November 24, 2018


The Camp fire burned homes but left trees standing. The science behind the fire's path (THOMAS CURWEN  and JOSEPH SERNA, NOV 20, 2018, LA Times)

[T]he popular perception is that wildfires burn through these communities like a wall of flames. In fact, small, burning embers -- firebrands -- blown in advance of the fire are the primary cause of structural fires.

"When we look at the big flames but not the firebrands, we miss the principal igniter and pay attention to the show," Cohen said.

Billions of these embers fly into neighborhoods, landing onto flammable roofs, into vegetation around the structure and rain gutters choked with leaves and needles.

Big flame fronts, on the other hand, are less effective in igniting structures because they burn fast -- often consuming their fuels in about a minute or less in one location -- and move along often so quickly as to not consume the structures themselves.

Yet in the face of increasingly severe and deadly wildfires throughout the country, Cohen maintains that it is possible to decrease the vulnerability of urban development in the face of these events.

"Uncontrolled extreme wildfires are inevitable," he said, "but does that mean these disasters are inevitable? No. We have great opportunities as homeowners to prevent our houses from igniting during wildfires."

Pangburn's assessment -- that the Camp fire in Paradise was an urban conflagration, structure to structure -- opens the door for fire behaviorists to consider the strengths and weaknesses of the state's codes for protecting property in fire-prone, rural environments.

The mandate in California, as stated in Public Resources Code Section 4291, is clear: A 100-foot perimeter of "defensible space" must be maintained in "land that is covered with flammable material."

While the 100-foot requirement is appropriate, it is important to begin thinking closer to the structure itself and work out in concentric circles, Cohen said.

"We have to take care of everything from five feet out," he said, "so that when it burns, it doesn't produce enough radiation to ignite the structure or produce enough flames to contact the structure."

The goal is to distinguish between structure fires and wildland fires and to understand that communities can be separated from wildland fire.

We don't have to live in ammo bunkers, Cohen said, and we don't have to entirely eliminate fire from within the perimeter, just ensure that fires that occur within 100 feet don't burn long enough or intensely enough to ignite other objects.

A defensible perimeter also provides residents with more safety options as fire approaches.

Cohen refers to the story of the medical staff and patients from the hospital in Paradise who took refuge in a home. Climbing on the roof with hoses and clearing pine needles from the rain gutters, they were able to survive.

"A house that doesn't burn is the best place to be during a wildfire," he said.

However, the 100-foot requirement in California stops at the property line, which creates a situation where homes can be built beside one another within that perimeter.

If multiple homes share this perimeter, then each home is a potential ignition source, and homeowners cannot create a defensible space beyond their property line if that means trespassing on someone else's property.

"All it takes is one house to catch on fire, and the heat and embers put the other houses in jeopardy," Pangburn said.

Posted by at November 24, 2018 8:59 AM