Review of Fire Behavior during Passage of Sandy Lake Fire 13 (NWT) across a Northwest Territories Power Corporation Transmission Line

Transcription

1 Wildland Fire Operations Research 1176 Switzer Drive Hinton AB T7V 1V3 Review of Fire Behavior during Passage of Sandy Lake Fire 13 (NWT) across a Northwest Territories Power Corporation Transmission Line Fire Report FR Dave Schroeder and Jim Thomasson March 2009 Copyright 2009, FPInnovations, FERIC Division, WIldland Fire Operations Research Group

2 Table of contents Introduction... 1 Objectives... 1 Methods... 1 Site description... 1 Data collection... 1 Results and discussion... 3 Vegetation types adjacent to the right of way... 4 Fire behavior... 5 SM-013 Sandy Lake Fire... 5 Effect of right of way on fire behavior... 8 Infrastructure heat exposure... 9 Use of right of way for fire operations Conclusions Future References Acknowledgements Appendix Page ii FR

3 Introduction Power transmission line vulnerability to wildfire is a concern shared by all of the provincial and territorial agencies, and utility companies. Parks Canada and the Government of the Northwest Territories (NWT) brought this issue to the FPInnovations Wildland Fire Operations group, specifically asking if infrastructure could be damaged directly by wildfire, or through firefighting operations such as back burning. Of particular concern was a Northwest Territories Power Corporation (NTPC) transmission line running between Fort Smith and Pine Point and through Wood Buffalo National Park. The towers on this line were built from an aluminum alloy and may not have the same resistance to heat damage as steel towers, more commonly used on major transmission lines. This report documents post fire conditions at locations where the Sandy Lake Fire 13 crossed the NTPC power line over the period of July 22 to 30, Protection of wooden power line structures are not addressed in this report nor is the effect of the fire on power transmission as the line was not energized during the fire passage. Objectives The objectives of the study were to: Document post fire conditions at locations where Sandy Lake Fire 13 crossed a power line. Estimate fire intensity where it crossed the power line. Determine if the power line infrastructure suffered any damage (visual observation). Methods Site description The fire was located north of Sandy Lake, NWT approximately 160 km northwest of Fort Smith. The terrain in this area is flat and vegetation is characterized by boreal pine and black spruce uplands with black spruce and tamarack in lower areas and adjacent to wetlands. Fuel types, following the Canadian Fire Behavior prediction system (FCFDG 1992), are predominantly immature pine (C4), mature pine (C3), and spruce (C2). Stands of conifer/aspen mixedwoods (M2) also occur. Final fire map and location is shown in Figure 1. Data collection FPInnovations researchers recorded the following information at selected towers: A photo set at each transmission tower including post-fire oblique aerial photos Vegetation type (overstory and understory) surrounding the tower Page 1 FR

4 Fire direction based on information from firefighters and evidence from vegetation. For example, conifer branches bend and freeze in the direction of the fire travel. Fire intensity based on vegetation consumed surrounding the tower and estimates using the Canadian Fire Behaviour Prediction system (FBP) (FCFDG 1992) Figure 1. Map of fire NWT ENR SM showing the progression of the fire and the location of the area surveyed by FPInnovations. Courtesy of Parks Canada. Page 2 FR

5 Results and discussion Although an intense wildfire crossed sections of the power line (Figures 2, 3 and 4), it did not result in observable damage to the towers or lines. All towers north of the highway to the northern fire edge (35 towers) were inspected by Northwest Territories Power Corporation following the fire; crews noted some soot deposits on insulators, but no other damage was apparent during the inspection. We assessed burned vegetation at 26 towers. Of these, 16 experienced a crown fire adjacent to the right of way (with fire direction from East South East). Nine towers experienced surface or intermittent crown fire, and one tower in unburned fuels was included for comparison. Appendix 1 lists each tower with vegetation assessment and photos. Figure 2. Fire burning near the transmission line, 25 July Figure 3. Fire on 25 July Page 3 FR

6 N Fire direction Figure 4. Aerial view looking toward the north end of fire. Power line runs north-south. Fire direction from East South East. Vegetation types adjacent to the right of way The tower line right of way width varied from 20 to 24 m and was surrounded by mature and juvenile conifer (mainly jack pine, Figure 5) as well as conifer/aspen mixedwood. Vegetation within the right of way was dominated by shrubs (willow, bog birch, wild rose and potentilla), cured and green grass, feather moss and herbaceous plans (Figure 6). There was little dead and down woody debris and the right of way had been mowed in Figure 5. Juvenile jack pine adjacent to the right of way. Note abundant Cladonia sp. (reindeer moss). Page 4 FR

7 Fire behavior Figure 6. Unburned right of way fuels. Canadian Forest Fire Weather indices (Lawson and Armitage, 2008) indicated the potential for extreme fire behavior during the period of July 24 31, 2008 in the southern NWT (Table 1). Table 1. Fire Weather Indices for Fire SM :00 Weather Chronology SM-013 Sandy Lake Fire Date Temperature (º C) Relative humidity (%) Wind direction Wind speed (km/h) 24 h rain (cm) FFMC DMC DC ISI BUI FWI Fire size (ha) 24-Jul 28 (F) 20 (F) SW (F) 9, Jul 27(F) 25(F) N(F) 10G15 22, Jul Jul , Jul , Jul Jul , Jul (same) 1-Aug (same) 2-Aug (same) (F) denotes forecast weather (not observed). FFMC Fine Fuel Moisture Code, DMC Duff Moisture Code, DC Drought Code, ISI Initial Spread Index, BUI Buildup Index, FWI Fire Weather Index Page 5 FR

8 With the fuel and moisture conditions present at the time of the fire, potential fire behavior for the C4 and C2 fuel types would result in continuous crown fire with head fire intensity ranging from to kw/m. The C3 fuel type is predicted to produce an intermittent crown fire but with similar fire line intensity to the other fuel types. Mixedwood stands would experience intermittent crown fire (where conifer greater than 50%), and a surface fire where conifer component is less than 50%. Our observations of fire behavior in the conifer types reflect the FBP predictions. The fire intensity was very high and crown consumption complete (Figures 4 and 7). We also observed abundant reindeer lichen in the C4 fuel although it is not a characteristic of this fuel type (FCFDG 1992). Observations at another test burn in the NWT (Schroeder ) indicated that presence of reindeer lichen resulted in higher rates of spread and fire intensity than that predicted by the FBP system. Therefore fire intensity greater than 40,000 kw/m was likely along some sections of the right of way. Figure 7. Post burn conditions in juvenile pine along the right of way. Pine mixed with an aspen component greater 50% had a mitigating effect on fire behavior, although these stands did experience vigorous surface fire (Figures 8 and 9). Scorch heights of 1-2 m were observed on aspen stems. 1 Fire behavior in thinned Jack Pine: Two Case Studies of FireSmart treatments in the Northwest Territories, Canada. FPInnovations, Feric, Advantage Report in preparation. Page 6 FR

9 N Mixedwood Juvenile pine Fire direction Figure 8. The mixedwood pine and aspen is partially burned (top center) and the juvenile pine is burned (either side of mixedwood). Mixedwood Juvenile pine Figure 9. Mixedwood stand (left) adjacent to right of way (foreground). Juvenile pine is completely consumed (right background). Page 7 FR

10 Effect of right of way on fire behavior Vegetation consumption within the right of way seemed to be a function of adjacent forest types/fire behavior. Figures 7 and 8 illustrate sections where crown fire completely consumed the crowns and also completely consumed right of way fuels. Figures 9 and 10 illustrate areas with intermittent crown fire and right of way fuels were not burned or partially burned. Note, however, the fire was not stopped by the right of way (burning was from left to right in Figure 10). Also, prior to the fire, a bulldozer had removed a swath of vegetation on the east side of right of way along this section of the power line. Hwy 5 Fire direction N Intermittent crown fire Figure 10. Intermittent crown fire. Highway 5 to Fort Smith is at top of the picture. We also observed an edge effect along the west side of the right of way. Trees assumed to be the same age were approximately 2 m taller than further back from the right of way, presumably because light exposure was higher. Frequently along the west side, no crown fuels were burned for the first tree length into the stand (Figure 11). The fire will have changed from crowning to a surface fire as it crossed the right of way and then a tree length distance may have been required to generate enough energy to reestablish an active crown fire. Page 8 FR

11 Fire direction N Figure 11. Edge effect on west side of the right of way. Infrastructure heat exposure The primary source of heat exposure was from fuel burning adjacent to the towers and transmission lines. Work done by Butler et al (2004) at the ICFME research site indicated temperatures that reached 1330º C within the burning fuels, but for less than 30 seconds. Data from a 2005 test fire in boreal pine forests (see Schroeder 2006) indicated that at 10m from the fire edge and 1 m above ground 2, temperatures rose quickly, spiked to 700º C, and stayed above 500º C for approximately 2.5 minutes. They then cooled quickly to 100 Cº within 5 minutes (Figure 12). Since the tower was 10 m from the edge of the right of way, we can assume a similar temperature profile on the side with the approaching fire. The tower arms extend closer to the edge of the right of way at 20 m above the ground, and likely would have experienced the higher temperatures during the flame front passage. Flame heights at 2.5 times the canopy height would have reached 25 to 40 m and could have contacted the towers directly because flames do not extend straight up wind, especially gusts, will force the flames up from the canopy at an angle. We could not estimate heat/energy output from the burning right of way fuels. It was assumed that fire residency adjacent to the towers would have been brief (less than one minute) and that flame heights were less than 4 m (vegetation height was a maximum 2 m and 1 m average). Research to determine temperature profiles above burning debris indicates that the transmission line would not have been negatively affected because of its height above the ground (FPInnovations 2007). 2 Recorded at a NWT research burn in 2005 by FPInnovations and Mark Ackerman (University of Alberta) Page 9 FR

12 Temp (C) 10 m from edge of crown fire (= right of way) Time (seconds) Figure 12. Temperature versus time 10 m from edge of a crown fire in boreal fuels. (Sampling height = 1 m) Towers were 20 m tall and the lines came to within 15 m of the ground in some places. Although the towers are made of an unknown aluminum alloy, most alloys have a melting point in the º C range ( referenced August 06, 2008). Certainly temperatures were high enough to cause damage to the towers had they been sustained. The aluminum would have heated and cooled very quickly because the lattice design of the towers does not trap heat. As temperatures reach 150º C, the yield strength for aluminum and aluminum alloys begins to degrade (presentation to Feric Wildland Fire Operations Group advisory in 2005). Although aluminum does not have high heat tolerance, it does have very low emissivity, which means that most thermal radiant energy is reflected. Therefore tower component temperature would not likely have risen to a damaging temperature, without prolonged flame contact or other exposure (e.g., convective heat transfer) 3. The change from crown to surface fire during passage across the right of way would have greatly reduced the heat exposure to the towers, as compared to unmanaged fuels (natural stands). Heat from the wildfire was not sustained long enough to cause immediate damage to the tower structure. All towers north of Highway 5, where the fire crossed the transmission line, were physically inspected by NTPC and no structural damage was reported. Some sooting of the insulators and the tops of the towers was observed. Any reports of tower failure in the future will be compared to fire behavior around the tower in question (Appendix 1). We also observed unburned bird nests in the center of three towers where crown fire had burned to the right of way, further indicating the heat in the towers was not sustained. Use of right of way for fire operations Our observations indicated little to no effect by the right of way on final fire size (Figures 8, and 11) due to ember transport and right of way fuel consumption. Where a surface fire stopped at the right of way, the leeward side still burned due to ember transport across the gap (Figure 10). However, the right of way still had potential to be used as a control line or as an ignition line to burn a fire back to the main fire front. Back burning was done along the highway and, if fire behavior had not become too 3 Mark Ackerman, University of Alberta. Personal conversation, March Page 10 FR

13 extreme, it may have been attempted along the power line as well. Narrow right of ways, such as this one, present a problem for equipment operation as little room is available for equipment to maneuver around towers and guy wires. Conclusions An intense boreal forest crown fire burned up to and across a power line right of way in the Northwest Territories during the end of July, The fire did not result in any immediate damage to the power line infrastructure. Management of right of way fuels may have reduced heat exposure to the transmission towers, but did not stop the fire. The transmission line was de-energized during the fire passage so potential fire effects on power transmission could not be documented. Use of power lines for fire operations should consider right of way width in order to assess equipment maneuverability, especially around tower guy wires. Future FPInnovations will follow up with NTPC to find out if any towers fail. References Butler, B.W.; Cohen, J.; Latham, D.J.; Schuette, R.D.; Sopko, P.; Shannon, K.S.; Jimenez, D.; Bradshaw, L.S Measurements of radiant emissive power and temperatures in crown fires. Can J For Res 34: FCFDG Development and structure of the Canadian Forest Fire Behavior Prediction System. Forestry Canada Fire Danger Group, Forestry Canada. Ottawa, Canada. Inf. Rep ST-X pp. FPInnovations Temperature profiles above burning piles: a report based on data collected in the Northwest Territories in June FPInnovations, Feric Division, Wildland Fire Operations Research Group. Contract Report CR Lawson, B.D.; Armitage, O.B Weather guide for the Canadian Forest Fire Danger Rating System. Natural Resources Canada, Canadian Forest Service, Northern Forestry Centre. Edmonton, AB 87pp. Schroeder, D Effectiveness of forest fuel management: A crown fire case study in the Northwest Territories, Canada. FPInnovations, Wildland Fire Operations Research Group. Web report. Acknowledgements The authors thank Government of the Northwest Territories (Sholto Douglas, Kris Johnson), Northwest Territories Power Corporation (Aaron Martin), Parks Canada (Dan Perrakis) for allowing access to the fire, sharing information on the power line, and providing data on the fire weather and progression. Page 11 FR

14 Appendix The appendix is html formatted and will be posted on along with this report. Page 12 FR