Wednesday, August 25, 2010

Debris Flows

Just a couple of days ago I mentioned The Control of Nature, one of my favorite books ever. The third and final essay in that book is "Los Angeles Against the Mountains," and deals with the physical, economic, and human angles of debris flows. I have never witnessed a significant debris flow first hand, but I've seen many, many deposits from them, both in the geologic record, and in the soon-afterward (days to months) sense. In my current location, a high, flat Willamette river terrace (which means there is the modern floodplain and at least one lower terrace to absorb floods before they reach my favorite coffee shop or my apartment), the natural hazard I think about most is the great Cascadia Earthquake. But when I've been out and about- particularly in the rainy months- the hazard I pay closest attention to is the risk of debris flows. They can wipe out roads and bridges in seconds; I have seen innumerable examples of such destruction, especially during the spring after the February, 1996 floods.

Earlier today, Dr. Dave Petley at Dave's Landslide Blog posted this clip of a debris flow. Respect its au-thor-i-tie. This isn't even a very big one. The action gets wild at about 1:15.

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