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Unit 11: Fire Behavior Prediction Systems

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Systems Pt. 1  ::   Worksheet & Field Guide   ::   Practice   ::   Systems Pt. 2   ::   Estimating   ::   Exercises

Objectives

Upon completion of this unit you will be expected to:

  1. Describe the differences between point source and line source fire behavior predictions.
  2. Given weather observations, fuel model, dead and live fuel moisture, slope percent, and midflame windspeed calculate rate of spread, fireline intensity, flame length and probability of ignition using fire behavior tables and formulas.
  3. Given rate of spread, projection time, midflame windspeed, slope percent, and fuel model calculate area and perimeter of a fire originating from a point source.
  4. Describe effective windspeed and explain how it is determined for both upslope and downslope fire spread situations.
  5. Given effective windspeed, rate of spread, and prediction time plot the perimeter of a fire originating from a point source.
  6. Describe four ways in which actual rate of spread can be measured on a fire perimeter and two ways of estimating flame length.
  7. Give three ways in which adjustments to predictions of rate of spread and flame length can be made as a result of differences between actual and calculated values.

Introduction

In this final unit, you will acquire the tools necessary to make basic predictions of fire behavior from the environmental assessments and input data described in earlier units. Please note the instructions to students for this unit are somewhat different from previous units. Here you are required to use the Fire Behavior Field Guide with the workbook. A small calculator is desirable, but not necessary, for the calculations.

The materials in this unit may appear to be highly technical and over-whelming upon first inspection. This should not be the case, since the system presented is relatively simple, and we will take you through it slowly, step by step. The job aids in the field guide are designed to make your assignments as easy as possible. You should have little trouble meeting the unit and course objectives.

This unit will be confined primarily to initial attack fire problems. The predictions system can be applied to larger wildfires; however, the greater variety of fire environmental factors and input values on these fires complicates the process. We'll leave those calculations to the Fire Behavior Officer or the Plans Chief, both of whom have had much more intensive training and experience.

Fire Behavior Prediction Systems

Phases to performing a fire behavior analysis:

  1. Gather and assess fire environment data using forecasts, photos, local observations, descriptions, and maps.
  2. Determine principal inputs using tables, formulas, keys, and descriptions.
  3. Calculate principal outputs using a worksheet, tables and other processing aids.
  4. Make fire behavior assessments using input and output data.

 

What is involved in the predictions system? The list above breaks the job of performing a fire behavior analysis into four phases. In the earlier units, you learned to gather and assess fire environmental data using forecasts, photos, local observations, descriptions, and maps. Then you were required to determine principal inputs using tables, formulas, keys, and descriptions. This was phase two.

Unit 10 jumped ahead to phase four, making fire behavior assessments using input and output data. This was done to give you a greater appreciation for making the calculations before actually doing them. You may never be required to do calculations on the job; however, you will be aware of their value and how to use them in fire suppression and other fire management efforts.

Phase three will be covered in this unit. Here you will calculate principal outputs for fire behavior using a worksheet, tables, and other processing aids.

You should note that the field guide contains all of the tables, formulas, keys, procedures, and other aids as introduced in the units for performing phases 2, 3, and 4. It is designed to give you a quick and easy reference for Unit XI, the final exam, and for future field use. Please take a few moments to inspect the field guide. Notice how the field guide is organized into sections to help you in the various phases. When you have finished, return to the text.

Fire behavior prediciton model : A set of mathematical equations that can be used to predict certain aspects of fire behavior when provided with an assessment of fuel and environmental conditions.

Considerations when using the fire behavior prediction model:

  1. It is not always possible to predict exactly what a fire will do, but it is possible and useful to obtain a reasonable estimate.
  2. The accuracy of model predictions is dependent upon and limited by the accuracy of the input values.
  3. Fire behavior models assume that: (1) the fuel strata carrying the fire is continuous and uniform, (2) the fire is spreading in surface fuels, and (3) the windspeed, slope, and fuel moisture values remain reasonably constant.
  4. Limitations of model are that: (1) it does not consider fire spread due to spotting, (2) crown fires are not modeled, but the potential is predicted, and (3) firewhirls and other similar extreme, fire-induced atmospheric disturbances are not modeled.

 

We will be dealing with the fire behavior predictions model developed at the Northern Forest Fire Laboratory at Missoula, Montana. The prediction model can be described as a set of mathematical equations that can be used to predict certain aspects of fire behavior when provided with an assessment of fuel and environmental conditions.

There are several considerations you should be aware of when using the fire behavior prediction model. First, it is not always possible to predict exactly what a fire will do, but it is possible and useful to obtain a reasonable estimate. Second, the accuracy of model predictions is dependent upon, and limited by, the accuracy of the input values. Third, the fire behavior models assume that the fuel strata carrying the fire is continuous and uniform; that the fire is spreading in surface fuel; and that the windspeed, slope, and fuel moisture values remain reasonably constant.

In order for predictions to be successful, the fire model must be used in applications for which it was designed. The model does not apply to smoldering combustion such as occurs in tightly packed litter, duff, or rotten wood. Predictions are for the flaming front of the fire and not for burnout that occurs after the front has passed. Complications of severe fire behavior due to crowning, spotting, and fire whirls are not predicted by the fire model, but the possible onset of severe fire behavior can be predicted.

Fire Behavior Calculation Aids

Listed below we have several fire behavior calculation aids that are available to fire managers. Each of these allows you to process input data using the mathematical equations in the fire behavior prediction model. The program was originally available on computer. Later, a set of fire behavior nomograms were developed as a cheap and convenient field processor for Fire Behavior Officers. A special program chip was also developed for the TI-59 programmable calculator, that gives the same outputs as the nomograms, plus more. The Texas Instrument Model 59 with fire behavior program chip has been popular; however, its availability is limited. Not every student of fire behavior or field technician has access to the TI-59. As a result, a set of fire behavior tables were developed for this course. The tables are easy to use and give you most of the output values available from the TI-59. Output values from the tables will be reasonably close to those from a TI-59.

 

  1. Microcomputer
  2. Fire Behavior Nomograms
  3. TI-59 Calculator
  4. Fire Behavior Tables

 

Stages of Fire Development

Calculation methods vary by the stage of fire development. Generally they are:

  1. Point Source : Predictions apply to an initiating fire burning during a time when conditions have been relatively constant, and where it can be assumed that the fire will maintain a basic elliptical shape.
  2. Line Source : Predictions apply to a fire that has become large, it no longer has the basic elliptical shape, the fire could have significantly different burning conditions along the perimeter, and the growth must be considered from more than one point along the perimeter.

 

Formulas

more

Formulas for calculating adjusted fire behavior values.

Copyright 2008, Michael Jenkins. Cite/attribute Resource . admin. (2005, November 16). Unit 11: Fire Behavior Prediction Systems. 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_11__Fire_Behavior_Prediction_Systems_1.html. This work is licensed under a Creative Commons License Creative Commons License