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Strong surf weakens ailing coast

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Winter waves pummeling the coastline are endangering a roadway and an apartment building teetering on the edge of a bluff — and storms this year could create further damage.

The mountains of water forecast to slam into the region are threatening homes and valuable infrastructure on the Peninsula and San Francisco coastline. Parts of Daly City, Pacifica and San Francisco are particularly vulnerable because they sit on less-solid sediment than other coastlines and subsequently erode more quickly, U.S. Geological Survey Research Civil Engineer Brian Collins said.

The sediment, known as “weakly lithified sandstone,” is too young to call sandstone but too old to call sand, and it was created from tectonic-plate movement several thousands of years after other bluffs along the California coast, with less time to solidify.

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The combination of the weak mineral composition and the large waves and swells produced by winter storms has caused several feet of a bluff to splice at 330 Esplanade Ave. in Pacifica. That required a temporary fix to try to save a 12-unit apartment building from falling into the Pacific Ocean.

Since Dec. 17, that building has been evacuated and construction crews have labored for long hours, placing 50 tons of boulders at the bottom of the approximately 100-foot-tall bluff to deflect the command of the waves. Four neighboring complexes are farther away from the edge of the receding cliff and have in the past installed boulders to deflect waves, but they are still subject to more erosion.

“Protecting the toe is part of the solution to making them safe, but you do have to consider the fact that a perfectly vertical bluff will continue to erode,” Collins said, referring to the base of the cliff.

Farther north, a section of roadway in San Francisco has been shuttered as the cliff between the street and the ocean has disappeared. The stretch of Great Highway south of Sloat Boulevard has been reduced to one lane in the southbound direction since the cliff next to the street washed away.

For at least five years, the Golden Gate National Recreation Area and the San Francisco Department of Public Works have been planning to build a sandbar in the ocean to minimize the impact of the waves before they reach the bluff. But that project is still at least a year away from fruition and the cost has yet to be determined, GGNRA spokeswoman Christine Powell said.

Public Works, which is in charge of the roadway, built a road parallel to Great Highway in 2003 as a backup for when this happened and is now considering whether it’s time to use it. In the meantime, the GGNRA and Public Works likely will put more sand at the base of the highway to protect it until the sandbar is built, “but erosion is just a natural process, and personally I don’t know if [the highway] can be saved,” Powell said.

kkelkar@sfexaminer.com

Risk factor no secret

The residents of an apartment complex in Pacifica have known for years that waves could eventually destroy the bluff on which their homes sit.

“Six months ago, there was rock starting to be put in behind some adjacent buildings. They piled some sand up as a temporary solution to the same problem, which [they] had been planning to do,” said Randy Nelson, the building’s manager of 14 years. “Over the years, we were told various things that halted the project and it was a little frustrating for the tenants.”

Pacifica residents who have been watching the coastline erode estimate it has lost up to 30 feet in the past decade.

“You better not sleepwalk if you buy property on the coastline,” said San Mateo resident Chet Clark, 61, who was taking pictures of the waves crashing against a temporary fix of boulders piled at the base of the cliff under the apartments.

Geological engineer William Godwin, who used to chair the Association of Environmental and Engineering Geologists San Francisco Section, said common sense is the best defense.

“The threat is there,’’ he said. “Anyone that wants to live down by the ocean, all they have to do is go out and monitor it happening.”

— Kamala Kelkar



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