Wednesday, September 14, 2016

Living Under the Knife

Review: Los Angeles Against the Mountains (from The Control of Nature) by John McPhee


Pulitzer Prize winning author John McPhee writes with a style informed by the journalist he was and the sciences he has explored for decades. When you read a piece by McPhee, you hear honest echoes of the people and places and concepts he explores.

I just re-read a favorite McPhee essay, Los Angeles Against the Mountains from The Control of Nature. Perhaps because when I first read it I was living near the "over-steepened slopes" of the San Gabriel Mountains, I remember a distinct frisson upon learning about their tendency to spawn killer debris flows, a little-advertised regional problem.
The San Gabriels, in their state of tectonic youth, are rising as rapidly as any range on earth. Their loose inimical slopes flout the tolerance of the angle of repose. Rising straight up out of the megalopolis, they stand ten thousand feet above the nearby sea, and they are not kidding with this city.

The source material for such a multi-ton flow builds up over decades, but needs a specific set of triggering conditions to mobilize it. It might take additional years for such conditions to be met—which makes this a genuine problem in a place where even last week is ancient history. Without realizing the danger, developers, realtors, home buyers, and other newcomers to the area are often led to occupy hazardous properties. 
People have been buried alive in their beds. But such cases are infrequent. Debris flows generally are much less destructive of life than of property. People get out of the way.

The good news, as they say, is that 30, even 50, years may go by without a killer debris flow. 

The bad news is that when it does come, you can't outrun it. And everything it sweeps past (or over) is plucked up and added to the tonnage of the flow: cars, refrigerators, garden sheds, iron gates. Los Angeles has tried to control these flows by trenching debris basins above the built-up areas. Emptying them periodically—theoretically, anywayserves to trap future flows before they can sweep through residential areas and businesses. 
A private operator has set up a sand-and-gravel quarry in the reservoir. Almost exactly, he takes out what the mountains put in.

And it would work, too, except that people keep building new houses above the elevation of the basins, along picturesque creeks below the mountain skyline. 


Right in the path of the next debris flow.