During recent debris burning Trials in the NWT, the WFORG took the opportunity to collect heat flux data as well as make visual observations of the burning debris piles. Our objective was to collect physical data of the heat flux created by debris piles and to document the change in heat flux with distance from the debris piles. The information is intended to help develop guidelines for pile spacing in cutblocks to reduce wildfire spread potential for both piles left permanently in the cutblocks and those piles waiting for winter burning.

Six piles were burned over a four-day period. The piles were composed of one or two year old pine slash and ranged in size from 3.0 x 2.7 x 1.3 m high to 8.0 x 4.5 x 1.8 m high. Sensors in radiant cubes were used to collect heat flux data and these were placed two, four and six metres from the piles (Figure 1). Observations of short-range spotting and flame lengths were also made for each pile burned.

Figure 1. Radiant cubes placed two, four and six metres from Pile 6.

When the largest pile was burned (8.0 x 4.5 x 1.8 m high), the fire moved into an adjacent pile 4.5 m away via short-range spotting and direct flame contact. Unfortunately, technical difficulties did not allow us to collect heat flux data. This fire scorched and killed a number of trees within 8 metres of the pile (Figure 2).

Figure 2. Pile-to-pile spread of fire. Fire moved from Pile 2 into adjacent pile 4.5 m away via spotting and direct flame contact. Note: scorched trees.

The following applies to this data:

• The data is from one pile only - the influence of multiple piles burning at the same time has not taken place.
  
• Ignition threshold values of 10 - 20 kW/m² are assumed for determining pile-to-pile distance recommendations.
  
• The influence of wind or short range spotting is not calculated when determining the pile-to-pile recommendations developed in this report; only physical heat flux data are used. Wind speed and spotting will be included in the final recommended distances to provide margins of safety.
  

Outcomes:

The data and visual observations show several outcomes. One, it appears the age of the debris has an influence on the heat flux generated by the piles. The one-year-old slash produced higher heat fluxes than the two year old slash even though some of the latter piles were larger. The one-year old slash retained most of the fine material with its volatile components while the two-year old slash had fewer needles and twigs as some had dried and had fallen or been broken off.

Second, at least three metres distance from a pile is required to lower the heat flux below ignition thresholds, based on the collected heat flux data for debris piles of this size.

Third, flamelengths ranged up to 10 m. These are sufficient to ignite piles within those distances, if the flames are bent by the wind. This did occur at Pile Two where an adjacent pile 4.5 m away was ignited by direct flame contact and short range spotting.

A general description of Heat Flux values and what they 'mean' is presented from Alexander 1999¹ (taken from Drysdale 1985²).

• 1 kW/m²: the level where firefighters can withstand indefinite skin exposure;
  
• 6.4 kW/m²: the level where pain will be experienced after 8 seconds on exposed skin;
  
• 10.4 kW/m²: the level where pain will be experienced if the skin is exposed for 3 seconds
  
• 16 kW/m²: the level where exposed skin will blister after 5 seconds
  
• 52 kW/m²: the level at which fibreboard will spontaneously ignite.
  
• 10 - 18 kW/m²: the range at which plywood and hardboard will ignite. (Principles of Fire Behavior; J.G. Quintiere 1997³). (Ignition times range from 60 - 300 seconds exposure).
  

Slash is assumed to have lower ignition thresholds than processed wood due to the fines associated with the debris piles (needles/bark), which ignite more easily.

This research still requires the collection of data from debris piles consisting of aspen and from piles that are much larger than the ones burned. This work provides some valuable information that can be applied immediately. For example, using measured heat flux data and observed flame lengths (which includes pile-to-pile spread), piles should be built, at minimum, the distance of observed flame lengths (up to10 m) apart to minimize the probability of fire spread due to direct contact (Figure 3).

Figure 3. Flamelengths of approximately 4 metres. If these flames are bent over by the wind they can ignite an adjacent pile.

Figure 4. The energy flux measured at set distances from each pile based on size and age.

² Drysdale, D. 1985. An introduction to fire dynamics. John Wiley & Sons, Chichester, England. 424 p.

³ Quintiere, J.G. 1997. Principles of Fire Behavior. Delmar Publishers, New York. 258 p.