Archive for the ‘oakland hazards’ Category

The fire zone

20 October 2011

oakland hills fire

I was across the bay on October 19, 1991, doing a freelance job at some client whose name I forget. It was a hot day, and I saw a tall plume of smoke in the Oakland hills. The fire people worked on it, and that evening as I headed back across the bay it seemed to be over. But the following day, 20 years ago today, an ember got loose and took out a huge swath of the hills with some three thousand homes on it. It was a terrifying day, like something out of Lord of the Rings. Thick smoke covered the whole sky, and the east wind wouldn’t let up. The city was at nature’s mercy until the winds turned, late in the afternoon. For years afterward we would find pieces of charred wood in our garden that had dropped from the sky that day.

Apparently the same thing had happened in 1970. Historians reminded us that it happened in 1923, too. Before that the native inhabitants made a practice of torching the hills often to keep the land clear. But in the aftermath of 1991 the hills were hastily repopulated and reforested by residents whose driving urge was to make the pain stop. They had the eager help of insurance companies, placing their bets on the enduring value of view lots.

Today these fireprone hills are platted out for houses forever, even the impossible slopes of Charing Cross Road. Now the hills are in an unsustainable cycle of building expensive homes cheek by jowl on inadequate streets, growing inertia as fuel builds up and preparations lapse, one dire day of conflagration, and heedless rebuilding. The earthquake cycle is just a slower version of this fire cycle. If madness is doing the same things in the face of futility, then Oakland has gone mad. It doesn’t have to be this way.

Hayes Creek – Dracena Park walk (#34)

10 August 2011

Last week I bought a copy of Secret Stairs East Bay, by Charles Fleming, and met the author in person at a book event at the Solano Street branch of Pegasus. As I leafed through the book and heard the author, it was clear that while the walks offer lots of insight into Oakland’s history and culture, the geologic stories to be seen on these walks were yet to be told. I thought, Well, I can try that.

Sunday my wife and I took one of the walks, number 34, to Dracena Park (featured here before) in Piedmont. It begins at Chapel of the Chimes on Piedmont Avenue and takes you on a pathway across the valley of upper Glen Echo Creek (which I’ve called Mountain View Valley). Here’s the view back toward the Chapel from the other end.

glen echo creek valley

The stream was culverted long ago, but on the 1897 topo map it’s shown as Hayes Creek. Today it’s Glen Echo Creek. The valley floor is so flat because it was graded and planted to houses. But in my unprofessional opinion, its flood hazard today is as low as anywhere.

Onward! The book directs us to the head of this little gorge, part of Pleasant Valley Creek’s watershed, and thence to the old quarry pit now known as Dracena Park.

pleasant valley creek

You should always suspect humans as a land-shaping agent in Oakland, and indeed Walter Blair, who ran the quarry and before that a dairy at this site, may have had a flume or a transport line of some sort here. But its original form appears to be intact.

We turn into the park proper, and glorious bedrock appears—Franciscan sandstone, ready-fractured for its purpose.

sandstone

Go ahead and inspect the stuff; no hammer is needed (and none allowed anyway) when it crumbles so readily. Fracturing and tectonic movements—and surely some seismic work, like a bartender’s cocktail shaker—has rubbed and even polished parts of the stone.

hand specimen

Fleming says that the stone went into the homes of Oakland, but that is not true. This is not dimension stone by any means, but rather the usual quarry of Bay Area stone hunters in general: crushed stone and aggregate for roadbeds, underlayments and concrete mixes.

What was once a noisy scene of dynamite and dust is now a green bowl punctuated by the cries of children.

dracena bowl

The walls of the park are pretty well greened over, but watch out anyway: bedrock exposures are not forever. Maybe in a marble or granite quarry, where solid rock is sawed away in blocks, but here rockfall is a continuing potential hazard.

rockfall

These are not decorative boulders emplaced by landscape designers, but fallen rock. And that ivy-covered fence at the left? It’s really a safety measure to keep landslides away from picnickers. Here’s the whole thing.

slide guard

Dracena Park is a worthy way to remake an old quarry. But if you’re here when an earthquake strikes, get away from the walls.

The Oakland seismic landslide map

3 August 2011

A few years back, two guys at the U.S. Geological Survey did an exercise with a database that was subjected to a mathematical version of the Big One on the Hayward fault in the middle of the wet season. Their result was published as Miscellaneous Field Studies Map MF-2379, “Seismic Landslide Hazard for the Cities of Oakland and Piedmont, California.” Here’s a piece of it.

oakland landslide map

Dark-green areas are relatively fine, red areas are relatively awful, and the in-between colors are in between. Gray areas have slopes less than 5 degrees and don’t count. There’s a purple line running from top left to bottom right representing the Hayward fault; students of our street patterns may recognize the upper left corner as the intersection of 580 and 13. The lower right corner is 580 at the exit to the zoo. The left edge of the band of orange down the middle is Outlook Avenue. There are two bits of blue; the upper left one is the pond at Mills College and the lower one is the big hilltop reservoir near Toler Heights.

This map is of very little real use, because it’s just one worst-case scenario of one particular simulation, but it’s well worth studying anyway. (For real uses, like assessing your own property, you should hire a pro.) One big point is that bedrock matters. The big dark-green swath represents the solid metavolcanic rocks that are exposed in the Leona Quarry; they also underlie King Estates Open Space and part of the hill with the reservoir on top.

Oakland gets a (small) tsunami

18 March 2011

tsunamis

Oakland is fairly safe from tsunamis. Not as safe as San Ramon, but safer than most Pacific ports. First of all, we sit rather far from any dangerous subduction zones. When you track the subduction zones of the Pacific’s “ring of fire,” you get a continuous zone from Chile up to Puerto Vallarta, then jump to Cape Mendocino where the Cascadia subduction zone begins. We’re in between, and huge thrust-type quakes are not tectonically possible here.

A giant quake in Cascadia or the Aleutian Islands subduction zones would send waves down this way, possibly up to 2–3 meters high (this is the height out at sea; as they hit shore they would rise much higher). That’s where our second line of defense comes in: the Golden Gate. Anything coming through there would first be throttled by the narrow strait, then would spread out once it enters the bay. The people who prepared the Alameda County tsunami inundation maps also considered a few local extreme earthquakes that may have components of thrust. These could raise local tsunamis, but not of the horror-inducing size we saw in Japan.

I don’t want to minimize the possibilities, but the Sendai scenario cannot occur here. The harbor, however, is liable to damage even in a modest event, as we saw in Santa Cruz. Ships will bang into docks and each other; a less obvious hazard is that swift currents may pluck ships off their anchors and wash mud into our carefully dredged channels. You won’t want to go down to Jack London Square and watch if there’s a big local quake or a great event in Alaska or Cascadia. But last week would have been OK. Did anyone see the water there?

Water underground

4 January 2011

In today’s Chronicle, the Oakland writer Jon Carroll was musing about fire: “Usually it’s an obedient little creature, about the size of a cocker spaniel—until one day it turns into the largest, meanest cocker spaniel on Earth, and there goes the house.” Water, another of the four ancient elements, is the same way.

water main burst

Water is great in metered doses, delivered by tank and faucet. But “water dissolving, water removing” is no tamer than fire. Some time you should see firsthand what keeps it constrained: giant dams and stout mains, treatment plants, intricate feeder lines. The antique examples of dams and treatment facilities in Oakland are not the state of things today. Every now and then something breaks, like this line under Santa Clara Avenue in 2005, and a hint of chaos leaks out.

Last week a handyman had our water turned off for most of the day, with no word about when he would finish. As sunset approached we panicked enough to go out and buy 48 pounds of jugged water. But we didn’t need to use most of it. And so another bit of our earthquake preparedness is in place, a little lurch of progress. After the next big-enough one there will be water, water everywhere.

The ancients had a handle on things with their notion of four elements. Fire and water are worthy of the status, both full of motion and power. As for air, every weather report vouches for it. But it took someone more observant than most of us to see earth the same way and sum it all up as panta rhei, everything flows.


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