City of Benicia Vegetative Fuels Management Plan
Planning for community wildfire resilience
Plan Context
The City of Benicia Vegetative Fuel Management Plan (VFMP) was created on behalf of the fire department's Vegetation Management program to better understand and reduce wildfire risk within city limits. This plan demonstrates the City of Benicia's commitment to increasing resilience to natural hazards through mitigation, as outlined in the Solano County Hazard Mitigation Plan ; and will serve as a tool to more effectively direct financial, technical and administrative resources.
Beginning in the 1960's, expanding suburban development into the Southampton Hills has created large areas of wildland-urban interface (WUI).
Historic patterns of wildfire occurrence throughout California demonstrate that where there is dry vegetation and people, there will be fires- a trend which is only exacerbated by prolonged drought and climate change.
Benicia has firsthand experience: according to the city fire department, from 1984 - 2004 the number of vegetation fire responses in WUI areas tripled. Steep topography and abundance of dry vegetation pose additional challenges for fire suppression.
Lithograph of Benicia, 1885. Source: Library of Congress
The City currently manages over 1,700 acres of open space parcels and lots - the majority of which are partially or completely encircled by residential neighborhoods.
For the last several decades, the city's fire department has operated a vegetation management program to reduce the overall risk of wildfire ignition and spread from open space areas to adjacent properties. The primary mitigation strategy is the creation and maintenance of defensible space by way of reducing vegetative fuels (dry grasses, shrubs and trees) within a 30-50 foot "buffer zone" of the open space perimeter.
Each of Benicia's 50+ open space areas and lots vary in size, vegetative fuel load, and topography. That is to say: all open space areas are different, and some may present a greater wildfire risk than others. With these differences in mind, this Plan will help the fire department move away from a "one size fits all" strategy and towards a site-specific approach to wildfire mitigation.
The goals of this Plan are to:
- Provide long-range planning and guidance to evaluate costs & benefits of vegetation management into the future;
- Develop a robust site-specific approach to determine wildfire risk and best management practices (BMP's)
- Leverage public engagement to set management goals which reflect the values and concerns of Benicia residents
History of Vegetation Management in Benicia
The City of Benicia Fire Department has employed disking,grazing, and (herbicide) spraying as strategies to manage fuels in open space areas over the last two decades. The department has focused on creating 30 to 50 foot-wide "buffer zones" around the perimeter of open space areas bordering residential neighborhoods.
Image Credit: Winnemucca District BLM
Disking usually entails operating heavy equipment to perform shallow tillage, creating a uniform strip of reduced biomass or bare ground.
While disking is incredibly effective at reducing fuels and slowing fire spread, its challenges are many: few operators are qualified and willing to work in Benicia's steep topography, contributing to rising contract costs. Property owners also absorb negative impacts including air and noise pollution, dust, and erosion ( City of Benicia, 2021 )
Goats grazing a hillside in Benicia in early summer 2021. Image Credit: DP+S
Grazing has a long history in California grasslands dating back to pre-colonial America, when Tule elk and pronghorn roamed the Central Valley and played a role in determining overall community composition and fire occurrence. ( Edwards 1992 ) Over centuries of land use changes, hunting, and species introductions (such as European annual grass species, cattle, and horses), California's remaining grasslands look vastly different - but grazing still plays an active role in shaping the landscape, from commercial dairy operations to vegetation and wildfire management in WUI areas.
Since 2004, the city has stewarded an informal agreement with Goats R Us to keep goats in City-owned open space areas during the offseason (winter) from paid grazing work around the Bay Area. This relationship has benefited both parties, and in addition to reducing fuel load the goats have become loved members of Benicia's community. In 2020 the City ended its disking contract and officially hired Goats R Us to provide grazing for vegetation management and fuel reduction in open space areas during the spring and early summer.
Herbicides have been used to a more limited extent across the plan area, especially for invasive and/or noxious weeds with a high wildfire risk potential that respond poorly to manual control.
Vegetation characteristic of Benicia's open space areas.
Benicia's Natural Landscape
Benicia's rolling hills can mostly be characterized as Inner Coast Range - grasslands which today are mostly dominated by a suite of nonnative European grasses. Previous survey work from Benicia's General Plan Update and the Lake Herman Solar Project Proposal EIR indicate the following main players: primarily annual nonnative grasses and "weedy" herbs such as ripgut brome (Bromus diandrus), Italian rye (Festuca perrenis), black mustard (Brassica nigra) and star-thistle (Centaurea calcitra); as well as emphemeral creek corridors and shrub and tree canopy including Eucalyptus.
Measuring Open Space Risk
Fire Risk: More than just fuels
Dry vegetation provides fuel for fire to spread from open space areas to nearby homes and neighborhoods; but sites differ in other important ways that create different risks and warrant different mitigation strategies.
To create a framework for the fire department to prioritize and customize site-specific wildfire mitigation strategy across the 51 open space areas in its jurisdiction, parcel-level risk scores were assigned to neighboring residential parcels, then aggregated across each open space area. The goal of parcel scoring is to capture the severity and extent of vegetative and topographic hazards present in the neighboring open space area that influence probability of wildfire ignition and spread to any given parcel.
Conducting manual (transect) surveys across the entire Plan Area would be a logistical and financial challenge and provide inconsistent spatial coverage. Instead, high-resolution remote sensing imagery layers made available through programs such as USDA NAIP and USGS 3DEP were used as inputs for risk scoring.
1) Parcels encircling open space areas (green rectangles) were assigned a 200-ft "buffer" sampling area (blue circles) extending into the OSA, from which topographic and vegetation data was sampled to estimate parcel-level wildfire risk.
2) Raster data layers were converted to point values and appropriate summary statistics (mean, mode, sum, etc.) were calculated within each parcel's buffer zone.
Explore the Data
Plan Area Overview
Use the map navigation tools to the right to explore the location and acreage of open space areas around Benicia, along with input data layers used to determine wildfire risk.
Heavy Vegetation Cover
Areas of heavy vegetation cover were by reclassifying land area on the basis of the Normalized Difference Vegetation Index ( NDVI ), using LiDAR imagery from the USDA NAIP program.
NDVI is an indicator of “greenness” commonly used in agricultural and landscape sciences. NDVI ranges from -1 to 1, and is calculated using portions of the near-infrared and visible light spectrum reflected by vegetation. ( NASA Earth Observatory 2000 ). In addition to measuring vegetation health, NDVI has also been applied broadly in the context of urban resource management to detect changes in land cover using remote sensing data. ( Hashimet al. 2019 )
Overall, NDVI proved an effective means of differentiating across four land cover classes: water, roads and buildings, low vegetation (grasses) and high vegetation (trees and shrubs). Grasses constituted most of the land cover across the planning area. In the context of understanding parcel-level risk, we are especially interested in identifying areas with fuel loads that warrant management prioritization or the addition of mitigation strategy beyond the existing prescribed grazing regime. For this reason, we focused on just the highest vegetation cover class (NDVI ≥ -0.12) to classify parcels into binary categories of fuel risk based on vegetative cover.
Steep Terrain
Fire spread rate increases as it travels up steep slopes due to the “preheating” effect of ascending flames on upslope vegetation. Depending on fuel conditions, spread rates increase by two times on 10˚ slopes and four times on 20˚ slopes, compared to spread rates over flat terrain. ( Butler etal. 2007 ) Steep slopes present additional access issues for deploying firefighting resources and may preclude certain mitigation strategies such as disking with heavy machinery.
Elevation data from the USGS 3DEP elevation model was used to determine three classes of terrain steepness (low, moderate, and high steepness).
Aspect
A landscape’s aspect describes the direction (azimuth) a slope is facing. South and southwest-exposed slopes receive more sunlight year-round are of special concern for wildland fire risk. In the City of Benicia, the prevailing wind direction is from the southwest, with the strongest winds occurring in the late afternoon during summer and fall. Therefore, homes which are situated uphill of SW-exposed OSA parcels are particularly vulnerable to fire spread from an open space area, especially where vegetative fuels are a concern.
Elevation data from the USGS 3DEP elevation model was used to identify portions of the Plan Area with SW-facing slopes.
