A naturally occurring event, the likelihood of cliffs collapsing is greatly increased by high surf and during — and after — winter storms.
It’s a recurring problem for residents and visitors alike in and around Sunset Cliffs Natural Park, a 68-acre city park adjacent to the Pacific Ocean.
Sunset Cliffs is renowned for its sheer cliffs, surfing spots, and for its interlocking network of well-worn cliffside walking trails especially popular for sunset viewing.
But the undeveloped cliffs can be as dangerous as they are gorgeous. People falling from them have injured themselves — and died.
The cliffs are an accident waiting to happen, which is why Point Loma Nazarene University on the bluff top issued a warning to students at the end of 2017. That warning asked them to avoid using “unofficial footpaths” on collapse-prone cliffs to get to the beach below.
“Our public safety team sent an alert out to our students who frequent the area of the park on Sunset Cliffs,” said Jill Monroe. “It was a reminder that students frequenting the cliffs need to be ‘very cautious’ about where they are on them, and to observe city signage present (warning of unstable cliffs).”
The process involved in cliff erosion is easier to understand than is to predict — or know how to handle.
The Beacon asked the city of San Diego, and retired SDSU geology professor Pat Abbott, about the “science” of cliff erosion.
“All coastal bluffs undergoing active erosion by the ocean have stability issues,” said city spokesman Alec Phillip. “The ocean erodes the base, and the upper bluff is undermined. Bluffs are made of different earth materials, and some are more resistant that others. In Sunset Cliffs Natural Park, the lower bluff materials are stronger, and more resistant, and when they are worn away, the upper bluff shales fail in blocks or rockfalls.”
In ancient times, much of San Diego was at the bottom of the ocean, which causes problems later on, said Pat Abbott, a retired professor emeritus at SDSU, who is the author of a college textbook, “Natural Disasters,” and is an expert on reconstructing ancient continental positions from coastal Mexico to coastal Alaska.
“Faulting lifts up all these things (ocean sediments), so you have all these soft sands on top, which is why you have weak sandstone that erodes so easy,” Abbott said. “The sea cliffs, attacked by waves on a daily basis, are being eroded. Then you add (rain and stormwater) runoff from the land to the ocean, coupled with the surf below … This eventually causes sections of cliffs to become unstable, and landslides to occur.”
Abbot noted periodic cliff collapses happen “about once a decade.”
Cliff erosion is an omnipresent problem, concluded Phillip.
“All coastal bluffs in San Diego are prone to block failures or rock falls, both large and small,” he said. “Hence, they are dangerous if you are near the edge, or near the toe of the slope. This is a common condition throughout California.”
Is anything being done to monitor and mitigate unstable oceanside cliffs?
“The City of San Diego provides safe access to beach areas, and provides signage to warn passersby of the danger and risk,” said Phillip. “Failures that threaten structures are evaluated, and the bluffs are evaluated for safe setbacks for viewing.”
Nonetheless, Phillipp noted people ignore the signs and traverse the bluffs “at their own informed risk.”
“To completely mitigate the risk,” added Phillip, “the coastline would have to be armored with seawalls or rock revetments similar to parts of the coast along Sunset Cliffs Boulevard, a methodology contrary to the natural character of the park.”
Abbott had a suggestion to offer in dealing with cliff erosion.
“Nobody should build on them (cliffs),” he said. “And if they do, they should run piers (a solid support designed to sustain vertical pressure) into the harder sandstone below.”
Abbott pointed out a common misconception is that spraying gunite, liquid concrete, can help with crumbling cliffs. “It does not stop it (erosion),” he said.
The famed geologist offered one other piece of advice.
“Don’t water lawns on the cliffs,” he warned. “It just soaks below the surface and causes much erosion. In the coastal zone, there should be no sprinklers used on lawns. That just accelerates erosion.”
Gene Berger, chairman of the Sunset Cliffs Natural Park Council, a citizen’s advisory group, claims Sunset Cliffs’ erosion is also caused by “subsurface water” percolating down from the bluff top.
“That water goes through the upper soil, hits an impervious layer, then goes downhill and out the cliff face: That’s one reason why it’s unstable,” Berger said.
Berger concurred with Phillipp and Abbott that people need to look out for themselves on Sunset Cliffs, and that the landform itself needs tender loving care.
“It’s a very fragile soil, and it’s very steep, so you get a highly erosive action happening,” Berger said adding SCNPC and the city “have a drainage plan we’re working on right now.”