Last week’s third and final storm was a doozy, setting all-time record low pressure in a number of cities. Here in the Bay Area we got 3 to 8 inches of rain, and wind gusts of over 80 mph.
All along the coast, 20 foot waves pounded the shore:
causing significant beach erosion:
And as I mentioned in my last post on the storms, the first of the three storms even produced a tornado (and some waterspouts) in Southern California. (see Berkeley Hills-El Nino Storms Hit Hard)
And of course, snow in the Sierras was measured in feet—8 to 10 feet in some spots!
After the mid-week second storm, I wanted to hike into the Berkeley Hills to see the effects of the storm on the canyons. But because it was so muddy, instead of taking a fire trail, I took one of my favorite paved roads up into the Hills, Panoramic Way:
I usually walk south on Piedmont, past the Memorial Stadium:
And then take a left up Bancroft, just past the International House:
When Bancroft dead-ends, I walk up the Bancroft Steps and then cross over to Panoramic Way, which winds up into the Hills:
As soon as I got up into the upper, less-developed parts of Panoramic Way, I saw lots of rocks on the road and erosion like this:
But I was really shocked when I came around a bend of on the upper part of Panoramic Way and found this:
A landslide! Not a huge one, but still, impressive to see first-hand. Over a hundred feet of the steep upper slope adjoining the road had given way:
And flowed down Panoramic Way for quite a distance:
I was struck by how intact the top layer was; the whole section had been slipped down the slope pretty much in one piece, carrying along most of the shrubs intact. Although small in scale, this little landslide had all three of the basic elements of any landslide:
Of course, this landslide on Panoramic Way in the Berkeley Hills was a small one compared to the huge landslides California is famous for. In the hills and on the cliffs all along the California Coast, heavy rains and steep, unstable land create a deadly combination that leads to highly destructive and often lethal landslides and debris flows. One of the worst in recent memory was the La Conchita landslide in 2005, which killed 10 people.
In Southern California, this vulnerability is only made worse after wild fires, fanned by the infamous Santa Ana winds, denude the hills of vegetation. Without vegetation to slow it down and trap it, water runs off too quickly, causing flash floods and debris flows filled with huge rocks and tons of sand and gravel. Sometimes truly enormous boulders are set loose:
You don’t have to have barren hills to be at risk for a landslide. Doing research for this article, I came across this amazing image of a landslide in England. Known as the Holbeck Landslide, it occurred south of Scarborough in North Yorkshire:
Fortunately, this particular landslide didn’t happen in a moment. It took place over a two-day period, so people were able to evacuate when the first signs of movement became evident. The scale of the movement of land is hard to fathom, until you know that the the cliffs in the depositional area are over 180 feet high! Geologist estimate over a million tons of loose glacial till (sediment) flowed down to the sea shore.
Looking at the Holbeck Hall image, I have to admit that the little landslide on Panoramic Way seems pretty insignificant! (Although I wouldn’t have wanted to be in its way when it broke loose!) Still, the soil physics involved are very similar. All you need for a landslide is instability—relatively loose soil and rock, a steep-enough slope, and some sort of triggering mechanism, such as too much rain, an earthquake, or erosion of the base of a slope of land. As this USGS diagram shows, there are all kinds of landslides:
The bottom line is that at some point, the friction and cohesion that hold the soil on a slope are simply overcome by gravity, and the soil and rock take off down hill, acting more like a liquid than “solid” land. In California, the majority of landslides caused by rainstorms, though earthquakes and tremors are a not-too-distant second.
Because of the unique geology and weather of California, landslides and debris flows are always going to be a part of the California experience, just like earthquakes, wildfires, Santa Ana winds, and El Niño events. Because Californian love their hills and the vistas they offer, hill dwellers are always going to be in harm’s way.
We can do what we can to be safe, but as nature writer John McPhee makes so clear in The Control of Nature, even our best efforts may be inadequate. In the last section of his outstanding book, he shows how residents of the San Gabriel Mountains in Southern California have had little success in preventing debris flows from destroying their houses in spite of spending millions on creating man-made diversion pits and dams.
From the standpoint of geology, landslides, mudslide, and debris flows are simply the more spectacular forms of the ceaseless erosion that shapes our beautiful planet, wearing down mountains and creating the sedimentary rock and soil so much of life depends upon. Walking in the Berkeley Hills, you can see evidence of this ceaseless erosion of wind and rain up-close and personal.
National Geographic has a fantastic video on YouTube about landslides. The opening scene of a landslide in Portland, Oregon is simply amazing. Watch as a fellow in a truck outruns a landslide coming down the street, sweeping away cars and everything in its path!
In this next amazing video, caught on camera by a Japanese research team, you can watch a whole mountainside slide across a road with the forest riding along intact! Amazing!