RWF-002 2002
January 3 - 10 2002
Nordegg, Alberta
A Case study of an excursion from top-piles
Background
RWF-002 initially escaped containment on Wednesday, January 3 2002 due to an increase in windspeeds caused by a chinook event. At least two piles escaped containment and joined together as RWF-002. The first fire remained within the cutblock in 2-3 year old cured fuels and was fought until Sunday, January 6th when another, stronger 'unpredictable event' (chinook winds) occurred near Nordegg,. At this time another pile flared and raced off into a stand of trees. The fire grew from 10 hectares in size to a final size of 20 hectares before it was contained in an area of blowdown along a ridge top. The top-piles in this block were ignited three weeks earlier, during a stretch of seasonal weather and when the ground had 100% snow cover of up to 25 cm in some places. Many piles were ignited at this time to take advantage of the good burning conditions. Just north of this area, between 6000 and 8000 piles were burned during the same time period (with only a few very minor excursions). A warm period followed New Years, and this combined with strong winds evaporated the surface snow and exposed the fuels which were capable of carrying fire. The cutblock containing the piles was directly to the east of a ridge that acts to deflect or bounce the chinook wind and is situated in a location that frequently catches the descending 'wave' at full force. It is in these areas that extreme care must be taken when pile burning at any time during the winter.
Fire Environment
1. Topography
The Nordegg area is rugged country containing steep slopes that are primarily aligned with east-west exposures. This area is influenced by the mountain pass through to the Saskatchewan River Crossing and a mountain ridge directly to the west. The fire location has a western exposure and is protected from the pass from the Rocky Mountains by the ridge and therefore is not influenced directly by the chinook wind coming from SW, but is susceptible to catching the bounced air forced up from the ridge. The fire initially started in two or three piles within the cutblock situated roughly 1/3rd the way up the slope, just out of the lee of the standing trees. The cutblock site had a relatively gentle slope (<20%) facing west.
2. Fuels
The fire began in up to three top-piles consisting primarily of spruce/pine debris and travelled through cutblock fuels that had been harvested 1 to 2 years prior to the fire. The moss layer was also very important in the spread of this fire. The fire progressed into a stand of timber that had experienced some blowdown as a result of the harvesting and the fact that it was located along a ridge top where winds tend to be the strongest. The fires quickly moved through the cutblock (containing 2-3 year old cured fuels) into this area of blowdown. This was a surface fire, but did burn with moderate intensities and was very difficult to control due to the strong winds after it entered the blowdown area. Blowndown trees still retained some 'red needles' and therefore had extensive cured fine fuels in which to carry the fire. Within the stand, moss was the primary fire carrier as it was almost completely exposed and was very dry. Fuel consumption of the larger fuels was estimated to be only ~ 30%, while moderate sized fuels and the fine fuels, including grass, was higher, but did not approach 100% consumption (see following photos).
3. Weather
A 'rare' stretch of seasonal weather in December was followed by mild, windy weather in early January. The near-normal snow pack that had covered 100% of the ground surface quickly opened up and exposed the fine fuels. The December weather was ideal for pile burning and because the eastern slope region of the province was more than two-years behind on its burning program due to the two previous warm, dry winters, many top-piles were ignited during this window of opportunity. The cutblock where this fire occurred is prone to strong chinook winds along its ridgetop due to winds skipping over a more western ridge and then touching down again along this ridge. The area of blowdown at the top of this ridge provides testament to this.
Fire History
Standard burn practices. These piles were one of many hundreds ignited during ideal pile burning conditions over the Christmas season in 2001. The weather was cool and stable and the ground was completely covered with snow that was approximately 10 - 25 cm in depth. The piles were ignited and allowed to burn, hoping to be extinguished by snowfall or to burn out completely before weather becomes a problem. The specific piles from which the fire escaped from were ignited and left to burn out. Industry became concerned with the piles only after the first pile made a small excursion.
On Thursday January 3 2002, the fire moved from one of the burned out piles into the cutblock. It then spread through the slash within the cutblock and threatened an oil camp located within the block, as well as causing smoke problems. At this time, the company attempted to control the fire on their own. Finally, on Sunday, January 6, the winds increased to approximately 80 - 100 kmh (on site estimate) causing the original fire to escape containment and another pile closeby to blowup and make a run. The first fire raced through the remainder of the cutblock, crossed a road and progressed quickly uphill into an area of blowdown. The second fire moved quickly upslope (25m) into a stand of timber. It was at this time (Sunday 5 am January 6th) that the company requested assistance from the province to help deal with the fire. About 3 hectares of standing timber was consumed in the blowdown area. The combined area, was now 20 hectares.
Observed Fire Behaviour
Scorch heights from a number of trees were estimated (using photographs) and these were used to calculate an estimated fire intensity. Scorch heights ranged from 3- 7 m in height in two different areas of standing forest. Moderate intensities were calculated based on scorch height and flamelength observations. Little or no snow was on the ground surface within the stands above the block. Spot fires were observed within the stand roughly five metres from the fire perimeter. Moss was the major fire carrying fuel within the standing forest. The fire moved approximately 1.5 tree lengths into the stand.
Comments from personnel on site mention that in the early morning flamelengths were in the order of 0.75 m high in the main piles. At this time the wind was roughly 25-30 kmh and the temperature was +6C. In the afternoon the wind increased to 90-100 kmh and flamelengths increased to 1.2 m and the fire was spotting 25-30 m ahead of the headfire into moss fuels. The temperature climbed to +12C (estimated on site). These flamelengths equate to fire intensities ranging from 138 to 386 kW/m and compute to a scorch height of 7.5 m. These are not extreme intensities, but under these conditions produced fire behaviour that was very difficult to control.
Resources used
At the peak of the action, 4 bulldozers, 4 water tankers, 2 vehicles from Rapid Fire and a number of industry staff and LFS staff were on hand in an attempt to control DWF-002. Initial attack efforts on January 3rd, were limited to the use of two water tankers and some local forestry equipment. This proved not to be enough resources given the prevailing conditions. The total cost of the fire to industry could be as high as $100,000. This value does not include the loss of resources. Sprinkler systems were utilised effectively along the top edge of the fire to prevent the fire from moving towards an active drilling rig located only 200 m from the fire boundary. The water source for the sprinklers was supplied by portable water bladders and the pumps were kept running to prevent freeze-up of the equipment.
When the fire escaped control on the Sunday, the resources on site consisted of 800 ft of fire hose, one water truck, one bombardier c/w 600-gallon tank and Honda fire pump. There were 4 firefighters, one water truck driver and one fire boss.
Predicted Weather
There was at least four-days notice that warmer weather was approaching from BC, and in this region that almost always means the development of chinook conditions (rapid increase in temperatures accompanied by strong and gusty dry winds). The actual strength and timing of the surface wind is difficult to predict in advance, but the likelihood is that at least moderate strength winds (40-60 kmh) should be expected. Four days provides a company plenty of time to check the conditions of their burn piles and to take any precautions deem necessary to prevent an excursion.
Lessons Learned
The main concern with pile burning along the eastern slope region of the province is and always has been the unpredictability and strength of the chinook wind. It is capable of removing the protective snow cover and exposing freeze-dried fuels to a fire. It is also able to 'bring back to life" a fire that is believed to be out, or blow in suddenly and catch contractors who are burning off guard. Until these winds can be predicted accurately in terms of their timing and force, erring on the side of caution should be the preferred management option. Fire DWF-002 is an example (although a rare one; less than 8% probability of a fire this size) of the potential losses a company may suffer resulting from a pile fire escaping as a result of these winds. The fact that more than one pile escaped from the same block should set off some alarms regarding current pile management practices.
The following is a small list of some of the lessons that may be gathered from this event.
a) Increase supervision of piles or extinguish them when the probability of a chinook occurring in your area becomes greater than 50%.
b) Only ignite as many piles as you can physically handle given the present environmental conditions and the long range forecast.
c) In areas prone to strong winds (ie. known ridgetops), minimum supervision should be required. Companies can assist in identifying these locations.
d) Up to three piles escaped from the same block identifies the location of this block as high risk. Layout design and aggressive action taken on piles in such a sight is required.