Fire Behavior and Prescribed Fire

Heat Fuel Oxygen Fire Triangle

Three Principal Environmental Elements Affecting Wildland Fire Behavior

Fuels Fuel Type Fuel Moisture Size and Shape Fuel Loading Horizontal Continuity Vertical Arrangement

Fuel Moisture The amount of water in a fuel expressed as a percentage of the oven-dry weight of that fuel

Fuel Types Nonburnable Fuel Type Models (NB) Grass Fuel Type Models (GR) Grass-Shrub Fuel Type Models (GS) Shrub Fuel Type Models (SH) Timber-Understory Fuel Type Models (TU) Timber Litter Fuel Type Models (TL) Slash-Blowdown Fuel Type Models (SB)

Fuel moisture time lags: (Time it takes for a fuel to lose 63% of its moisture) Time lagFuel diameter 1-hour (fine fuels) <¼ inch (twigs, dead grass, leaves, needles) 10-hour ¼-1 inch (twigs, small branches, cones) 100-hour 1-3 inch (branches, tops) 1000-hour >3 inch (large branches, tops, logs) Fuel Size

Fuel Loading The quantity of fuels in an area Generally expressed in tons per acre

Horizontal Continuity: Uniform vs Patchy Vertical Arrangement Ground Surface Aerial

All combustible materials lying beneath the surface including deep duff, roots, rotten buried logs, and other organic material Usually called a “PEAT FIRE” Ground Fuels

All materials lying on or immediately above the ground including needles or leaves, grass, downed logs, stumps, large limbs and low shrubs. Surface Fuels

All green and dead materials located in the upper forest canopy including tree branches and crowns, snags, moss, and high shrubs. Aerial Fuels

Weather Temperature Wind Speed and Direction – Increases supply of oxygen – Drives convective heat into adjacent fuels – Influences spread direction and spotting – Carries moist air away replacing it with drier air – Dries fuels – Raises fuel moisture if the air contains moisture

Weather Relative Humidity (RH) – As RH increases, fuel moisture increases Precipitation – Increase fuel moisture

Topography Aspect Slope Position of Fire Shape of Country Elevation – Relates to curing of fuels, precipitation, length of fire season, etc. Aspect Slope Position of Fire Shape of Country Elevation – Relates to curing of fuels, precipitation, length of fire season, etc.

Aspect

Steep Slopes Cause Rapid Fire Spread

Slope Affects Fire Behavior Preheating Draft Faster Ignition and Spread Burning Material Rolling Downslope

Position of Fire on Slope

Box Canyon & Chimney Effect

Radiant Heat Across Narrow Canyon

Spotting Across Narrow Canyon

Mountains Cause Channeling of Wind

Elevation

Characteristics of Fire Behavior Fire Intensity: Heat release per unit time (BTUs) Fire intensity affected by – Fuel loading – Fuel moisture content – Compactness or arrangement of fuels

Flame Length: The distance measured from the average flame tip to the middle of the flaming zone at the base of the flame

Rate of Spread (ROS): The distance a fire travels during a given period of time

Rate of spread = distance/time Burned Area Increased fire intensity Windspeed Steepness of slope Primary factors affecting rate of spread?

Slope Reversal

Running Types of Fire Behavior – spreading quickly Creeping – spreading slowly with low flames Smoldering– burns without flames; barely spreading Spotting– sparks/embers carried by wind or combustion column or moved by gravity Spot fires– new ignition points Fire brand– a piece of burning material

Torching Types of Extreme Fire Behavior – surface fire moves into crowns of individual trees Crowning – spreads from tree crown to tree crown (dependent, active, or independent) Flareup – sudden acceleration of fire spread or intensity (short duration, for portion of fire) Blowup– dramatic change in the behavior of the whole fire (rapid transition to a severe fire) Fire Whirls– vortex (gas mass with rotational motion)

Fire Effects

Prescribed Fire

Prescribe Fire Plan Burn prescription written Pre-burn site treatments completed Equipment designated & ready Personnel identified & trained Approvals & permits in hand Authorities & interested parties identified & notified* Latest forecasts checked* * Day before & day of burn

Ignition Devices Hand-held/ground-based – Drip torch – Propane torch – Fire fuse (flare) – Flame thrower

Drip torch fuel: 3:1 or 3:2 Diesel/gasoline

Ignition Devices Aerial – Helitorch – Delayed aerial ignition device (“ping-pong balls”)

Helitorch

Delayed aerial ignition device

Ignition Devices Lightning

Ignition Techniques Backfire (backing fire) – Least intense & most predictable – Short flame lengths – Narrow burning zone – Slow rate of spread – Low smoke output

Backing Fire

Headfire (heading fire) – Most intense and unpredictable – Long flame lengths – Wide burning zone – Fast rate of spread – High smoke output

Strip-heading Fire Distance between strips controls intensity

Spotfire – Intensity intermediate between back & head fires – Useful in shifting winds – Spots merge to form a strip head fire – What you get using a DAID

Spotfire Distance between spots controls intensity

Flankfire (flanking fire) – Intensity intermediate between back & head fires – Fire spreads at right angle to wind – Limited to steady wind conditions – Requires careful crew coordination – Used primarily for securing flanks of back or head fires

Flanking Fire

Headfire Flankfire Backfire Wind

Smoke Management

Principles of smoke management Have clear, defensible objectives Comply with local pollution regulations Notify local fire & law enforcement officials, nearby residents, & adjacent landowners Obtain the best available weather forecasts Don’t burn under highly stable conditions Burn during midday; avoid night burns Use caution near, upwind, or up-drainage of smoke sensitive areas

Principles of smoke management (continued) Use test fire to estimate smoke output & behavior Use backing fires if feasible Burn in small blocks if dispersion marginal Do not burn when fuel moisture high Don’t burn organic soils Mop-up along roads first Have an emergency plan!