Wildland Urban Interface Risk Class Maps
The Abbott-Chapman report on the 2017 wildfire and freshet seasons (Addressing the New Normal: 21st Century Disaster Management in British Columbia) recommended that the provincial government identify risk management strategies to guide and prioritize funding for wildfire mitigation activities, based on risks to communities (Recommendation #81).
In 2018, the BC Wildfire Service developed the Wildland Urban Interface (WUI) Risk Class Framework to support initiatives related to wildfire risk reduction. This process was updated in 2021.
Wildfire Risk Framework
A risk-based framework considers the likelihood of an unwanted wildfire event and the consequences for communities and high-value resources and assets, as a measure of risk, as follows:
- Likelihood is the probability of the unwanted wildfire event occurring.
- Consequence is the amount of damage occurring as a result.
- Risk is measured as the product of likelihood and consequence. Multiple inputs are required to effectively quantify risk (including severity, type of value, and vulnerability to wildfire).
By identifying risk levels, wildfire threat mitigation priorities and opportunities to increase community resiliency are both enhanced.
Differing risk levels require tailored risk management to minimize the negative impacts of wildfires on communities and high-value resources and assets (HVRAs). The intent is to enable the development of cost-effective wildfire risk reduction strategies for communities and HVRAs at two different scales – local and provincial.
WUI Risk Class Assessment and Maps
In British Columbia, structure density (i.e. the number of buildings located within a given area) is used to define the boundaries of the wildland urban interface (WUI) for the purposes of wildfire and risk management planning purposes. It also identifies transition zones between unoccupied land and human development. A two-kilometre-wide buffer zone was applied to the edge of structures located in the WUI to indicate the distance that embers from a wildfire could reasonably expected to be carried by the wind and possibly ignite a structure.
The data that is currently available in B.C. only supports a fire threat analysis for provincial Crown land, so there are large tracts of private land within the WUI for which limited data is available. Since it’s important to consider how much private land is present in the WUI when completing a risk analysis, the buffer zone was expanded to 2.75 kilometres for structure classes with a density of more than 25 structures per hectare.
The spatial WUI attributes were combined with the PSTA wildfire threat layer (for Crown land) to identify at-risk areas at a strategic scale. The level of risk (“risk class”) reflects the analysis of weighted PSTA threat components within the individual WUI Risk Class polygons. Five risk class ratings were applied to the WUI polygons, with “1” being a higher relative risk and “5” being the lowest relative risk. The application of relative risk does not imply “no risk” since the goal is to identify areas where there is higher risk.
The WUI Risk Class Assessment was revised in 2021 to reflect updates to the 2021 Provincial Strategic Threat Analysis and the 2020 wildland urban interface structure density data and mapping. A number of WUI risk class ratings were modified based on changes to underlying fuel types related to land-based activities (e.g. wildfires, harvesting, fuel treatments, development), updated vegetation resources inventory or changes to fire weather inputs (e.g. increases in threat levels due to shifts in the weather data).
The resulting WUI Risk Class Map highlights patterns and trends in the WUI in a format that is easy to understand. WUI risk class polygon ratings represent structures on the landbase, not administrative boundaries. This means that a WUI area may include multiple jurisdictions (e.g. regional district, municipality, Treaty Settlement lands and/or First Nations reserves). This high-level analysis supports the initial identification of at-risk areas (around communities, for example).
In addition, to support planning fuel management treatment data polygons (from 2012 to current) are added to the WUI Risk class maps. This information will help determine the most effective risk control options, including the development or updating of a Community Wildfire Resiliency Plans or Crown Land Wildfire Risk Reduction (CL WRR) Fuel Management Tactical Plans that includes an assessment of local threat conditions and wildfire risk reduction priorities.
In cases where local assessments provide evidence of a higher wildfire risk than is indicated by the WUI Risk Class, that information should be used to guide risk reduction activities.