An Assessment of Fuel Treatment Effects on Fire Behavior, Suppression Effectiveness, and Structure Ignition on the Angora Fire

The Angora Fire started southwest of South Lake Tahoe on the afternoon of June 24th from an unattended campfire. It burned under some of the most severe fire danger conditions experienced in this area during the last 20 years.

The fire spread four miles in three hours and burned over 250 structures on private property. Most of the 3,072 acres within the fire perimeter involved National Forest System lands, however about 300 urban lots owned by the United States Forest Service (USFS), California Tahoe Conservancy (CTC), and Eldorado County, and 231 acres of private property burned.

Ed Hollenshead, USFS Region 5 Fire Director requested a team to assess:

Effects of fuel treatments on:
~ fire behavior
~ fire suppression
~ structure ignition
~ public safety/egress

Fire behavior in non-treatment areas and other vegetation management treatments units.

Areas evaluated within and adjacent to the fire perimeter included all of the 480 acres of USFS area fuel treatments, and about half of the approximately 300 urban lots. About 405 acres of USFS area fuel treatments burned with surface fire intensity. Over eighty percent of the urban lots burned as surface fire. Almost all of the non-treatment and other vegetation management areas burned with crown fire intensity.

Key Findings:

Most of the area fuel treatments reduced fire behavior from a crown fire to a surface fire. Area fuel treatments adjacent to subdivisions provided important safety zones, increasing suppression effectiveness which saved houses. Urban lot treatments reduced ember production, and reduced heat and smoke allowing firefighters to be more effective. A large number of houses burned from firebrands generated from other burning houses rather than wildland fuel.

Fuel treatment units on steep slopes burned at higher intensity than those on flat ground. Some fuel treatment units burned at high fire intensity because they were adjacent to and downwind from untreated units. Crown fire momentum carried high fire intensity partway into these treated areas before the more widely spaced crowns and reduced surface fuel caused the fire to fall to the surface.

(For a thorough USDA presentation on the Angora fire click on the green Web Link below.)

Web Link: http://www.fs.fed.us/r5/angorafuelsassessment/