Avalon Canyon Access Road and Storm Drain

Daly City, California


City of Daly City

Avalon Canyon

Alternatives analysis led to a simpler solution that was
more reliable, less expensive, and could be constructed rapidly.

Avalon Canyon is located along the Pacific Coast and, because of an unfavorable geologic setting, has been the site of significant landslides and erosion. A major landslide in 1999 threatened to undermine homes along Skyline Drive located 300 feet above sea level. In 2000, Daly City implemented a major re-grading project for the canyon and installed a new 28-inch diameter storm drain to convey storm water runoff from Skyline Drive to an outfall on the beach at the base of Avalon Canyon.
Another major landslide occurred at Avalon Canyon in December 2003 and an extensive geologic study of the conditions indicated that the new landslide could grow and undermine a portion of the Canyon Access Road constructed in 2000 and the 28-inch diameter storm drain. Protecting the storm drain was critical because it is the only way to manage stormwater runoff from a large area at the top of the bluff. As recommended by the geologic study, Daly City requested proposals for designing a retaining structure to protect the storm sewer with the requirement that the protection be in place before the start of the rainy season in the winter of 2004-2005.
TERRA made an initial evaluation of the site and concluded that the proposed retaining structure was probably not constructible and would be very expensive. As an alternative, TERRA recommended relocating portions of the storm drain and access road so that they were outside the zone of risk of future landslides. This solution was less expensive than the retaining wall originally considered and could be designed and constructed before the start of the rainy season.
TERRA was given a notice to proceed in mid-August and completed the design and contract drawings in six weeks so that the contractor could start work by October 1 and have the project substantially complete by November 15. All this was accomplished on schedule and within budget.

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