Archive for the ‘oakland hazards’ Category

Slump at Round Top

28 July 2009

slump

This slump sits at the edge of a level area in Sibley Regional Volcanic Preserve. Notice the well-formed curved headscarp on the right side and the flow of material exiting the slump at its toe on the left. A classic textbook slump has a rotational aspect, because it moves along a scallop-shaped underground surface. This is easier to show than to tell, using this USGS image:

slump diagram

So the Sibley slump has aspects of an earthflow—basically it lacks some of the features of a slump. In a classic slump, that middle part would be tilted slightly backward instead of slightly forward.

But why is it here? First, I’m not sure whether this area is bedrock or a big tailings pile; that would involve walking down there and poking around, or looking for old aerial photos (which I could do from my desk). If it’s bedrock, that would be terrestrial mudstone and conglomerate of the Orinda Formation, the stuff you see flanking Route 24 just east of the Caldecott Tunnel. This doesn’t resemble that at all, so my working hypothesis is that it’s tailings: waste rock and soil from the days when this was a quarry. It has little strength, and its slopes tend to give way.

And why is it a slump, and not just a washout or a slide? That involves water and the consequences of human acts. This tailings pile appears to have been graded without due regard for drainage. Rainwater and runoff would collect on and infiltrate into this flat top surface. Then the groundwater would trickle its way through the pile and exit as seeps and springs near the bottom, where Fish Ranch creek is happy to accept it. But in a wet year the water table would build up and exert pressure around its edges. Water would rise and buoy up the precarious sediments at the base of this slump, easing the gravitational force holding it against the ground beneath. The details are important, and that’s why landslide management requires the services of a geotechnical specialist.

That reminds me: the new California budget plan consolidates the state Board for Geologists and Geophysicists into the State Mining and Geology Board. In itself, this should not threaten the state’s licensing system for geoscience professionals, but it does give the governor an opportunity to eliminate board members whose scientific advice tends to contradict business as usual. I don’t have any scuttlebutt to pass on, but I do know that commerce does not always look kindly on the costs of living in the real world, on the real Earth.

Two dams

1 June 2009

I was asked the other day about the safety of living below Oakland’s dams. We have two of them, both of which I happened to photograph in March 2003 (back when I was still using film). Anthony Chabot built them both. This is the dam at Chabot Reservoir, at the very south end of Oakland. The Hayward fault is a couple hundred meters downstream.

chabot dam

This is the dam at Lake Temescal.

temescal dam

Both are earthen dams, basically massive piles of clay and boulders. The first was built in 1874-75 and the second in 1868. As the “great San Francisco earthquake” occurred on the Hayward fault in 1868, seismic safety was high in people’s minds. Here’s a page about their construction. These dams are generally considered sound and able to withstand another big one. Lake Temescal straddles the Hayward fault, but the dam is so massive and the water it holds so modest that even a 2-meter displacement on the fault will not lead to a dangerous failure, as I understand it.

The Calaveras Dam, farther south near Milpitas, is also of earthen construction. It crosses the Calaveras fault and is being replaced with a safer design; in the meantime the Calaveras Reservoir has been drained to half its volume.

Rock failure

27 April 2009

quarry failure

The Rockridge Shopping Center quarry is in pretty good shape . . . for a Bay area quarry. Most of the rocks in the Coast Ranges are battered and shattered to various degrees, what with being ripped apart and shaken and squeezed into hills by the activity along the San Andreas fault system. Most of this former quarry is holding up well. But the whole length of the rock face has a chainlink fence in front, with piles of rubble trapped behind it. This spot is near a zone of dark, serpentinite-looking rock that has been failing more than the sandstone and lava around it. If these stout timbers can’t deal with it, I doubt the fence will help much when the rest of the slope starts to give.

Rock naturally deteriorates when you strip the soil off it. Actually the soil is made of deteriorated rock itself. Solid rock may be overrated.

Vantage Point Park

15 February 2009

vantage point park

This tiny park is at East 12th Street and 13th Avenue, on a low rise next to a lot of activity. Between here and the water—Brooklyn Basin, the innermost part of Oakland Inner Harbor—run East 8th Street, I-880, Embarcadero Street, BART and the Southern Pacific rail line, plus I’m sure a number of underground power and water lines and what not. It’s a highly concentrated lifeline corridor. Across the way is the Coast Guard base on Government Island. In the distant left corner in the 800×600 version (click the image) you can just see Alameda Island. Except for the hummock in the foreground, everything visible is made land, artificial fill, with a high water table. It was created more or less haphazardly starting a century ago, and under strong shaking a lot of old fill of this type is prone to liquefaction.

Past the left edge of this photo, a little buried creek valley running down 14th Avenue reaches the bay. It’s all filled in, too. All the crowded life lines I mentioned cross that creek bed. In the next big Hayward fault earthquake, this is a highly vulnerable spot.

I took this shot on 19 November 2008 during a walk I took the length of Oakland, from the San Leandro to the Ashby BART stations. Lots more photos from that day on my Fotothing site.

Scars of the Hills Fire

9 January 2009

eucalyptus scars

The hills south of Hiller Highlands, just across route 24, were swept bare by the 1991 fire. Anything that was there besides eucalyptus has been unable to compete, and now it’s a fire-prone monoculture. That suits eucalyptus fine—keeps down those riffraff oaks and madrone, thank you.

The hillside forest was partially cut down maybe ten years ago, then again just a couple years ago. Each wave of attack remains obvious on the landscape. Eucalyptus is like the hundred-headed Hydra in the Odyssey: every head you cut off grows two new ones in replacement. Hercules defeated the Hydra with his sword in one hand and a torch in the other, cauterizing the wound after each head he struck off.

We can only reclaim these slopes by poisoning the stumps, herbicide-haters be damned. For a successful example, see the new slopes along Skyline Boulevard between Broadway Terrace and Elverton Drive, a wedge of land belonging to the Sibley park that has been reclaimed from just this state, exposing some excellent outcrops.

Landslides

7 December 2008

landslide scarp

This fire trail in the hills is beginning to disappear downslope. The biggest mover of sediment is not erosion, it’s landslides—or mass wasting, to use the geologist’s more general term. The land is like a building in that respect: neither of them wear out, the way an ice cube melts; instead they get more and more rickety, then collapse.

This headscarp will concentrate the infiltration of rainwater at the same time it admits air underground. The comfortable stasis of the underground is now broken. The alternating wetting and drying and the rapid loading and unloading give gravity more advantages until one day it will pull the road down. Then the city will help mass wasting along by cutting a fresh track into the hillside. This will never end until the hill or humanity is gone. For us it’s just part of the cost of living.

Once you know these crescent scarps, you’ll see them everywhere in the hills.

Earthquake day

17 October 2008

cypress structure mandela gateway

October 17 always has an ominous ring to it, because of the Loma Prieta earthquake of 1989 (or “the big-enough one“). In Oakland, we were distant from the epicenter, which is just visible on a clear day in the mountains beyond San Jose. But it was on this spot where the double-decker Cypress Structure, part of the Nimitz Freeway, felt its soft ground give way and collapsed, the deadliest single place in the whole disaster. I remember riding BART into the Oakland West station (remember when they called it that?) and sensing the whole carful of riders hold its breath as the wreckage came into view.

It was an ugly, traumatizing mess for years and years. In 2005, when I took this photo from the BART station on January 28, the Mandela Gateway complex at the base of Mandela Parkway was new, landscaping along the road was under way, and the area seemed nearly finished. But if you know where to look as you ride west from the downtown stations, you can still see the curved trace of the old freeway in the lines of the buildings and lots. Earthquakes are forever.


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