Fire fuel-reduction projects completed prior to the Rim Fire saved structures and helped suppression efforts by reducing the intensity of the blaze in those earlier “treated” areas, according to a U.S. Forest Service and National Park Service report released Wednesday.
The preliminary analysis on fuel treatment effectiveness found that more than 17,000 acres in the Stanislaus National Forest and 18,000 in Yosemite National Park that had previously undergone prescribed burning or mechanical thinning to reduce potential fuels were less impacted than untreated areas.
The Rim Fire, third largest wildfire in California history, started Aug. 17 in the Stanislaus National Forest when a hunter lost control of an illegal campfire about 5 miles west of Lumsden Bridge. It threatened several communities to the west as it grew and also pushed north and east into Yosemite National Park over the next several weeks.
Acreage burned has not measurably increased since Wednesday morning and remains at 256,895. Containment remains at 84 percent.
At its worst, the Rim Fire burned through 30,000 to 50,000 acres on two consecutive days. The fire’s behavior was considered to be generally “high to extreme,” the report said.
The report analyzed the effectiveness of four previously “treated” areas and the effects they had on the Rim Fire.
A fuel break north of Groveland helped firefighters establish a dozer line and “possibly had a significant role in the successful defense” of communities along the southern edge of the fire, according to the report.
The Southwest Interface Team, or SWIFT, collaborated with federal, state and local agencies to treat 420 acres for what is called the “Rim Truck” fuel break. The project was largely completed in 2012, the report said.
A summer-home tract featuring several leased cabins and a developed campground on Stanislaus National Forest land was spared partially thanks to the 742-acre “Peach Growers” treatment area that surrounds it, the report stated.
Treatment activities included mechanical thinning of trees as well as hand thinning, hand piling, tractor piling, pile burning and broadcast burning. In addition, the removal of small-diameter and insect-damaged trees in and around the cabins helped create more open stand conditions.
Firefighters may not have been able to defend the cabins had it not been for the fuel treatment, which provided a safe place to work and relatively strong chance of success, the report said.
According to the report, the treatment also contributed to a transition from a “very high-intensity” blaze to a “low or moderate,” after it passed through the treated stands.
The Bear Mountain area saw little, if any, firefighting operations as the Rim Fire moved aggressively to the east through 1,813 acres that had previously received treatments, which involved mechanical thinning using selective tree removal and a variety of other techniques.
The Forest Service found “significant” differences between the effectiveness of each treatment unit depending on a number of factors, such as slope, type of treatment and proximity to untreated vegetation.
The report concluded that the treated units saw reduced fire intensity, especially where prescribed burning was a follow-up treatment to mechanical thinning.
In Yosemite National Park, the Hodgdon Meadows Residential area was right in the fire’s main path, but a combination of treatments gave crews more time to build fire lines to protect structures. Facilities located in the area included an office building, employee housing, sewage treatment plant, maintenance yards and campgrounds.
“The time afforded by the reduced fire behavior allowed for significant structure protection to be put in place in this developed area,” the report said. “This allowed fire crews to direct the Rim Fire around Hodgdon Meadow with little to no fire impacts and no loss of structures.”