S290 Unit 12 part 1

12-1-S290-EPGauging Fire Behavior and Guiding Fireline DecisionsUnit 12 Part 1

Unit 12 – Part 1

Gauging Fire Behavior & Guiding Fireline Decisions


Unit 12 Objectives

1. Describe how to apply fire behavior information to safety and suppression decisions.

2. Demonstrate how to calculate the size of safety zones.

3. Identify the importance of changes in fire behavior to firefighter safety.


4. Discuss what drives large changes and identify the “next big change.”

5. Demonstrate a simple but systematic method for gauging change and estimating fire spread time.

6. Identify other fire behavior prediction tools.

Unit 12 Objectives


Fire Behavior Affects Safety and Suppression

Decisions


Basic Fire Behavior Measures

• Flame Length (FL)—a measure of how fast energy is being released at the flaming front.

• Rate-of-Spread (ROS)—a measure of how fast the fire front is moving.


The distance from the middle of the flaming zone to the average flame tip.

Flame Length

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Flame Length

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Applying Flame Length

• Flame length affects choice of fire suppression tactics and methods.

• Dictates the dimensions of safe zones.


Fire Suppression Limitations

Fires can generally be attacked at the head or flanks by persons using hand tools. Handline should hold the fire.

Fires are too intense for direct attack on the head by persons using hand tools. Handline cannot be relied on to hold the fire.

Fires may present serious control problems; torching out, crowning and spotting. Control efforts at the head will probably be ineffective.

Crowning, spotting and major fire runs are probable. Control efforts at the head of the fire are ineffective.

Flame Length

0 - 4 ft.

4 - 8 ft.

8 - 11 ft.

>11ft.


Fire Characteristics

Chart

(light fuels)

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Fire Characteristics Chart(heavy fuels)

FIRE BEHAVIORFire Characteristics Chart

(scale for heavy fuels)

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Safety Zone Calculations


Definition of a Safety Zone

“An area cleared of flammable materials used for escape in the event the line is outflanked or in case a spot fire causes fuels outside the control line to render the line unsafe. They are greatly enlarged areas which can be used with relative safety by firefighters and their equipment in the event of blowup in the vicinity.”


“…used when fire conditions are such that escape routes and safety zones have been compromised. Deployment zones are the last ditch areas where fire shelters must be deployed to ensure firefighter survival due to the available space and/or fire behavior conditions at the deployment zone location.”

Definition of a Deployment Zone


Safety Zone Guidelines• Avoid locations that

are: • downwind from fire• in chimneys, saddles,

narrow canyons

• Take advantage of heat barriers

• Burn out safety zones prior to flame front approach


Assumptions

• Safety Zone size calculations are made on the “worst case scenario.”

• All sides of the safety zone will receive the same heat flux.

• Based on a crew of 3.


Fire surrounding a circular shaped safety zone gives the worst case (most radiant heat transfer).

Fire and safety zone edge more closely resemble a straight line. Radiant heat transfer from the “sides” is low.

x feet

x feet


Assumptions

Flame Length = Flame HeightSafe separation is a straight line from

the heat source to the firefighter


USDA Forest Service, Fire Behavior Research

Firefighter Safety Zones

DistanceSeparation

Firefighter

Flaming FrontFlame Height


Safety Zone Equations

Flame Height Input: 4 x flame height

Consider increasing the distance separation when more than 3 people and 1 engine will occupy the safety zone.


Circle Safety Zone-Example Distance Separation = the radius

of the circle.

Flame Height = 20 feet

20’ x 4 = 80’ radius

Distance separation = 80 feet

Diameter of circle = 80 X 2 = 160’


Safety Zone Size Assuming no Slope and no Wind

Flame Distance Area in

Height Separation Acres

10 ft 40 ft 1/10 acre

20 ft 80 ft 1/2 acre

50 ft 200 ft 3 acres





• More than 3 people and 1 engine will increase the distance requirement.

• Convective heat from wind or terrain will increase the distance separations requirement.


Rate of Spread (ROS)Rate of advance of the flame front

• Can be given in absolute terms, such as feet-per-minute (ft/min)

• Can be given in relative terms, such as twice as fast as before (2X)

Spread Time:The time it will take the fire to move a given distance; it can be determined from ROS.


ROS is a key safety factor…if a fire can catch you

it can hurt you!

Rate of Spread


Changes in ROS are very important to safety and suppression.

A universal factor in fireline accidents is a rapid change in ROS.

Rate of Spread


ROS can also be used to express how fire spread rate will change

Compare the ROS before with the ROS after a change…how much faster or slower will the fire move?

ROS-ratio=(bigger ROS)/(smaller ROS)

Example: ROS changes from 10 to 60 (or 60 to 10), ROS-ratio = 60/10 = 6X


ROS Ratio Example

= 6XDIVIDED BY


ROS-ratio can indicate the degree of coming danger, and can be used to

predict future fire spread time.

Rate of Spread Ratio


The idea of predicting fire spread in the future, using the “complete application” of FLAME

The “next big change”

30 minutes 30 more min.

If fire moves 3X faster onthe slope, it will take 1/3 of 30, or about 10 min. to spread up the slope

observeobserve

predict

projectThe s

lope is about a

s

long as the 3

0-min.

spread on th

e flats.

Note: these are Note: these are ‘spread times’, ‘spread times’, rather than rather than ‘spread rates’, and ‘spread rates’, and are very practical for use onare very practical for use onthe fireline…how long will it takethe fireline…how long will it takethe fire to go from here to there?the fire to go from here to there?

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What a Change in ROS Can Mean

• ROS changes ranging from 60x to 500x have accompanied fireline fatalities.

• ROS-ratio of 60x means that fire spread that has taken place over hours can suddenly take place in minutes.

• If walking-pace represents “slow” ROS, an increase of 500x would be the equivalent of twice the speed of sound…a huge relative change.

• Changes are not instantaneous, and can involve a transition over time (often minutes to 10s of minutes)…

But always be aware, change is coming!


First Consider “Current” Fire Behavior

• Current fire behavior demonstrates the effects of current fuels, terrain and weather, and provides a baseline.

• Factors such as live fuel moisture or 10-hour FM do not change rapidly—they are important overall, but vary over longer time scales.


Unforeseen Changes Kill Firefighters

Rapid, large increase in fire ROS is a common denominator in fatality fires.

– ROS increases of 60x and more have been associated with fatality incidents.

To be safe requires firefighters:– foresee changes well ahead

– have a sense of the size of the change

– understand both “current” and “expected” fire behavior


Quotes from Fatality-Fire Reports“…benign appearance of fire…no appreciable wind during decision making…”

“Look what it’s doing now.; …the fire didn’t look that bad…” “…assuming they would see the fire coming at the same rate they had all day.”

“…right flank was backing into a light up-canyon wind. …numerous observers reported that the wind had remained light and steady…”

“…intensity and rate-of-spread were much greater than had been anticipated…”

“…burnout was conditioned upon light upslope winds holding…”

Revealing a reliance on impressions of current behavior, a tendency to not foresee the coming dangerous fire behavior.


Foreseeing Changes in Fire Behavior


It is critical for a firefighter to anticipate the “next big change” in plenty of time to confront it.

Look ahead at the factors that cause big changes.


Anticipating the ChangeSupports LCES

Lookouts- What to be looking for, and using the most appropriate lookout locations

Communications- What key things to communicate and how often

Escape routes- Where and how long

Safety zones- Close enough and big enough


Dominant Change-Makers

Litter

Crowns (timber & brush)

Grass

FUEL TYPE is a significant contributor to big changes, changes of 15X or more.

WIND (effective wind, which includes slope) is the biggest change-maker, ROS changes of 200X or more.

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Another Important Factor 1-hour FM can change fairly rapidly and can cause changes up to 1.6x or so (usually less), but it is a minor change-maker compared to fuel-type or wind.

Very important:

1-hour FM is a key factor in the potential for crown fire.

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Typical “Big Change” Events

By drawing in the simple pictures both current and expected fire behavior must be consider.

If either picture is blank, your evaluation of the fire behavior is incomplete.


Sudden Wind Change

• Current conditions

- Light winds

• Expected conditions - Strong winds, direction change

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Change in Fuel Type

• Current conditions - Litter fire

• Expected conditions - Crown fire

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Slope Reversal a major change, often in both fuel and wind

• Current conditions - Upslope crown fire

- Wind-exposed

• Expected conditions - Downslope litter fire - Wind-sheltered

Favorable change

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• Current conditions

- Downslope litter fire

- Wind-sheltered

• Expected conditions - Upslope crown fire - Wind-exposed

Dangerous change

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Slope Reversal


Gauging the Changes in Fire Behavior


How much does ROS change as fuel factors, wind, and slope

change?

We’ll look at each factor, to develop our sense of change, and eventually learn to apply guidelines to assess

changes in fire behavior.


A scaled diagram can help develop a sense of change to expect as conditions change.

Specific guidelines will allow us to consider any change in:

- fuel type - wind - slope


Consider first the effects of changing fine dead fuel moisture (FDFM), a fairly large change, equivalent to a

substantial change in relative humidity.

Later we’ll introduce a guideline for gauging the effect of FDFM on ROS.


FDFM: Timber Litter

1-hour fine dead fuel moisture = 2%

ROS = 6 ch/hFL = 3 ft.



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FDFM: Logging Slash





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FDFM: Short Grass





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FDFM: Tall Grass





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FDFM: Shrub





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Changes in Fuel Type Produce Large Changes in

Fire Behavior


Fuels are grouped into fuel-types based on their similarities in

ROS.Litter is a dead, compact,horizontal fuel

Crown foliage is a live,vertically arranged, open fuel

Grass is a dead, verticallyarranged, open fuel

slowest

fastest

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Effect of Fuel-type on Relative Rate-of-Spread

Litter1x

ROS in crown fuels about4x faster than in litter

ROS in grass fuels about3x or 4x faster than in crowns

Other factors being equalThis is only about fuel-types

Crown4x

Grass15x

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Fuel-Types often Change with Aspect

SN

SN

SN


Transition Fire Behavior:an Important Change in Fuel-Type

Can become crown fire

Such changes between fuel beds that are stacked vertically can be very rapid and may be difficult to foresee.

Surface fire (litter or grass)


Indicators of Crown-Fire Potential

• Seasonal drought period prevails• Overall drought makes matters worse• Recent crown fire • Relative Humidity 35%-20%, or less• Backing fire produces torching• Fire moving up ladder fuels• Torching and short crown runs


Effective Wind Speed (EWS)

Combines the effects of wind and slope.

Think of EWS as the midflame

wind speed (MFWS).

It is a huge driver of change!


Key Questions

1. How does wind vary from place to place?

2. How does wind at flame level vary with fuel type?

3. How do we account for slope?


Wind speed is the component of the wind that is pushing the fire ahead into

new fuels.

Across the heel, wind has least effect on fire spread

Across the flank, wind parallels the flame front; little net wind influence

Across the head, full wind effect

Midflame wind speed

Most of the time we’ll consider either head fire or backing/flanking fire.

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How We Express Changes in Effective Wind Speed

As the ratio of the bigger to the smaller wind speed(very much like the ROS-ratio)

For example: 6 mi/hr

The bigger wind speed

ex: 2 mi/hr

The smaller wind speed= 3x in our example; (i.e. the wind speed will triple)

The ratio of the wind speeds,

the EWS-ratio.=Divided by

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Calculating EWS-RatioSmaller EWS

Lar

ger

EW

S

In this example, the bigger wind is 6 and the smaller

wind is 2… a tripling of EWS.

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Effects of Doubling the Wind Speed on ROS and FL


Midflame Wind: Timber litter

Midflame Wind = 4 m.p.h.




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Midflame Wind: Logging Slash





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Midflame Wind: Short Grass





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Midflame Wind: Tall Grass





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Midflame Wind: Shrub





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Effects of Slope Alone(from Gentle to Steep)


Slope: Timber Litter

Slope = 8%


Slope = 24%


Slope = 72%


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Slope: Logging Slash

Slope = 8%


Slope = 24%


Slope = 72%


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Slope: Short Grass

Slope = 8%


Slope = 24%


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Slope: Tall Grass

Slope = 8%


Slope = 24%


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Slope: Shrub

Slope = 8%


Slope = 24%


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Add the Influence of Slope to the Wind to Get the Effective Wind Speed

Slope acts like a little more upslope wind, increasing the efficiency of heat transfer to new fuel.

As a guideline, add a little bit to the upslope wind speed to account for slope (mostly on upslope fires).


Effective Wind, the Slope Contribution

20%

40%

80%

60%

Less than 20%, no adjustment

1 mph

20% to 40%, add 1 mph

2 mph

40% to 60%, add 2 mph

3 mph

60% to 80%, add 3 mph

Add the contribution for slopeto the upslope wind component

5 mph

Over 80%, add 5 mph

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Combine Slope Contribution and Mid-Flame Wind in EWS

Wind Blows UPSLOPE: Add6

MFWS

2+

slope

=8

EWS

Wind Blows DOWNSLOPE: Subtract

6

MFWS-

slope

2=

EWS

4

NEVER USE EWS < ½ For fires backing upslope into the wind set EWS = ½

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Potential Magnitude of Fuel and Wind Effects on Short-Term Changes in ROS

• FM in large and live fuels ~ 1.2x

• FM in fine fuels ~ 1.6x

• Fuel type (litter, crown, grass) ~ 15x

• Effective Wind Speed ~ 200x

It is clear that fuel type and effective wind speed dominate changes in ROS. The FireLine Assessment MEthod,

FLAME concentrates on those factors.


End of Unit 12 - Part 1

Education

S290 Unit 12 part 1