Open Space Advisory Board Agenda and Packet 2023 07 12

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Mitigation efforts are unlikely to stop urban conflagrations driven by extreme windspeeds and producing prolific ember cast like the 2021 Marshall Fire. Fuel treatments cannot prevent embers from blowing into treatment areas or onto surrounding structures. Uncontrollable factors will always play a role in home loss during extreme wildfires. Minute -to -minute shifts in wind directions, unexpected wind gusts, and extreme fire behavior and growth that overwhelm suppression efforts can result in home loss not explained by mitigation efforts prior to the fire. How wide should a fuel break be? The general guidance for fuel breaks is that "wider is better and less fuel is better". Guidance on fuel breaks widths include 30 feet around structures (CSFS, 2021), three times the vegetation height (Trauernicht & Kunz, 2019), and widths of 210 feet, 300 feet, 330 feet, or 1,300 feet (Rossi et al., 2019). Unfortunately, there is little scientific evidence behind these specific guidelines and few management tools to determine effective fuel break widths (Rossi et al., 2019). The following research is available on the effectiveness of fire breaks of different widths in grasslands (linear features devoid of vegetation). There was not comparable information available on the effectiveness of different widths of fuel breaks (linear features with some vegetation left intact, such as a mow line): • Fire breaks over 15-feet wide were not breached by experimental grassland fires, but embers could travel across the fire breaks. The probability of fire break success was lower for narrower fire breaks and decreased with the presence of trees within 65 feet of the fire line (Wilson, 1988). • Fire modeling simulations suggest fire breaks must be between 1 to 10 times flame length or 10 to 80 times the fuel height to allow firefighters to suppress the fire safely and effectively, with greater widths required for more extreme fire behavior under hotter, drier, and windier conditions (Frangieh et al., 2021). • Fire modeling simulations suggest fire breaks must be about 2 times the flame length to have at least an 80% chance of preventing fire from igniting fuels across the fire break. Fire breaks >_26 feet were effective for most flame lengths. When designing fuel breaks, fire and fuel managers need to answer the following questions — questions that are as much of a social nature as a scientific one: • What intensity of fire weather conditions are we designing fuel treatments to be effective against? • Are we designing fuel treatments to stop the forward spread of fire or to slow the spread of fire and create tactical opportunities for firefighters to engage? • How much uncertainty are we willing to accept in terms of predicted effectiveness? • What tradeoffs are we willing to accept? For example, wider fuel treatments with less remaining vegetation might be more effective at slowing the spread or reducing the intensity of wildfire, but they might degrade wildlife habitat, reduce privacy for adjacent property owners, and be more costly than narrower, less aggressive treatments. • What marginal returns are acceptable? For example, are we satisfied with a treatment requiring 2x the amount of work to increase potential success by 50%? By 10%?