new landslide hazard map

The California Geological Survey has released a new map of landslide hazard that reinforces some of the incontrovertible facts about the equation that governs landslides in the Bay Area:

slope angle + clay expansion + weak rock + rain = landslide

The last variable, rain, is one that is peculiar in the Bay Area. We're moderately arid--not as many inches as Seattle, far more than Barstow. But the signature of Bay Area rains is that they come all at once, in catastrophic storms that funnel into the Bay Area across the Pacific as if by a pipeline. The effect of this is saturation, and the effect of saturation is overland runoff, water literally unable to soak into the receiving clay. This runoff can have an ferocious erosive effect, transforming stable slopes in a matter of hours to undercut, unstable, moving landscapes.

As the San Jose Mercury News has reported, this is bad news for Marin County. Marin is blessed with steep, interesting hills of schist and serpentinite and chert. But the vertiginous fractured Franciscan landscape are both weak and steep, simply waiting for a slight nudge, in the form of water weight and lubrication. And unlike the East Bay, where the mountainous regions are sparsely developed, Marin has extensive building on very steep slopes.

As bad as this is, it gets worse. As was recently discussed at a conference in Sacramento, California faces a risk from periodic "megastorms," called ArkStorms, which can deluge the state with biblical rain.