**NATIONAL ADVANCED FIRE AND RESOURCE INSTITUTE**

**INSTRUCTOR:** Noel Livingston and Dennis Winkler

**LESSON:** 24 -- Fire Behavior Guidance for Firing Operations

**UNIT:** IV -- Application

**COURSE:** S-590 -- Advanced Fire Behavior Interpretation

**OBJECTIVES:**

Upon successful completion of this lesson, the participant will be able
to:

1\. Identify the fire behavior information that is relevant to the
success and safety of firing operations.

2\. Provide specific fire behavior information to support the choice of
the time of firing and the needed width of the burned zone.

3\. Provide specific information on the fire's behavior that can be
expected from various ignition patterns.

4\. Provide specific information on spot fire potential.

5\. Identify hazards and fire safety information to fireline personnel.

6\. Evaluate the results of the firing operation to improve fire
behavior information.

**NARRATIVE:**

> **I. Introduction**

A.  Fire behavior sets the time window when firing operations can be
    conducted based on:

-   The time the main fire arrives.

-   The period of appropriate flammability of the fuels.

B.  Behavior of the set fire determines:

-   How rapidly and effectively the fuels will be consumed.

-   The potential for collateral damage to the overstory vegetation and
    for firebrand production.

C.  Fire behavior information is a critical input on which to base the
    plan for a safe and successful firing operation, and should be
    provided by the FBAN to those planning and firing.

> **II. Developing burned zones**

A.  What is sufficient width to stop the encroaching fire?

-   Radiant ignitions and direct flame contact usually occur within one
    flame length (FL), or the downward projection of the outer tip of
    the flames. Less wind means less ignition distance.

-   Guideline: develop a burned zone (plus control line) that is at
    least 2X maximum FL, or about 4X the average FL.

-   Spot fires can extend much further, many FLs, so \"wider is
    better\".

B.  Time required to develop the burned zone depends on:

-   Rate-of-spread (ROS) of the set fire.

-   Ignition pattern employed.

-   Rate-of-advance of the firing operation along the line.

C.  Intensity of the set fire must be moderated to be:

-   Intense enough to remove the fuel.

-   Low enough to avoid torching/scorching.

-   Low enough to maintain control.

D.  Backfire vs. Burnout: Definitions & Characteristics

Backfire: To set fire to widen the effective control zone ahead of the
main fire. Backfiring is difficult because you are:

-   Working against the wind and slope.

-   Often in the spotting zone.

-   Can be pressed for time by the oncoming main fire.

Burnout: To set fire to clean out fuel between the fire edge and the
control line, when the main fire is not an imminent threat to the line.
It is usually done with ample time and under conditions that allow
effective control of the set fire. Should be done concurrently with line
construction.

> **III. Ignition patterns use the natural sections of a free-spreading
> fire**

A.  Head fire.

-   Most rapid ROS; burns area quickly.

-   Highest intensity and FL; can easily lose control.

B.  Backing fire.

-   Slowest ROS (often a few ft/min), long burn times.

-   Lowest intensity and FL, easy to control.

C.  Flank fire.

-   Intermediate ROS; modest burn times.

-   Intermediate intensity & FL; modest control difficulty.

D.  Contrasts between head, backing, and flanking fire.

-   Degree of contrast depends upon wind and slope; as wind/slope
    increase, so does the contrast in ROS.

-   An example, with 6-8 mi/hr wind (20 ft) or on a 50% slope:

[Relative Spread]{.underline} [Relative Burnout Times]{.underline}

> Head fire 10 1

Flank fire 2 2 1/2 (2 flanks)

Backing fire 1 10

E.  Interior fires show all spread modes, as spot fires do.

> **IV. Basic ignition patterns**

A.  Type of pattern is described by its orientation relative to the fire
    line.

-   Edge firing - set along the edge of the control line.

-   Strip firing - parallel to, but away from, the control line.

-   Spike firing - driven in from the line, roughly perpendicular to the
    control line and into the wind‑slope influence.

-   Dot firing - discreet ignition points inside the line.

-   Chevron firing -

-   Center - convective firing

B.  Fire behavior depends on orientation of the lit fire relative to the
    wind/slope influence.

-   Across the wind/slope yields backing fire on one side and head fire
    on the other side (one side is usually held by the control line).

-   Along the wind/slope yields flanking fire.

C.  Examples of firing patterns and the resultant fire behavior:

-   In front of the main fire:

> Edge firing yields backing fire.
>
> Strip firing yields head fire toward the line (and backing fire).
>
> Spike firing yields flanking fire.

-   On the flanks of the main fire:

> Edge/Strip firing yields flanking fire.
>
> Spike firing yields head fire and backing fire.

-   Across the heel of the main fire:

> Edge firing yields head fire.
>
> Spike firing yields flanking fire.

Note: When wind/slope drives the set fire away from the control line it
is not necessary to light multiple strips (except when reducing
intensity under the overstory).

D.  Basic ignition patterns can be combined, as in these examples:

-   Edge firing to buffer the control line, then.

-   Spike firing to deepen the burned zone, and finally.

-   Strip firing against the burned zone to widen it.

E.  Some examples of the basic ignition patterns.

F.  Interior fires are essentially spot fires, and spread on all sides.

-   Useful for accelerating the burnout can help when travel in thick
    fuels is difficult.

-   Can be used to help moderate intensity (for example, lighting high
    on a slope to back the fire toward the control line).

G.  Be aware of changing fire behavior.

> **V. Regulating fire intensities**

A.  Reduce fire intensity (to reduce damage or enhance control).

-   Employ backing or flanking fire instead of head fire.

-   When strip firing, light narrower strips so the fire has shorter
    runs and does not gain so much intensity; go slower.

-   When spike firing or spot firing, spread out the ignited lines or
    points to minimize the mutual exchange of radiant heat and to
    consume an area of fuel more slowly.

-   Let fire back under, or outward from, trees.

B.  Increase fire intensity (to more completely consume the fuel).

-   Employ head fire.

-   When strip firing, light strips at least wide enough to allow the
    fire to reach its maximum intensity.

-   With spike firing or spot firing, concentrate the lighted lines or
    points so they radiate into each other, and so that the fuel is
    consumed more rapidly.

-   Light pockets of good fuel (such as wood rat nests, or piled fuels)
    or improve the fuel bed.

-   Use hotter firing devices (knowledge of firing devices).

C.  A note on spacing vs. intensity to allay confusion:

-   When lines/spots of fire are burning simultaneously the mutual
    exchange of radiant heat adds to fire intensity, therefore
    concentrating lines/spots will increase fire intensity.

-   When a line of fire burns out before another is lit, there is no
    exchange of radiant heat. Therefore, as when narrower strips are
    fired, a shorter run of the fire will reduce overall intensity
    because it does not have time to build up to the full potential
    intensity.

D.  Match the firing time to the desired burning conditions.

-   To reduce intensity fire with higher humidity, lower temperature,
    and lighter winds.

-   To increase intensity, fire with less humidity, higher temperatures,
    and stronger winds.

> **VI. Airflow in the fire area** (consider the practical effect of the
> indraft)

A.  Fire-induced indrafts.

-   Are generally weak in low to moderate intensity fire.

-   Can be up to several mi/hr and extend out several hundred feet in
    very intense fires (note calculations & Flambeau data).

B.  Airflow in the fire area has three main contributions:

-   Fire-induced indraft (the whole story in calm conditions).

-   Ambient wind (the overall wind flow, apart from the fire).

-   Interaction of the wind with the convection column.

C.  Wind shifts the indraft balance and creates turbulence.

-   Winds of several mi/hr can override the indraft.

-   Flow toward the fire will be greatest at the back, less on the
    flanks, and weakest or even reversed at the head.

-   Turbulence in the lee of the column often results in winds near the
    fire head that are very localized and changeable.

D.  Operational implications depending on the indraft:

-   Interior firing can create more problems (hot runs and firebrands)
    than helpful indraft.

-   Consider the potential gain (additional advance of your firing by
    the expected indraft) vs. the risk (airborne firebrands or hot runs
    of the main fire or interior fires):

> The gain in litter is typically a few extra feet.
>
> The gain in grass is typically a few extra tens of feet.

Note: The above examples are based on the following assumptions:

-   In litter: main fire ROS = 40 ft/min; 3 mi/hr indraft at the fire
    front extends out 120\', declining uniformly to 0 mi/hr; Fuel Model
    #9.

-   In grass: main fire ROS = 100 ft/min; 4 mi/hr indraft at the fire
    front extends out 200\', declining uniformly to 0 mi/hr; Fuel Model
    #1.

-   Similar advance of the fire can usually be generated by using
    appropriate ignition patterns, instead of depending on indrafts.

-   By the time the indraft is significant, the main fire or your
    interior fire may be too close. You\'ll often be in the spotting
    zone before you are in the indraft zone.

E.  Provide information when appropriate of the possibility that:

-   Ambient wind may greatly diminish any helpful indraft.

-   Spotting distance may exceed the reach of the indraft.

-   Firing patterns might develop as much burned zone as the indraft
    influence could.

> **VII. Predicting the potential for spot fires**

A.  Burning crown foliage is a common source of firebrands.

-   Torching trees/brush produce firebrands (aided by low foliage fuel
    moisture and/or concentrations of dead fuel at their base).

-   Dead, attached foliage (such as on thistle plants) contributes
    firebrands.

-   Try to determine the FL that will cause torching and estimate when
    that will occur. Use SPOT to determine spot distances.

B.  Probability of Ignition (PI)

-   Evaluates the ease of ignition of the fuel bed.

-   PI of roughly 40% separates unlikely ignition from likely ignition.

-   Guideline: When relative humidity is below 20%-30% spot fires in
    grass or litter fuels become likely.

> **VIII. Evaluating the burn and the FBAN predictions**

A.  Very valuable information can be gained by observing weather and
    fire behavior during the firing operation.

B.  Observations:

-   Wind, temperature, and relative humidity.

-   Times and conditions when surface fuels will carry or not carry the
    set fire.

-   Performance of the various ignition patterns; burn-out times.

-   Conditions and FLs that produce torching; spot fire distances.

-   How equipment and personnel functioned.

C.  Evaluate and refine your predictions.

-   Was the predicted weather correct?

-   Were the predictions of FL and ROS correct?

-   What were the threshold fuel moistures for adequate burning
    conditions in the surface fuels?

-   What conditions and FLs were associated with torching?

> **ix. Closing comment**
>
> Provide the specific and relevant fire behavior information, about
> both the main fire and the set fire, which will allow those conducting
> the firing operation to be systematic, effective, and safe.