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.

Well, that happened in California in the winter of 1861-1862, and scientists say it will happen again, bringing massive flooding, landslides and property damage across the state.

To help emergency agencies plan for such an event, the US Geological Survey released the “ARkStorm Scenario” in Sacramento this week, detailing the repercussions of a storm that produces up to ten feet of rain and forces the evacuations of more than a million people.

“Vast floods would basically take out all the farm land,” said Marcia McNutt, director of the USGS. “They would destroy homes, animals would die, roads would be impassable, infrastructure would be rendered unusable, dikes would fail, levees, would fail.”

Scientists created this hypothetical storm by combining two actual storms, one from 1969 and one from 1986, and putting them back to back.  Together, they rival the 1860′s storm in magnitude, which was the last time California saw a storm this size.

Lucy Jones, the chief scientist of the USGS Multi-Hazards Demonstration Project and the architect of ARkStorm, said that storms of this size happen roughly every 100-200 years, or with about the same frequency as a major earthquake on the San Andreas Fault.  Jones said the geological record shows that six storms larger than the ARkStorm have occurred in the last 1800 years. (McNutt noted that climate change could cause this frequency to change.)

The name ARkStorm comes from the term “atmospheric rivers,” which are a meteorological phenomenon that draw moisture from the tropics and funnel it into temperate regions.

“In essence, they act like a fire hose, pointing tropical moisture in at California,” said Jones.  “They’ve been the source of some very devastating storms in the past, and when we look across the country, we can recognize that the most severe storms in the country are in the hurricane states and in California.”

McNutt said that an ARkStorm could cause close to $1 trillion in damages.  The USGS report estimates $400 billion in property damage and $325 billion in business-interruption losses, and it predicts that one-quarter of California’s buildings could be flooded as a result of a storm like this.  Additional costs would include loss of life, public health, loss of tourism and recreation dollars, and damage to historical and cultural artifacts and buildings.

Below is a video released by USGS explaining the project.

Mike Dayton, acting chief of the California Emergency Management Agency, said that science-based scenarios like ARkStorm are a key tool in disaster planning.

“The scenario is going to help us refine existing plans, and it’s going to point out some weaknesses, and we’re going to incorporate that into revised plans,” he said.

Cal-EMA plans to base its annual major planning exercise on the ARkStorm Scenario.

Spillway at Alpine Lake on Mt. Tamalpais. File photo: Marin Municipal Water District

A version of this post also appears on Dan Brekke’s personal blog, Infospigot. Also see our updated map of reservoir conditions at the end of this post.

By Dan Brekke

Is California’s drought over? OK, let’s take a step back. Yes, I realize one could debate whether the last three years in California actually constitute a drought. But that’s a discussion for another time. For now, I think everyone can agree that we’ve had lower-than-average precipitation for the past three years.

The only reason to ask the question is that, after the first half of the wet season delivered only spotty rain, we’ve had a pretty solid week of downpours. Water is sluicing into our reservoirs, and the hills are greening up. Some counties, like Marin, have water tumbling down the spillways. All of that is a sign of what we think winter should be here.

My favorite water statistic from last week: when the storms were at their heaviest around Lake Shasta, California’s biggest reservoir, water was flowing into the lake at about 500,000 gallons per second. That’s 1.5 acre feet, or about enough for two-to-three “average” households for a year, every second.*

Amazing numbers like that aside, the people who get paid to think about whether the drought is over say “not yet.” Last week, Quest managing editor Paul Rogers wrote a good summary of the situation, for The San Jose Mercury News.

Rogers’ story does contain one bit of quirky California thinking about rain and water, though. He quotes a well established local meteorologist, Jan Null, about where we stand in terms of normal rainfall, saying: “This is a great start, but we need to keep it going.”

Of course, Null recognizes better than most that the amount of rain we get and when we get it is out of anyone’s control. But once you understand the importance of water in California, once you get how crucial the winter rains are, there’s a score-keeping aspect to weather-watching here. It becomes second nature to study the rain gauge and the seasonal precipitation table as an index of performance, a reflection on whether a great collective goal is being attained. Lots of rain means we’re doing well (and that we can put the complexities of water supply out of our minds). A dry spell means we’re failing (and the prospect of hell to pay, or at least the strong possibility of stringent conservation measures).

But in reality, there’s no performance going on. The rain is the rain, and the climate is the climate. California’s rainfall is famously variable. Dry spells can be counted on and the current run of dry years is the third we’ve had since I arrived in Berkeley in the 1970s.

My first California winter, 1976-’77, was bone-dry and was in fact the second year of the driest two-year period ever recorded here. A decade later, from roughly 1986 through 1992, we had another run of dry years. And if our winter rains were to stop now, we’d be in the fourth year of drier-than-normal years. In between these periods we’ve had average years and very wet years and years that didn’t quite hit the average. That might not be too different from anywhere else. The reason it’s a bigger deal here than it might be in, say, Wisconsin, is that we have a six-month dry season. We need to store water to get through that. We have 37 million people and millions and millions acres of farmland that need water, whether it’s falling from the sky or not. Thus the need to believe we can wish the rain to keep going during the wet season and the tendency to feel disappointment when the winter turns into a string of dry, sunny days.

*500,000 gallons per second. Here’s the arithmetic: California Department of Water Resources figures show that in the hour between noon and 1 p.m. on Tuesday, January 19, net inflow into the lake was 66,288 cubic feet per second. That’s the highest inflow figure for any single hour that week. One cubic foot equals 7.48 gallons. 66,288*7.48 = 495,834.24 gallons. One acre-foot = 325,851 gallons. And 495,834.24/325,851 = 1.52 acre-feet. Per second. For the entire 24 hours of the 19th, Lake Shasta’s inflow averaged just over 1 acre-foot a second.

View KQED: California Reservoir Watch in a larger map