USGS & Don Knabbe

Options for fixing the San Pedro Slide in southwestern Los Angeles range from several million to 62 million dollars.

It’s a long and inconclusive list of usual suspects that appear in the final draft report [PDF] released by the City of Los Angeles this week. The Department of Public Works tapped the Glendale geotechnical consulting firm Shannon and Wilson to investigate the slide that sent a 600-foot section of seaside roadway toward the Pacific last November.

My radio report and blog post back in April explained the slippery mix of water and soft sediment that permeates the Palos Verdes Peninsula, and what happens when too much water gets between those unstable layers of earth.

Shannon and Wilson’s report lists the following as “contributors” to the slide: irrigation — both residential and watering done inside the White Point Nature Preserve adjacent to the slide — coastal bluff erosion, precipitation, road construction and underground utilities. Just above the Preserve is a 13.2-acre complex of U.S. Air Force housing, and watering from those homes could be “influencing groundwater” near the slide.

Precipitation was a “significant effect,” investigators say: a smaller slide in December of 2009 “associated” with heavy rains may have destabilized the 2011 slide area. The report also mentions the two inches of rain between October, 2011, and last November when the most recent slide occurred. City of Los Angeles sewer lines, storm drains and gas and water lines built back in the 1960′s may have played a role. A city survey with video cameras late last summer and early fall found “some damage” to the storm drains. Interestingly, the only factor the investigators could rule out was the Nike Missile Base built into the hillside above the slide area.

Next come the discussions and decisions about solutions and their price tags. Check out some possible designs in a city video on L.A. City Council member Joe Buscaino’s website. He’s the Chairman of the Public Works Committee, which will be moving the repairs and reconstruction forward. The choices range from a low of several million dollars to leave the slide “as is” and build safe turnarounds for traffic on either side and several mid-range options including grading the slide or re-routing the roadway into the nature preserve for between four to eight million dollars. Two high-range options, 42 million to 62 million dollars, include restoring the roadway without a retaining wall or building a bridge across the slide.

Bureau of Engineering / City of Los Angeles

November's sea cliff failure took out 600+ feet of roadway and sidewalks.

The latest Palos Verdes Peninsula slide may calve, and the main slide mass is likely to keep “moving oceanward.” That’s according to a preliminary draft of a geotechnical study commissioned by the City of Los Angeles in early winter, but that’s the extent of the news for now. The same report says based on the studies completed to date, the risk of landslide movement behind last November’s slide — landward into a nature preserve and beyond a new chain link fence — is low.

That’s just the latest from an area southwest of downtown L.A. that has been generating geological news for decades. According to a landslides map by the California Geological Survey, the PV Peninsula boasts 175 slides, 49 of them active.

Two studies, one published recently in Environmental Research Letters and another I reported on late last year by Scripps Institute of Oceanography and the U.S. Geological Survey, are predicting more frequent coastal flooding in California caused by rising sea levels and an increasing number of extreme Pacific storms that hit the coast.

Both trends could impact the PV Peninsula: one from above and the other at the base, according to Lisa Collins, a lecturer in environmental studies at USC. “There’s a potential for increased storm activity and storms cause a lot of erosion. The cliffs of Palos Verdes are eroding anyway.” Up top, the threat is from precipitation. When the volcanic layers of the peninsula weather or degrade due to rain, explains Collins, they morph into two kinds of very slippery clay. The clay stops the water, but the layers of earth above it get saturated and heavy.

Factor in another element: the Peninsula is an “uplift.” When the Pacific tectonic plate slammed into the North American plate, the earth folded into vertical layers, just like the hood of a car might accordion in an accident. Combine soft and saturated earth sitting on slippery clay, with an incline tipped toward the ocean, and you get the formula for these landslides.

Collins uses tilted slabs of plywood and sand castles to teach her students about the Peninsula’s geology. “You can take a bucket and fill it with some damp sand and invert it and it will stay standing. But if you artificially make it rain… the mixture essentially turns liquid and flows again. They can’t keep that steep angle… and we see a lot of those angles on the coast of California.”

Take an audio trip to the PV peninsula on Monday with The California Report, and hear from residents and scientists about this oceanside geology lesson.