Archive for the ‘Oakland hazards’ Category

Two lessons about floods

19 October 2015

As we anticipate the strong possibility of heavy El Niño rains, my attention will be on Oakland’s streams this winter. Last week parts of southern California were hit by “thousand-year” rainfall events, cloudbursts that washed thick sheets of mud over roads and properties. We can expect such things here too, in any given thousand-year period.

Arroyo Viejo, the stream that crosses Knowland Park, offers two lessons about floods. The scene below is at the northern edge of the park, looking upstream: a streambed piled with boulders, some as large as sofas. (All photos 800 px)


Notice: these rocks have been tumbled by the stream. How much water would it take to do that? Let’s make a rough, arm-waving estimate.

The rainfall in last week’s cloudburst was almost 4 inches in one hour. Had it fallen on the watershed of Arroyo Viejo above this point — say, half a square kilometer — it would represent an input of roughly 30 cubic meters of rainwater every second.

Picture in your mind that volume of water — no, it would be mud and therefore that much greater — funnelled through this narrow valley. Do a little geometry and it’s easy to see the floodwater would be well above the tops of the boulders.

Hidden in plain sight in this photo, then, is a single hour of tumult that might have happened a thousand years ago or five hundered years ago — or perhaps during the dreadful winter of 1861-62, when it rained for 43 straight days and much of the Central Valley became a lake.

The lesson is that most of geology’s hard work gets done in rare spurts of extraordinary activity.

Okay, the second lesson is hidden in these rocks. All of them, like this boulder as tall as me, are made of conglomerate.


These rocks, assigned to the Knoxville Formation of Late Jurassic to Early Cretaceous age, were laid down by ancient floods in a nearshore or terrestrial setting. I’ll show you three different specimens. Notice the large clasts and the fine-grained matrix that surrounds them.


This boulder displays a wide range of clast sizes. It was probably laid down by what’s called a hyperpycnal flow, a slurry of sediment that carries everything along with it. We’ve watched them happen offshore in Monterey Canyon. Here’s another example.


Instead of an underwater landslide, as seen in the first specimen, this represents something gentler and more organized, like a mudflow, or like the mudslides we saw in the news. The clasts are aligned with the current that carried them here.

The key observation in both cases is that the large clasts are floating in the matrix. In geologist’s terms, they are matrix-supported conglomerates.

Then we have this.


Here’s a beautiful clast-supported conglomerate. It represents a clean bed of well-rounded cobbles, all touching each other, like you’d see in a rushing stream or a rocky beach, nicely infiltrated with clean silt or clay after it was laid down.

None of these stones were made by ordinary sediment wafting down streams during ordinary rainy seasons. They were assembled by floods of all sizes.

Soft-story buildings

8 June 2015

Let’s take a minute to think a bit about public policy. Think about when Oakland’s next major earthquake strikes. Remember, the Hayward fault is considered capable of causing an earthquake of magnitude 7.5 or even larger. According to a compelling memo by acting manager Henry Gardner issued last year on the 25th anniversary of the Loma Prieta earthquake, Oakland could expect tens of thousands of dwellings to be lost in such an event. An estimated two-thirds of the people left homeless will be those living in Oakland’s stock of about 1100 residential buildings built with soft first stories—with garages or retail shops on the ground floor—that are less sturdy against earthquake shaking.


I’m not saying that the particular buildings shown in this post are deficient—only a building inspector can determine that—they just caught my eye during a recent walk. Soft-story buildings that collapse will endanger more than just the people living in them. A large fraction of buildings that collapse will catch fire and threaten their neighbors. Each multi-resident building lost will displace many more people than a single-family residence will. And all of these buildings are owned by landlords. (See approximately 1400 of them in this preliminary app from Open Oakland.)


Steps that save these buildings from collapse will go a long way to reduce the misery after a big quake. With intact homes, people can stay where they are, sleep at night, resume their jobs as soon as possible, avoid becoming someone else’s problem and help get the city back on its feet.

The solution being slowly considered by city officials, last I heard, would loan landlords the money for specific quakeproofing projects, at nominal interest. The tenants would pay back all or most of the loan over a period of 20 years.

Let’s look at that. Let’s say a $10,000 project would strengthen a soft-story building by bolting it to a beefed-up foundation, bracing the walls with plywood and so on. It’s elementary stuff, but of course it requires permits and inspections and so on. Say the city makes it easy: interest-free money, rapid permitting/inspection, simple paperwork, protections for tenants against gouging and eviction.

Who benefits? Well we all do, from the state of California on down. The city ensures itself (and by “it” I mean “us”) of one more intact residential building after the Big One. The neighbors benefit even more, and of course the landlord does too. But the tenants get the greatest benefit. Tenants get to live in a building that won’t kill them or send them to the streets.

Divide the cost of that benefit into 240 payments—once a month for 20 years—and that $10k becomes $41.67. It’s not nothing, but the benefit is immediate, significant and permanent.


There is never any urgency felt about this kind of preparation. The ground is quiet and we have so many other problems. In the city manager’s memo, the city was making noise about putting a plan to the City Council early this year. Two weeks later, I wrote a long post on the subject for KQED Science.

And now it’s no longer early 2015, there’s nothing online, and all I’ve heard is a Twitter post a month ago or so quoting a staff recommendation that tenants pay 100 percent of the retrofit loans.

What do you hear? What do you think?

Trees and serpentine

29 March 2015

There’s a stretch of Castle Drive, up in the Piedmont Pines neighborhood, lined with huge trees. On the Walk Oakland map, it’s even labeled “Colonnade of Eucalyptus.”


These give me mixed feelings, as tree removal projects have aroused lately elsewhere in Oakland.

First, there’s the experience the trees provide. For one thing, you basically can’t walk here, so the colonnade is not a realistic attraction for walkers. Its main effect is a momentary diversion for drivers, who really don’t need one at this location.

Second, there’s the effect on the surroundings. As you climb up in this valley, the trees emerge as a very tall fence that blocks the view of the hills and the city and the bay.

Third, there’s the geologic setting. This part of the roadway runs along a very steep 40-degree slope through pure serpentinite, visible in the small landslide scar on the right side of the photo. Serpentine rock is poor footing for these massive trees. The trees may seem like they’re buttressing the roadway, but when they inevitably tip over in a storm or earthquake, they’ll uproot it instead, forcing the locals to drive up and down Ascot Drive for many months.

But how about that rock? Here’s a hunk of it that spilled across the road.


And here’s a hand specimen. I love this stone, but roadbuilders don’t.


It’s not my problem, since I don’t live there, but I think the best thing to do is to turn this colonnade into a line of ground-level stumps. The root systems would bolster the soil for another decade or so, giving the city time to plan and execute a properly engineered roadway. And bollards set in the stumps would preserve the trees’ most useful current function of keeping cars out of the canyon.

Trees are supposed to be wonderful stockpiles of carbon, sequestering it from the atmosphere. For me, that argument shouldn’t apply to individual trees or even individual groves of trees. What do we do, in the long run, with the carbon in trees—pile the trunks in pyramids? Carbon is best stored in the soil, where it provides excellent tilth and maintains a thriving ecosystem that resists fire and drought. It’s like circulating money in an economy: do you hoard it in vaults or spread it around among people ready to use it as a medium of exchange? Humans have spent thousands of years degrading the world’s soils, and I’d rather we begin to restore them.