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

Introduction

The Lake Tahoe Basin encompasses about 500 square miles of which more than a third (122,600 acres) is the lake itself. The basin is located 150 miles east-northeast of San Francisco on the California-Nevada border. The lake, at 1645 feet deep and maximum elevation of 6,229, is most famous for its crystal clear water. Recreation and scenic beauty attract 3-4 million visitors to the National Forests in the Lake Tahoe Basin each year.

Map of Lake Tahoe Area detailing Angora Fire

Figure 1: Vicinity Map of Angora Fire

The U.S. Forest Service administers the Lake Tahoe Basin Management Unit (LTBMU) and is the Tahoe Basin's largest land manager, responsible for 78% of basin lands, approximately 165,000 acres. As such, the Forest Service has the largest single role in ecosystem and watershed management and protection. The LTBMU is a unique inter-mix of forest and urban communities, presenting a variety of challenges and complexities. The Forest Service works in collaboration with other federal, state and local agencies to reduce hazardous fuel in the Lake Tahoe Basin.

National Forest System Urban Intermix Parcels

The Santini-Burton Act of 1980 (Public Law 96-586) authorized the Forest Service to acquire environmentally sensitive lands within the Lake Tahoe Basin to protect these lands

from development. The Act has resulted in federal acquisition of over 3,500 parcels, varying from a fraction of an acre to several hundred acres in size. The primary focus of Forest Service activities on these lots has been fire hazard reduction, hazard tree management, and watershed restoration. Nevada Division of State Lands and the California Tahoe Conservancy manage more than 6,500 additional urban lots within the Lake Tahoe Basin.

Fire History

Tree ring studies in the Lake Tahoe Basin indicate that the natural fire return interval was approximately 5-20 years in the lower elevations. A century of fire exclusion has interrupted the natural fire cycle resulting in dramatic changes in the forest. Tree density has increased, ladder fuel has developed, and surface fuel has accumulated.

In the last twenty years, there has been an annual average of 62 fire starts on National Forest System lands within the Lake Tahoe Basin. Twenty one percent of those have been caused by lightning and 79% by humans.

Prior to the Angora fire, the Gondola fire (673 acres) and the Showers fire (294 acres) were the largest recorded wildland fires in the Lake Tahoe Basin. Both of these fires occurred in 2002. The Angora fire grew to 3,072 acres and most of that growth was during the initial burn period under dry, windy conditions.

Objectives

The objectives for the assessment were to evaluate:
- 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.

Methodology

Facts and circumstances surrounding the Angora fire were determined through on the ground and aerial reconnaissance, interviews with homeowners, firefighters, fire scientists and fire behavior experts. Videos and photos taken prior to, during and after the Angora fire were also reviewed. In addition, data collected by staff of the Lake Tahoe Basin Management Unit and others were used in the review.

Executive Summary

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 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.