Wednesday, June 2, 2010

Debris Flows

I mentioned yesterday that I was pleased to see a NOAA/NWS weather alert stress the danger of debris flows associated with heavy rains in and near steep landscapes. I often find myself enjoying OregonLive's weather reports; they're good, accurate, and talented summaries, with the added benefit of frequently being written with a sense of dry humor. The latter is ever so appropriate for our frequently moist environment. That humor is in evidence in today's report- the current weather set-up is more that of deep winter than late spring or early summer, so yesterday and today they're referring to this week's precipitation as our "Juneuary Storm."

Still, what got my attention was that a sizable portion of the article was a description of what debris flows are, how they work, and how they can be triggered in the conditions we're currently having. Then, to cap it off, they have a sidebar of debris flow warning signs:

Warning signs of debris flows
Stay alert. Listen to the radio, TV, or a weather radio for flood watches, which include the potential for debris flows and if told to evacuate, do so immediately.

Listen for unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of falling mud or debris may precede larger landslides.

If you think there is danger of a landslide, leave immediately.

If water in a river or stream suddenly turns muddy or the amount of water flowing suddenly decreases or increases, this is a warning that the flow has been affected upstream. You should immediately leave the area because a debris flow may soon be coming downstream.

Assume highways are not safe. Be alert when driving, especially at night. Don't overdrive your headlights. Embankments along roadsides may fail, sending rock and debris onto the road.

Landowners and road managers should check road drainage systems and conduct needed maintenance in case the predicted heavy precipitation does occur.

Cleaning up after landslides can also be hazardous. "When it's this wet outside, people need to be careful when they're cleaning up the mess. A small mudslide can actually be part of a larger landslide," explains Roddey. "Cleanup should not be done until after the storm."
Well done! I would add that valley floors are a bad place to be in heavy rain events, and western Oregonians seem to have a predilection for building houses at the top of alluvial fans where canyons emerge from mountains. This is precisely the spot most likely to be impacted by a debris flow. The alluvial fans are built up from previous events, and are evidence that they have happened and will happen again. Picking a building site to one side or the other, away from the mouth of the canyon, will decrease the risk of property damage, injury or loss of life in the event.

Some other factors that can increase risk of mass movement (defined as driven primarily by gravity, as opposed to erosion, which is defined as transport in a medium such as water, wind or ice):
  • steep slope
  • deep soil profile
  • sedimentary layering or jointing (in any kind of rock) in a direction similar to the surface profile
  • undercutting of a slope, either naturally (e.g. stream or wave erosion), or artificially (e.g. construction work)
  • removal of timber and other vegetative cover by logging or fire- this leads to increased risk for as much as a decade or more
  • high levels of soil moisture
  • flow of ground water laterally to the surface, particularly in saturated conditions.
There are probably additional factors that should be added to this list that I'm not recalling right now... if you can think of others, leave them in the comments.

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