Strong surf weakens ailing coast

By: Kamala Kelkar
Examiner Staff Writer
January 14, 2010

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.

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.

Read more at the San Francisco Examiner: http://www.sfexaminer.com/local/Strong-surf-weakens-ailing-coast-81423477.html#ixzz0cefYtPRp

Posted by Steve Sinai