Fire Hazard Severity Zone Development

Fire Hazard is a way to measure the physical fire behavior so that people can predict the damage a fire is likely to cause. Fire hazard measurement includes the speed at which a wildfire moves, the amount of heat the fire produces, and most importantly, the burning fire brands that the fire sends ahead of the flaming front.

The fire hazard model considers the wildland fuels. Fuel is that part of the natural vegetation that burns during the wildfire. The model also considers topography, especially the steepness of the slopes. Fires burn faster as they burn up-slope. Weather (temperature, humidity, and wind) has a significant influence on fire behavior. The model recognizes that some areas of California have more frequent and severe wildfires than other areas. Finally, the model considers the production of burning fire brands (embers) how far they move, and how receptive the landing site is to new fires.

Fire Hazard - A measure of the likelihood of an area burning and how it burns (example: intensity, speed, embers produced).
Fire Risk - A measure of the potential for damage. Risk considers the susceptibility of what is being protected. Factors like defensible space, non-flammable roofs, and ignition resistant construction reduce fire risk.
Fire Hazard Zoning - A map of the fire hazard without considering the value at risk.
Hazard Scores - All State Responsibility Areas are rated moderate, high or very high fire hazard.

Fire Hazard Elements

Vegetation - Vegetation is "fuel" to a wildfire and it changes over time. Fire hazard considers the potential vegetation over a 50 year time horizon.
Topography - Fire burns faster on steep slopes.
Weather - Fire burns faster and with more intensity when air temperature is high, relative humidity is low, and winds are strong.
Crown fire potential - Under extreme conditions, fires burn the up into trees and tall brush.
Ember production and movement - Fire brands are blown ahead of the main fire spreading the fire and getting into buildings and igniting.
Likelihood of an area burning over a 30 - 50 year time period.

Hazard map uses

  • Building construction standards on building permit
  • Natural hazard disclosure at time of sale
  • Defensible space clearance around buildings
  • Property development standards such as road widths, water supply, address signs
  • Considered in City and County general plans

Hazard Severity Zones are not intended for

  • Tactical fire fighting
  • Seasonal fire severity
  • Insurance
  • Setting project priorities



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