Monday, July 16, 2007 | Surfers’ footprints line this sandy path through Sunset Cliffs Natural Park. Rain-carved gullies cut through the soft soil.
Barbara Keiller and Ann Swanson, two leaders of the park’s recreation council, walk along a cliff-side trail, narrating the too-fast transformation of a place they hold dear.
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Sunset Cliffs Notes |
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The gray-haired women lean on decades-old memories. They will point to a massive gully and remember a time when their children walked there. Or they’ll recall a long-ago picnic that was held on land that no longer exists. Right there, they’ll say, and point to a giant 40-foot deep canyon that was once flat ground.
The memories are fleeting, and the park is, too. Each year, the 68-acre Sunset Cliffs Natural Park gets smaller. The culprit is a combination of natural forces — waves and rainfall — as well as runoff from residential irrigation that sweeps park’s soft soil into the sea.
Erosion is a fact of life along San Diego’s coastline. Eighteen thousand years ago, the coast stretched out between a half-mile to several miles further into the Pacific.
But what’s occurring at Sunset Cliffs is not entirely natural, highlighting the impact that humans have on San Diego’s fragile coastal geography.
The city of San Diego is in the midst of studying the problem, examining the sources of urban runoff and its drainage patterns throughout the 68-acre park, which stretches south from Point Loma Avenue in Ocean Beach along the coast to below Point Loma Nazarene University.
The erosion report, which has not yet been released, will produce a series of recommendations for what the city estimates will be a multimillion-dollar project to control erosion. The city will look for ways to intercept urban runoff and reroute it away from the soft soil.
“It’s going to be a pricey solution altogether,” says Paul Jacob, an associate civil engineer for the city’s Park and Recreation Department. “It will be a long-term, multiple-project solution.”
To understand why the cliffs are eroding, you must understand their geology.
The cliffs are made of two geologic formations. The soft sandstone — called the Bay Point formation — is about 120,000 years old. This gives the clifftops their golden hue.
The sandstone sits atop the Point Loma formation, a harder, darker rock made of shale and sandstone that’s 70 million to 75 million years old.
Where the two formations meet — about 25 feet to 30 feet above today’s sea level — was once an ancient sea floor, says Tom Deméré, curator of paleontology at the San Diego Natural History Museum.
About 120,000 years ago, during a previous period of global warming, the face of Point Loma — the hill that defines the peninsula’s western side — was the sea cliff. The warming period melted ice caps around the globe, causing sea levels to be about 20 feet higher than they are today.
Sea levels slowly fell as a colder, glacial period set in, leaving behind the layer of marine sands — the Bay Point formation — that constitutes today’s Sunset Cliffs. At the peak of that ice age, about 20,000 years ago, sea levels were about 400 feet lower than they are today, the result of the expansion of the polar ice caps, Deméré says.
As global warming again sets in — accelerated by the world’s fossil fuel appetite — sea levels are on the rise, climbing seven inches in the last century. And the cliffs are once again in retreat, sped up by the rising tides.
“It’s a natural process that’s on-going,” Deméré says. “The sea cliffs are dynamic.”
But the erosion has an unnatural component: runoff from development. When rain hits roofs or roads, it doesn’t soak into the ground, instead running downhill toward the ocean, carving paths as it goes. Irrigation from lawns adds to the problem, destabilizing the soil.
Storm water and surface runoff are major contributors to erosion, especially when bluffs are made of weaker material — like that at Sunset Cliffs, says Scott Ashford, a geotechnical engineering professor at University of California, San Diego.
Ashford says the water causes three problems:
- It increases the weight of rock in the cliffs, making it heavier and more unstable.
- t weakens the layers between geological formations.
- Water fills cracks in the cliff face, pushing those cracks apart.
“Water,” Ashford says, “is the worst culprit.”
But the human-induced erosion can be controlled, says Reinhard Flick, staff oceanographer at the California Department of Boating and Waterways.
“If the drainage is done properly, you shouldn’t have that,” he says. “If that’s storm-water-caused erosion, that suggests there’s something wrong with the drainage patterns. That’s a civil engineering problem.”
Walking through the park, Keiller and Swanson describe their vision for the park’s future, hoping it will some day resemble Torrey Pines State Reserve. Their vision of the park doesn’t include massive culverts or the occasional sinkholes that open up. (Nor the prolific nonnative species that have spread throughout the park.)
Keiller and Swanson lament development’s contribution to Sunset Cliffs erosion. They point to a large culvert — they call it Culvert Canyon — that bisects the 50-acre section of park that sits at the south end of Sunset Cliffs Boulevard.
“It’s the broken heart of our park,” Keiller says. “It’s been abused.”
The women say irrigation runoff from Point Loma Nazarene University has accelerated erosion in the canyon, though school officials dispute how much of a role it has played, pointing instead to delays in adopting the park’s master plan, which outlines goals for what the park should become.
Joe Watson, the school’s vice president of external relations, says the university employs several practices to reduce runoff, monitoring its irrigation and replacing some invasive species with native plants that can reduce runoff.
“When you have a geological formation like we do there that’s been there for thousands of years, how do you account for all that erosion and say the last 30 years has been the exacerbating factor?” Watkins asked. “The university has tried for years to cooperate.”
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