On our visit last week to Colorado to talk about all things Firewise, Molly Mowery and I were taken on a tour of the Fourmile Canyon Fire in Boulder, where 167 homes burned last September. Allan Owen, the District Forester with the Colorado State Forest Service Boulder District, led us to several areas affected by the 6400 -acre fire.
While results of an in-depth investigation into the impacts of the fire are still pending, I can tell you what we observed and heard about. First, this large fire was wind-driven. We could see the “needle freeze” on the burned pine trees that showed the direction of the strong winds moving the fire forward.
Wind-driven fire accounted for the sometimes erratic pattern of the burn and spot fires. Spotting (wind-driven embers landing on a site and starting a fire on the spot) accounted for much of the seemingly random pattern of home destruction.
Some of most vulnerable homes – wood-sided, wooden-roofed, surrounded by flammable vegetation and covered in dry pine needles – were spared because the winds shifted and the fire never entered the neighborhood. Others, even newer construction, burned because of their proximity to heavily overgrown steep slopes.
The physics of home ignition hold true: homes burned or did not burn based on whether the home ignition zone (the house and everything within 100 feet of it) met the conditions for combustion. Those “un-Firewise” homes that survived had the fuel and the oxygen, but no heat (no ignition in the area). Surviving homes where fire raged could be observed to have reduced fuel on and around the house. This stucco-sided, metal roofed house survived quite nicely, even with some trees nearby.
We’re anxious to hear what the investigators have found, about the behavior of this particular fire, about the ecological impacts of the burn, about the fire response, but mostly about why some homes burned while some survived. Check out ongoing coverage from the Boulder Daily Camera. And watch the Firewise blog for more analysis of the information coming out of this fire investigation.