Vectoring Fire Behavior
Surface Fire Section
Projecting fire spread with cross slope winds utilizes a vectoring process, where the effect of wind and the effect of slope on Rate of Spread (ROS) may be represented by separate vectors that represent both a magnitude and a direction. The resultant vector represents both a direction and magnitude of maximum spread in that direction.
- Slope Vector is drawn directly upslope and estimated by calculating ROS with the estimated slope steepness and Zero (0) windspeed for inputs.
- Wind Vector is drawn in the direction of the wind and estimated by calculating ROS with the estimated windspeed and Zero (0) slope.
- Maximum Spread Vector can be drawn as shown and measured to determine the resultant ROS and spread direction.
In example A here, wind is crossing more upslope, resulting in an enhanced maximum ROS.
In example B, wind is crossing more downslope, resulting in a reduced maximum ROS.
With winds blowing downslope (±30°), the difference between the spread rates is the resulting ROS using the direction from the larger vector.
If the vectoring process is completed manually:
- ROS is determined from the measured maximum spread vector (spread distance) and the time period used to obtain wind and slope vector estimates.
- Heat Per Unit Area (H/A) is the same for all component vectors.
- Fireline intensity (FLI) and flame length (FL) can be calculated from ROS and Heat Per Unit Area (HPA) using these calculations.
FLI = (ROS * HPA)/55
FL = .45 * FLI.46