Evaluating Wildfire Risk in Cull Canyon: The Mosaic Project

The Mosaic Project is a San Francisco Bay Area nonprofit that has spent more than 25 years bringing students together for outdoor learning experiences that help young people build the skills to create more peaceful communities. After nearly a decade of planning, Mosaic identified a permanent home in Alameda County on a site in Cull Canyon above Castro Valley.

The location sits in a fire-prone area. Diablo winds, steep slopes, and a community with vivid memories of the 1991 Oakland firestorm made wildfire risk a central issue in the approval process. When the Alameda County Board of Supervisors took up the project's conditional use permit in early 2026, the question before the board was direct: would the project make wildfire conditions better, worse, or unchanged for the surrounding community?

Mosaic had already done the work to design a responsible site. The project incorporated WUI code construction, irrigated defensible space, vegetation management, and Zone Zero principles. To support the approval process, Mosaic needed data demonstrating how those measures would perform under real wildfire conditions.

Bringing Science to the Approval Process

Mosaic engaged XyloPlan to conduct a wildfire risk assessment of the proposed site. The goal was to evaluate how wildfire is likely to behave at the site and whether Mosaic's mitigation measures would meaningfully reduce risk for occupants and the surrounding community.

XyloPlan used wildfire spread modeling and fire behavior analysis informed by more than 80 years of weather data specific to Cull Canyon. That allowed the modeling to reflect the frequency, duration, and severity of Diablo wind events at the project site.

What the Data Showed

The assessment confirmed that Mosaic had incorporated the right measures to reduce wildfire risk at the site.

The project included:

• WUI-compliant construction
• Irrigated defensible space
• Vegetation management
• Zone Zero principles around structures
• Operational protocols suspending programming during National Weather Service Red Flag and Particularly Dangerous Situation warnings

When those measures were incorporated into the modeling, ignition probabilities for the proposed structures were reduced to near zero under the modeled scenarios.

The assessment also evaluated evacuation concerns. Because consequential fires in the region are typically driven by Diablo winds from the north and northeast, modeled fire movement was unlikely to interfere with the site's primary evacuation route to the south.

The overall finding was clear: the proposed project, with its planned mitigation measures in place, would be more wildfire resilient than the existing unmanaged conditions on the site and would not increase wildfire risk to the surrounding community.

How Data Supported a Good Cause

XyloPlan Chief Wildfire Officer Dave Winnacker presented the findings directly to the Alameda County Board of Supervisors. He walked the Board through the fire behavior analysis, the evacuation assessment, and how Mosaic's mitigation measures influenced outcomes under the modeling scenarios.

Following the presentation and public review process, the Board approved the project 3-1.

The technical assessment concluded that the project does not expose occupants or nearby structures to significant wildfire risk, does not compromise emergency access, and provides a measurable wildfire resilience benefit for the surrounding community compared to existing conditions.

Mosaic had spent years designing a permanent home where students could learn, grow, and build community. XyloPlan's role was simple: provide the science and analysis needed to help decision-makers understand how the project's wildfire mitigation measures would perform in the real world.

Mosaic had already designed a safe, responsible site. XyloPlan gave them the data to prove it.

When wildfire risk becomes part of a permitting, planning, or development decision, data can help communities move beyond assumptions and evaluate what will actually change outcomes. To learn how site-specific wildfire analysis can support your agency, community, or project review process, connect with the XyloPlan team.