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

Fire Effects of Area Fuel Treatments

Descriptions and Prescriptions

Prior to the Angora Fire, approximately 480 acres of fuel treatments were completed on National Forest System lands within the fire perimeter (excluding USFS urban lots). Fuel treatments were purposely located adjacent to residential areas. Other vegetation management activities had taken place in the fire area, but these treatments were not designed to change fire behavior (see maps in Appendix A).

Fuel treatment prescriptions on NFS lands consisted of commercial and/or pre-commercial mechanical thinning, followed by hand thinning, piling, and burning handpiles. Prescription details can be found in Appendix B. Underburning for all of the fuel treatment areas, except the urban lots, was in the planning stages.

Fire Behavior

Fuel treatments are designed to be used together with suppression resources and "fire-safe" principles to effectively reduce risk to homes. The objective of fuel treatments is to change fire behavior from crown fire to a surface fire, reducing spotting distances and convective and radiant heat. Suppression resources are effective only if fire intensity is reduced to moderate or low intensity surface fire. This level of intensity allows firefighters to extinguish spot fires and allow safe "close-in" engagement.

Under the weather conditions present during the Angora Fire, all vegetation would need to have been removed for as much as one half mile to stop the fire without suppression action. Furthermore, fuel treatments on wildlands are effective only when adjacent homeowners have well-maintained, fire-safe property and when there are sufficient suppression resources present to extinguish spot fires and surface fire spread.

Fire behavior was analyzed for the 16 area treatment units on USFS lands that were wholly or partially within the fire perimeter. About 405 acres of USFS area fuel treatments burned with surface fire intensity and 75 acres burned as crown fire.

image showing results of untreated units ion Angora

Figure 11: Direction of fire spread is indicated by the red/yellow arrow. Initially, spread direction was to the north. Late afternoon shifting, gusty winds drove the flank of the fire to the northeast. Note the "browned", but intact tree crowns in the fuel treatment area (enclosed in yellow lines) and the completely consumed crowns in the untreated area to the south and west (top of photo). The fire entered the treatment area as a crown fire and immediately transitioned to a surface fire due to the thinning of small trees that would otherwise carry fire into the crowns. When the fire moved into the residential area as a surface fire it increased in intensity due to the increased flammability of the structures and combustible items adjacent to the houses.

Differences in fire behavior within treatment areas were dependent on steepness of slope, position on slope, and the intensity of the main fire when it entered the particular treatment area. A fundamental wildland fire behavior principle states that fire intensity and rate-of-spread increase as slope increases. Fuel treatments need to be more intensive (more surface fuels removed and wider crown spacing) on slopes to achieve the same effect as on flat ground. Unit 16 (see Figure 12) depicts a treatment unit which burned mostly by crown fire due to slope effect and its location in the path of the head fire. All of the fuel treatment units that burned with high intensity (crown fire) were situated directly in the path of the head of the fire on steep slopes. "Crown fire momentum" will carry high fire intensity into treated areas to a distance associated with the level of radiant and convective heat produced by the crown fire. Eventually, the fire will drop to the surface and burn with lower intensity.

Image showing crown fire momentum

Figure 12: Fuel Treatment Unit 16 which burned at high intensity due to steep slope and the effect of "crown fire momentum".

Image showing fuel treatment unit 17

Figure 13: . Fuel Treatment Unit 17 (outlined in yellow) is located below and to the east of Angora Highlands. The full force of the crowning headfire ran into this treatment unit, torching trees along the southern edge of the unit. After penetrating the treated area, the crown fire lost momentum and transitioned to a lower intensity surface fire due to greatly reduced amount of surface fuel, limited ladder fuels, and wider crown spacing.

Figure 13 depicts Unit 17 where crown fire momentum carried high intensity fire into a treated unit, but fell to a surface fire partway through the unit. Approximately thirty percent of Unit 8 (steep slope portion) burned as a crown fire. Smaller percentages of Units 21 and 22 burned as crown fire because of steep slope portions and the momentum of crown fire from upwind untreated areas.

The eleven treatment units that burned entirely as surface fire resulted in the desired fire behavior change (from crown fire to surface fire). Units 21 and 29 were located between the main head fire (in the south part of Angora Creek and on the west slope above Angora Creek) and the residential area defined by Cayuga on the south and the Mt Rainier Drive/Lake Tahoe Boulevard intersection on the north. Fuel treatments in these units slowed fire spread and reduced fire intensity. In addition, spot fire production decreased and spotting distances were reduced to less than 50 feet (as reported by eyewitness accounts and videos).

The overall effect was a significant reduction in radiant and convective heat as the fire entered the residential area and a reduction in the number of houses impacted by firebrands.

In general, the effects exhibited by treatments in Units 21 and 29 were evident throughout the fire area. The ultimate fire perimeter, especially on the east flank, was much smaller than it would have been without treatments. Units 18, 19, 21, and 29 reduced the fire to a surface fire, and reduced the number of embers impacting houses. Without the fuel treatment, the ember impact zone would have been several hundred yards further into the subdivisions to the east and northeast. Spotting estimates were determined from firefighter interviews comparing spotting from crown fire runs with spotting from surface fire runs.

Unit 20 (near the Angora Stream Environment Zone as it enters the residential area at Lake Tahoe Boulevard) was unique in that scheduled treatments were incomplete when the Angora Fire burned. This treatment unit had been thinned and the resultant slash was piled between the fall of 2004 and July, 2005. The "green" tree boles, branchwood and needles in the piles were left to dry out prior to burning. Green boles take a minimum of one year, sometimes two years to dry out enough to enable timely and complete consumption of the material during prescribed burning of piles.

Figure 14: Fuel Treatment Unit 29. Note intact tree crowns indicating a low intensity surface fire.

Unit 20 received the full force of crown fire momentum when the fire entered it from the upwind untreated stands. Most of this unit burned with crown fire intensity, however, a few trees within the unit did not crown (see Figure 15). Total heat output from the fire in this unit was similar to that of adjacent untreated stands. All of the fuel in the handpiles would have been located at a greater height (in upper branches and crown foliage) and would have burned just as this fuel burned in adjacent untreated stands. The greater height of the fuel would have allowed embers to be lofted higher and transported downwind to a greater distance. Very little difference in convective or radiant heat output would have occurred because the same amount of fuel would have burned.

Image showing crown spacing and reduced crown loads

Figure 15: Unit 20 west of Angora Creek Road. This unit contained unburned handpiles which had been generated by a thinning project. While most of this area burned with crown fire intensity it is apparent that some crowns did not burn, likely due to the wider crown spacing created by the thinning project. The wider crown spacing and reduced crown fuel loads (crown residue was lowered to the surface) reduced spotting distances. Spotting distance is related to height of ember loft and transport windspeed. Cutting trees and piling the "crown" residue at the surface had an effect of reducing ember lofting height.

The Tahoe Paradise Residential Area (see Residential Area Street Map in Appendix A) was a high priority for LTBMU hazard reduction activities. Hand piles in many fuel treatment units within and adjacent to the Angora Fire (units 7, 8, 11-14, 16-19, and 28) were burned beginning in the fall of 2006 and continuing through April 2007 (see Fuel Treatment Map in Appendix A). More than 850 acres of hand piles were burned within LTBMU last fall, winter, and spring. However, there were not enough available burn opportunities due to a short dry winter and air quality constraints to burn all "cured" handpiles within the Basin. The hand piles in Unit 20 were among those which did not get burned.

Suppression effectiveness, structure ignition, and public safety/egress

Many firefighters reported increased ability to take "close-in" suppression actions because of the adjacent treatments which provided "black" safety zones after the fire had passed. Many houses were protected from fire or had spot fires extinguished in areas that would have been unsafe. Units 12, 13, 17, 18, 19, and 29 (see Fuel Treatment Map in Appendix A) all burned as surface fire with less duration than had they been untreated, and therefore provided these important safety zones.

Units 6, 29 and 30 burned with reduced fire intensity that enabled firefighters to build and hold direct fireline. Unit 7 burned mostly with crown fire behavior; however, firefighters reported that they were able to use treated areas within this unit to construct and hold direct fireline.

Eyewitness and video evidence indicates that most houses ignited from ember fall or direct flame impingement by low intensity surface fire. On-site inspections revealed that many trees adjacent to houses had severe scarring on the side facing the house and much less severe scarring on the side facing away from the house. This indicates that the house fire caused the tree to burn and not the reverse.

image comparing scarred and unscarred section on the same tree

Figure 16: Note the severe scarring on the side of the tree facing the burned structure (enclosed by yellow circle) and intact bark on the side of the tree facing away from the structure.

Public safety, especially during evacuation, was enhanced by several treatment units. Units 8, 12, 13, 14, and 19, adjacent to Lake Tahoe Boulevard and Tahoe Mountain Road all burned as a surface fire, reducing the intensity and amount of smoke than would have occurred had these units been left untreated and burned as a crown fire. Firefighters reported that this provided greater visibility and enhanced an orderly evacuation.