Ember ignitions from burning structures

The vinyl siding on the upper portion of the surviving house was thermally damaged from burning structures directly across Fraser St (Fig. 20). As discussed above, the thermal exposure required to damage and even liquefy vinyl siding is significantly less than the thermal requirements for wood ignition. The wood fence at the sidewalk and closer to the burning structures shows no evidence of thermal decomposition (scorch/char).

Figure 20a: Thermally softened vinyl siding on the upper third of the front facing burning structures across Fraser St. Note the wood fencing closer to the thermal source without char. Figure 20b: Burned structures across Fraser St. produced the thermal radiation that damaged the vinyl. These structures are identified in the coarse dashed yellow rectangle of Figure 17.

a b

Ember ignitions from burning structures

Radiant heating across streets (10 m or more) from burning structures was insufficient to, and did not, initiate ignitions in adjacent blocks. If not flames, then burning embers principally from burning structures must have produced the ignitions from block to block resulting in the Village of Lytton fire spreading across streets. Key Findings (5) and (6) are consistent with observations from other WU fire studies (Cohen and Stratton 2008; Cohen 2010; Calkin et al. 2014) and with the science that developed the HIZ (Cohen 2004). Figure 21 provides an example of this typical mechanism of fire

Figure 21: Burning embers ignited the ignition vulnerable wood steps and decking at the front entry way (a) of the house (a, yellow oval). Another ember ignited structure (b) is in the background to the left of the house in front. This house is also shown to the right of the Post Office in Figure 14.

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spread, wherein burning embers were likely generated by burning structures in the adjacent upwind block, and windblown to the house in the photo. The paired images of before (left, October 2018) and during (right, 1830 hrs) the Lytton WU fire illustrates the principal vulnerability that likely contributed to the ignition. The highly weathered wood with openings beneath and between members of the wooden stairs and landing would have accumulated leaves and other combustible debris in addition to the inside corners of the stair/landing construction where burning embers commonly accumulate during extreme wildfire conditions.