach and Daly City with cliffs that had particularly high rates of erosion. He also found that cliffs with high erosion rates in recent decades were often preceded by periods of very little erosion.
Young has been busy in recent months with numerous cliff collapses in northern San Diego County. Sections of the cliffs in Del Mar collapsed three times in just a few weeks last fall, threatening the coastal railroad. A large landslide at Torrey Pines last month crashed down early enough in the morning that no beachgoers were hurt.
“It’s definitely been more active than it has been in the last few years,” he said. “We’re still trying to understand what happened.”