2.0 Methodology
2.1 Project Team
A project team including experts in the fields of fire behavior and suppression, geographic information systems (GIS), natural resource ecology, and forest health collaborated to complete a Community Risk/Hazard Assessment for each community. Teams included personnel with extensive working wildland fire experience in Nevada and resource specialists experienced in the natural resource environment of the Great Basin.
The teams used standardized procedures developed from the Draft Community Wildland Fire Assessment For Existing and Planned Wildland Residential Interface Developments in Nevada (Nevada’s Wildland Fire Agencies, Board of Fire Directors, April 2001; revised 2002) during the assessment process. This approach incorporates values for fuel hazards, structural hazards, community preparedness, and fire protection capabilities into an overall community rating.
A glossary of terms is included in Appendix B.
2.2 Base Map Data Collection
The project geographic information specialists compiled and reviewed existing statewide geospatial data to provide the assessment teams with maps for use and verification in the field. Data sources for the maps were the Nevada Fire Safe Council, the Nevada Department of Transportation, the Natural Resource Conservation Service, the US Forest Service, the Bureau of Land Management (BLM) and the Tahoe Regional Planning Agency (TRPA). Data sets and sources utilized include:
Land ownership
Vegetation communities
Topography
Roads
Fire suppression equipment and personnel (hydrants, stations)
Fire history
Fuel types
Wildfire hazard
Current aerial photos and satellite data
1-meter resolution satellite color imagery
Existing data was reviewed and pertinent information compiled on maps in geographic information system (GIS) format and then field verified during the data collection phase of the Community Risk/Hazard assessments. Geographic information system specialists provided data management to assure accuracy and effective analysis of the statewide geospatial data and the production of the maps in this report.
2.2.1 Wildfire History
Wildfire history information was obtained from US Forest Service Lake Tahoe Basin Management Unit GIS databases that locate multiple years of wildfire perimeters and ignition points on USFS and private land. Fires were mapped using GPS and screen digitizing, with the smallest scale being 1:250,000. The data set is intended to be a central source of historical GIS fire data to be used in supporting fire management efforts and land use planning.
This data set forms the base for the wildfire history table presented in the county description and provides the ignition point locations for the maps in this report. In many cases, the ignition point location is only accurate to within the section; in such cases, the point coordinate is located in the section center on maps.
The fire history data and ignition patterns were used to formulate risk ratings and to develop recommendations specific to areas that have been repeatedly impacted by wildland fires. Observations made from the project team members and comments from local fire agencies also allowed for the development of recommendations for areas absent of recent wildfire activity where a significant buildup of fuels or expansion of urban development into the interface area represents a growing risk.
2.3 Community Risk/Hazard Assessment
The wildland/urban interface is the place where homes and wildland meet. This project focuses on identifying hazards and risks in the wildland/urban interface areas District-wide, assessing each community individually. Site-specific information for the North Lake Tahoe Fire Protection District was collected during field visits conducted June 7 through 11, 2004. The predominant conditions recorded during these site visits were used as the basis for the Community Risk and Hazard Assessment ratings.
2.3.1 Hazard Assessment Criteria
The Community Risk/Hazard Assessments were completed using methodology outlined in the Draft Community Wildland Fire Assessment For Existing and Planned Wildland Residential Interface Developments in Nevada. This system assigns community risk values (Low through Extreme) based on the following scoring system:
| Hazard Category | Score |
|---|---|
| Low Hazard | < 41 |
| Moderate Hazard | 41-60 |
| High Hazard | 61-75 |
| Extreme Hazard | 76+ |
To arrive at a score for the community, four primary factors that affect potential fire hazard are assessed: 1) community design, 2) structure survivability, 3) availability of fire suppression resources, and 4) physical conditions such as fuel loading and topography. A description of each of these factors and the importance in developing the overall score for the community is provided below. Copies of the rating sheets used by the project teams and copies of the Community Risk/Hazard Assessment summary sheets are provided at the end of each community section.
Community Design
Community design accounts for 26% of the total score of the risk assessment. Many aspects of community design can be modified to make a community more fire safe. Factors considered include:
Interface Condition. Describes the density and distribution of structures with respect to the surrounding wildland environment. The four Interface Condition Classes are: Classic, Intermix, Occluded, and Rural. Definitions for each Condition Class are defined in the glossary (Appendix B).
Access. Design aspects of roadways influence the hazard rating assigned to a community. A road gradient of greater than five percent can imply increased response times for vehicles carrying water; roads less than 20 feet in width often impede two-way movement of vehicles and fire suppression equipment; and hairpin turns and cul-de-sacs with radii of less than 45 feet can cause problems for equipment mobility. The presence of secondary entrances and exits, and loop roads in a community can lower a hazard rating.
In addition, visible, fire-resistant street and address identification signs and adequate driveway widths are aspects of access infrastructure that also influence the hazard rating of a community.
Utilities. Poorly maintained overhead power lines are a potential ignition source. Fires have been known to start from arcing power lines during windy conditions. In the event of a fire, a burning power pole could contribute to a short, causing power failures downline. A power failure in a community without backup energy generation may leave residents without water for protecting their homes and leave firefighters without pumps for the community’s fire suppression water system. Energized power lines may fall and create an additional hazard for citizens and fire fighters. In some areas these downed power lines could block road access. Properly maintained rights-of-way (ROWs) underneath power lines greatly reduce the risk of fire ignitions along power line corridors during fire events.
Construction Materials
Construction materials account for 31% of the total score of the risk assessment. While it is not feasible to expect all structures in the wildland/urban interface area to be rebuilt with non-combustible materials, there are steps that can be taken to reduce the risks associated with hazardous construction in the interface area. Factors considered in the assessment include:
Structure Building Materials. The composition of building materials determines the length of time a structure could withstand high temperatures before ignition occurs. Houses composed of wood siding and wood shake roofing are usually the most susceptible to ignitions. Houses built with stucco exteriors and tile, metal or composition roofing are able to withstand much higher temperatures and heat durations.
Architectural Features. Unenclosed balconies, decks, porches, or eaves on homes can create drafty areas where sparks and embers can smolder and ignite, rapidly spreading fire to the house. A high number of houses within a wildland/urban interface with these features implies a greater risk to the community.
Defensible Space. Density and type of fuel around a home determine the potential fire exposure levels to the home. A greater mass of trees, shrubs, dry weeds and grass, woodpiles, and other combustible materials near the home will produce more intense heat during a fire, increasing the threat of losing the home. Additionally, fuels close to structures become a source of wind driven, burning embers that can blow into attics or crawl spaces through unscreened vents or accumulate in other unenclosed spaces. These embers can rapidly spread fire to the home.
Suppression Capabilities
Suppression capabilities account for 16% of the total score for the assessment. Knowledge of the capabilities or limitations of the fire suppression resources in a community can help the residents take action to maximize the resources available. Factors considered in the assessment include:
Availability, Quantity, and Training Level of Firefighting Personnel. When a fire begins in or near a community, having the appropriate firefighting personnel to respond quickly is critical to saving structures. Whether there is a local paid fire department, volunteer department, or no local fire department impacts how long it takes for firefighting personnel to respond to a reported wildland fire.
Quantity and Type of Fire Suppression Equipment. The quantity and type of available fire suppression equipment has an important role in minimizing the effect of a wildfire on a community. Wildland firefighting requires specialized equipment.
Water Resources. The availability of water resources is critical to fighting a wildland fire. Whether there is a community water system with adequate fire flow capabilities, or whether firefighters must rely on local ponds or other drafting sites may indicate whether firefighters will be able to adequately protect the community.
Physical Conditions
Physical conditions account for 27% of the assessment. Fire behavior is influenced by numerous physical conditions and is dynamic throughout the life of the fire. With the exception of changes to the fuel types and density, the physical conditions in and around a community cannot be altered to make the community more fire safe. Understanding these physical conditions, fire behavior specialists can predict fire growth patterns and help fire suppression personnel respond appropriately to a fire threatening a community. Physical conditions considered in the assessment include:
Slope, Aspect and Topographical Variations. In addition to local weather conditions, slope, aspect, and topographical variations can be used to predict fire behavior. West and south facing aspects are most prone to severe fire behavior due to preheated vegetation that has lower moisture content from daylong sun exposure. Steep slopes greatly influence fire behavior. Fire usually burns upslope with greater speed and flame lengths than on flat areas. Fire will burn downslope; however it usually burns downhill at a slower rate and with shorter flame lengths than in upslope burns. Canyons, ravines, and saddles are topographical features that are prone to higher wind speeds than adjacent areas. Homes built mid-slope, at the crest of slopes or in saddles are most at risk due to topography in the event of a wildfire.
Fuel Type and Density. Vegetation type and density around a community affect the potential fire behavior. Areas with thick, continuous, vegetative fuels are at a higher risk than communities situated in areas of mosaic or broken fuels. Weather conditions that dry the vegetation in combination with steep slopes or high winds can create situations in which the worst-case fire severity scenario can occur.
2.3.2 Hazard Mapping
The wildfire hazard data compiled for the project originate from a number of past projects. Data from these projects formed the foundation for the hazard mapping effort. The Wildfire Susceptibility Analysis conducted for the Lake Tahoe Basin Watershed Assessment and the Wildfire Hazard Rating dataset crated by Karl Kratuer for the Sierra Front Wildfire Cooperators were reviewed.
Information from each of these previous studies was useful, but had limitations. The Sierra Front Hazard data was a direct analysis, combining fuel model mapping with slope classes to develop a hazard rating. However the vegetation layer used in the fuels mapping was from the late 1980s. The Wildfire Susceptibility Index data used more current vegetation mapping and included more complex analysis; however a key analytical component was the ignition history which was inconsistent throughout the Tahoe Basin. Both datasets were used as a guide to help identify priority areas, but field verification was employed to completely address hazardous conditions.
2.3.3 Fire Behavior Worst-Case Scenario
The worst-case scenarios described in this document are based on the project wildfire specialists’ estimation of severe fire behavior that could occur given a set of weather conditions, observed fuel loading conditions, and minimum fire suppression resources. These scenarios describe a maximum potential for loss of property and in some cases human lives. The worst-case scenario does not describe the most likely outcome of a wildfire event at the interface, but illustrates the potential for damage if a given set of conditions were to occur simultaneously. The worst-case scenarios are described in this document for public education purposes and are part of the basis for the fuel reduction recommendations.
2.4 Interviews With Fire Personnel
The Project Team interviewed local fire department personnel to obtain information on wildfire training, emergency response time, personnel and equipment availability, evacuation plans, pre-attack plans, and estimates of possible worst-case scenarios. Local fire personnel reviewed maps showing the history of wildfires to ensure that local information on wildland fires was included. Refer to Appendix A for a list of persons contacted.
2.5 Recommendation Development
A wide variety of treatments and alternative measures can be used to reduce ignition risks, mitigate fire hazards, and promote fire-safe communities. Proposed recommendations typically include physical removal or reduction of flammable vegetation, increased community awareness of the risk of fires and how to reduce those risks, and coordination among fire suppression agencies to optimize efforts and resources. The project team met repeatedly to analyze community risks, treatment alternatives, and treatment benefits. Treatment recommendations to reduce existing risks were formulated based upon professional experience, quantitative risk assessment, and information developed in conjunction with the National Fire Plan and FIREWISE resources (National Fire Plan website, FIREWISE website and Nevada Cooperative Extension publications).
2.6 Hazard Mitigation Project Development
Modifying the fuel structure around communities is necessary to effectively address wildfire hazards. Based on field review of the existing conditions, the RCI Resource and Fire specialists developed detailed mitigation projects or prescriptions for areas surrounding communities in North Lake Tahoe Fire Protection District. Prescription areas were delineated based on continuity of fuel bed, appropriate treatment alternatives, and topography. Each prescription area was mapped, and detailed project worksheets specifying estimated costs, timelines, and material removed, were developed for each recommended project. The prescription areas are detailed on Figure 3-2. Recommended treatment methods are described in Chapter 7.
Methodology for Biomass Estimates
The ultimate goal of the mitigation projects is to reduce the wildfire hazard by altering the amount and characteristics of fuel in the forest. Fuels can be reduced by either burning or removing biomass. Given the amount of material to be removed and the amount of material that can typically be burned annually (due to air quality restrictions and burning windows), some of the material will have to be removed as biomass.
To estimate the amount of biomass to be removed in the project areas, variable plot sampling was used to count the number of trees, basal area per acre, and amount of material to be removed as biomass. A basal area factor of 20 was used in the sampling process. The basal area, the number of trees, and the cubic foot volume were calculated on a per acre basis for each sample plot. Volume estimates for material to be removed was consistent with a silvicultural prescription to thin from below, or remove smaller diameter, non-merchantable trees. A photograph and UTM coordinate (NAD 83) was recorded at each plot location. Further photo points were located in areas demonstrating important forest characteristics. The results of this sampling effort were used to calibrate the fuel load estimates for each recommended treatment area.
Methodology for Cost Estimates
The actual cost per acre for the recommended thinning treatments was obtained from recent projects completed in the North Lake Tahoe Fire Protection District.