GrokSurf's San Diego

Local observations on water, environment, technology, law & politics

Debris flows and the San Gabriel Mountains

Posted by George J Janczyn on October 7, 2009

In his book The Control of Nature, John McPhee wrote about places where people are engaged in a major struggle with nature. One of those places is Los Angeles, where the city meets the San Gabriel mountains. His is a vivid account of devastating debris flows following rain events resulting in tremendous damage and loss of life, and of the expensive efforts by the city to contain the flows. After the recent Station Fire, many people no doubt were already thinking about this and sure enough, today’s LA Times reports the U.S. Geological Survey has issued a forecast that major mudslides are highly likely during the winter rain season.

Of the San Gabriel Mountains, McPhee wrote:

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. Shedding, spalling, self-destructing, they are disintegrating at a rate that is also among the fastest in the world. The phalanxed communities of Los Angeles have pushed themselves hard against these mountains, an aggression that requires a deep defense budget to contend with the results.

McPhee published his book in 1989, at which time he counted “at least a hundred and twenty bowl-shaped excavations that resemble football stadiums and are often as large” that the city and county have constructed to catch debris flows that would otherwise smash into homes and carry them away and sometimes still do in spite of the basins. After a major event, even the large basins can fill up overnight, requiring fleets of earthmoving equipment to hurriedly try to empty the basins before they overflow. In a single “productive” year, he wrote, they may have to remove a million cubic yards of debris at a cost exceeding $60 million (the LAT report cited above says there are 678 drainage basins in the burned area).

And that is just par for the course. After a major fire, things are much worse. McPhee:

Wherever there has been an antecedent fire, debris basins are likely to fill to the brim. In 1978, every basin filled for fifteen miles under the slopes burned by the Mill Fire–eighteen basins in all. In some of them, the rocks were so big that they had to be broken by dynamite before they could be removed. Dunsmuir was cleaned twice that winter. Mullally and Denivelle three times. They filled as they were being emptied. Zachau was cleaned out three times as well, but it still let boulders ten feet in diameter get away.

For anyone interested in learning more about this and other great human battles with nature, I highly recommend McPhee’s book.

Postscript Feb 6, 2010: 41 homes damaged in La Canada mudslides
Postscript Feb 27, 2010: Convoys of trucks haul away mud from storms

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One Response to “Debris flows and the San Gabriel Mountains”

  1. Orhan Guldam said

    Thank you very much this was quite helpful

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