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Sept. 2 mid-day update on the Dixie: Focus on the escarpment — but what is it?

By Michael Condon

Former Fire and Aviation Chief for the Plumas National Forest

Special to Plumas News

 

There is a lot happening on the Dixie Fire today, but most of the focus is in the southeast corner of the fire near a location locals know as “the escarpment.”  This area is the center of attention today — both because there are communities at the base of the escarpment along the Highway 395 corridor and also because the escarpment is notorious for producing extreme and dangerous fire behavior.

What is the “escarpment” and why does it generate extreme fire behavior?

The escarpment is a geological feature.  Because of the geologic forces that shaped the Sierra Nevada Range, the west slope of the Sierras tend to have gradually rising slopes moving from west to east.  Along the Sierra crest, the slope to the east into the Great Basin is very steep and very dramatic.  This steep drop is what we call the escarpement.

The Honey Lake Valley is the other geographic feature that works in conjunction with the escarpment to generate unusual fire behavior.  The Honey Lake Valley is a broad expanse of mostly desert landscape.  The vegetation is very sparse and there is a lot of bare ground.

On a typical summer day this valley becomes very hot.  The valley floor heats the air above it and the large air mass begins to rise.  The rising air needs to pull cooler air in behind it to replace the rising air mass.  That cooler air comes off of the escarpment.

Honey Lake Valley sits at an elevation of about 4,000 feet.  The escarpment is generally a little over 6,000 feet with peaks along the ridge rising to 7,000 and even 8,000 feet.  The air is much cooler there and it rushes down the slope to replace the rising air in the middle of the vast Honey Lake Valley.

The volume of air needed is quite large, so lots of air has to run down the side of the escarpment to fill the valley.  That creates very rapid down slope winds.  Strong wind fuels the fire increasing the spread and intensity dramatically.

The normal gradient winds blowing in the Great Basin are channeled laterally when they hit the escarpment.  The winds can blow very strong to the northwest or southeast depending on the angle at which the winds hit the escarpment.

This means that at the base of the escarpment, a strong down-slope wind will likely meet a strong cross-slope wind.  This generates the turbulence that forms whirlwinds.  Add some fire and the rapidly rising air it creates and you have all of the ingredients of a fire whirl.  Fire whirls generate their own winds further increasing the intensity of any adjacent fire.  And they travel in whatever direction suits them.  It is a firefighter’s nightmare.

Let’s back up a minute.  Firefighters are taught basic fire behavior early in their training.  They learn that fire burns more rapidly uphill than it does downhill.  They learn that the safest location to fight a fire from is below the fire. That becomes ingrained in their basic instinct.  At first glance, the base of the  escarpment might appear to be a relatively safe location for fire fighting.  Local firefighters are very well schooled in the unique fire behavior of the escarpment and they are quick to educate out of area firefighters.  But more than once unwary crews have been caught off guard and found themselves in a dangerous situation.  It is very challenging to fight fire along the escarpment.  It can be very dangerous if you don’t know the sort of fire behavior that can develop very quickly.

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