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Unit 1: The Changing Fire Environment

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Environment   ::   Heat Transfer   ::   Behavior   ::  Predictions  ::   Summary   ::   Exercises

Firefighter's Predictions

There are four primary areas of concern to firefighters in predicting fire behavior: Forward rate of spread of fires, the future perimeter of the fires, the fireline intensities or flame lengths, and any unusual or extreme fire behavior such as crowning and spotting.

Firefighters are primarily concerned about predicting:

  • The forward rate of spread of fires
  • The future perimeter of the fires
  • The fireline intensities or flame lengths
  • Unusual fire behavior such as crowning and spotting

Note that we are now using the term "fireline intensity," rather than fire intensity. It's important that you understand that this is not the same as fire intensity. Fire intensity is a somewhat general term, referring to the entire heat release of a fire at a given time. It is very difficult to measure or to relate overall fire intensity to fire control activities.
In contrast, fireline intensity is a measurable and useful term that is related to flame length. In turn, flame length can be related to fire control jobs. We will explain fireline intensity and its relationship to flame length a little later.
The job of predicting fire behavior is indeed a complex one. How does one even start to analyze the many variables? Well, this course is intended to help you gain the basic knowledge required to assess fire behavior. It will also brief you about systems available to make fire behavior calculations or estimations

Assumptions of Fire Models

Let us take a closer look at the mathematical fire behavior prediction model. The model processes fuel and environmental conditions to give expected fire behavior. Methods of estimating the input values and interpreting the output values will be covered throughout the course.
In the final unit of this course, Unit XI--Predicting Fire Behavior, you will go through the entire process of assigning input values, using the model in the form of tables and graphs to obtain output values, and interpreting these output values.
An entire unit will be devoted to each of the following basic input values: Fuel bed description, fuel moisture, slope, and wind speed. These units will cover both the general information required for a firefighter to assess the fire situation and also the specific information that is required as input to the model.
In order to express the many interactions that occur during a forest fire in mathematical terms, certain simplifying assumptions have been made. Among these are the following: The model describes fire behavior only at the leading edge of a 'free burning fire; the fuels are assumed to be continuous, uniform and in a single layer contiguous to the ground; wind, slope, and fuel moisture content all are constant for the time period of the calculation; the fire is not spreading by spotting or crowning; and firewhirls and other fire-induced atmospheric disturbances are not occurring.

Assumptions made in the mathematical fire behavior prediction models:

  • The leading edge of a free burning fire is being considered
  • The fuels are continuous, uniform and in a single layer contiguous to the ground
  • Wind, slope and fuel moisture contents are constant for the calculation period
  • The fire is not spreading by spotting or crowning
  • Firewhirls and other fire-induced atmospheric disturbances are not occurring

It is important that you understand the implications of these assumptions. You know, of course, that fire does not occur in a continuous, uniform and constant environment. But predictions from the model can be used successfully in many situations. The closer actual conditions are to the model assumptions, the better the predictions will be. This is why human judgment is always used along with the model. Even though the model does not describe extreme fire behavior, you will see in later units that it can predict the potential for spotting and crowning.

Fireline Intensity

Fireline intensity is the heat released by a foot wide slice of the flaming combustion zone in one second.

Fireline Intensity

The figure above presents a diagram of fireline intensity to further clarify this term. Fireline intensity is the heat released in 1 second by a foot-wide slice of the flaming front. This represents the heat that would impact a firefighter just ahead of the front. Since it has a direct relationship to flame length, it can be related to the kind of control actions and size of fireline that must be planned for on the fire.
The relationships between fireline intensity and flame length will be explained in more detail in later units. Flame length is also an output of the mathematical fire model. It should not be confused with flame height. The figure below illustrates how each measurement is taken. Flames usually bend forward at the head of a fire, depending on wind slope factors. Researchers have determined that flame length is a better parameter for describing fire behavior than flame height.
As mentioned earlier, fire behavior predictions are useful for planning fire control actions. Such planning includes the location of firelines, use of direct or indirect attack methods, the type of control forces which will be effective, and the standards for fireline construction. Certainly, good planning in each of these areas will make the suppression effort more safe and effective.

Observing Flame Length

Uses of Fire Behavior Predictions

Fire behavior predictions are useful for planning fire control actions, such as:

  • Location of fireline
  • Direct or indirect attack methods
  • Type of control forces which will be effective
  • Standards for fireline construction

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 20). Unit 1: The Changing Fire Environment. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_1__The_Changing_Fire_Environment_10.html. This work is licensed under a Creative Commons License Creative Commons License