Tiny steps toward flood control

8 August 2016

More than once on this blog, including last week, I’ve described streams as sleeping creatures that wake up in floods. Kittens that turn into tigers, nebbishes that become the Incredible Hulk, pick your own metaphor — streams do most of their geological work in spasms. The downpour that happens once in a century, filling a creek and scouring its streambed a bit deeper and wider, is one hour in a million hours.

Last week one such rainfall made a shambles of Ellicott City, Maryland. Hydrologist Anne Jefferson looked at this flood with a geologist’s perspective. Her insights apply here as well as there.

Oakland’s Lake Merritt is prone to floods as well as high tides. The tides are slowly growing as sea level rises, of course, but the greater threat of flood comes from the land side. Oakland’s soil absorbs much less rainwater than it used to. As Oakland grew, its dusty streets were sealed under asphalt, its grassy lots occupied by homes with guttered roofs, its footpaths paved over in concrete. Today these impervious surfaces shed the rain, and the runoff drains swiftly away to the nearest body of water.

In October 1962 the Columbus Day Storm dumped over 4 inches of rain in a single day. Lake Merritt rose more than 7 feet and left the surrounding roads waist-deep in stormwater. A few years later a large flood-control station was installed at 7th Street that regulates the tidal lake.

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We can do better on the land side. Greener building practices, sustained from now on, will gradually offset our disruption of the local hydrology. This is one of those — panels of pervious pavement flanking the street trees near the lake on East 18th Street. They’re made of a porous concrete that lets water through, like a super mulch. They let the tree roots breathe, too.

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Another green practice you’ll see near the lake and elsewhere around town is rain gardens, shallow basins filled with vegetation that catch and absorb rain runoff before it can reach the lake.

Oakland alluvium

1 August 2016

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When someone opens up the ground in Oakland, no matter where, I think it’s interesting. This construction site on Telegraph Avenue between 29th and 30th Streets exposes alluvium, the stream-laid sediment that once supported productive farmland throughout Oakland’s flats. Mapped as “alluvial fan and fluvial deposits (Holocene)” or unit Qhaf on the geologic map, it covers more area than any other geologic unit.

The uppermost part, the brown stuff that the excavators have turned over in curls, is rich in organic matter and clay. A little deeper it turns tan as the organic matter thins out. It’s dense and firm, good ground for building.

The nearer you get to the bay, the finer grained this material gets — more clay, less sand and gravel. Streams have carried it down from their canyons in the hills over the last few hundred thousand years, pushing back the sea. And by “streams” I mean floods. The clear trickling streams we know are actually asleep. Floods are the one day in a thousand when streams awaken, picking up and carrying alluvium from place to place.

Occasionally the streams themselves jump their tracks. If you visualize the land in super-fast motion over geologically recent time, our streams whip back and forth over the coastal plain like firehoses out of control, winnowing the alluvium again and again. From the hills outward they build up low, cone-shaped piles of sediment called alluvial fans. Downstream, these coalesce into an alluvial plain.

The “h” in “Qhaf” refers to the Holocene time period. The Holocene (“fully new” in scientific Greek) began when the latest pulse of the ice ages ended, about 12,000 years ago. It’s been a mostly pleasant time. Many geologists argue, with good reason, that the Holocene has given way to a new permanent state of wrenching climatic changes. Because the natural balance of climate is strongly influenced by human activities, they argue, the climate system today is a writhing firehose we may be able to control. They propose to call our new era Anthropocene time.

A greenstone boulder in Lakeside Park

25 July 2016

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Lakeside Park holds a scattering of boulders and plaques. The plaques are always interesting, and sometimes so are the boulders. This one sits at the west side of Bandstand Cove by a grove of redwood and oak trees. I can tell at a glance — the greenish color, even texture and lack of sedimentary fabric — that this rock consists of metamorphosed lava, informally called greenstone. There’s a lot of it in the Coast Range. There’s also some in the Sierra foothills, and I suspect that this was quarried over there.

One side of the boulder displays a nice slickenside, a sign that the rock was cracked and wrung underground.

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Emily Brodsky down at UC Santa Cruz studies these fault surfaces and has been finding deep clues in them (see the latest paper from her team).

Elsewhere the boulder shows stretch marks — little extensional fractures filled with quartz. Like a run in a stocking, these are evidence of the stresses that affected this body of material once upon a time. Since the boulder has been ripped out of its original setting, these scrape marks and stretch marks have lost their geological meaning, but they’re still pretty.

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Oh yeah, the boulder has a message on it. The plaque announces that the three fountains in Lake Merritt were installed or renovated by Madeleine and Andrew Wong as a gift to the people of Oakland.

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And not least among its functions, the boulder punctuates the most peaceful view on the whole lake, whether the fountain is running or not.

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Lake Merritt needs a lot of human management to stay clean and pleasant, and the fountains are a key part of that.