CROWN FIRE EXPERIMENTS

Planning is now under way for the second annual International Crown Fire Modeling Experiments, according to Bret Butler, research engineer with the Fire Behavior Project at the USDA Fire Laboratory in Missoula, Montana. This session follows last summer's crown fire modeling work in Canada's Northwest Territories.

Local and territorial governments in NWT are reportedly eager to see further progress on the experiments, which are scheduled for June 22 through July 13. Tests during the 1997 burns measured weather conditions, spread rates, and general fire behavior. Weather stations posted around the burn plots showed fire guards' effects on wind flow patterns, and fire shelter tests showed that shelters set up inside the burn plots suffered more damage than what generally occurs on fire entrapments; temperature and heat flux data showed that the thermal environment inside the plots is much more severe than originally thought.

"Temperatures and associated radiant heat fluxes were significantly higher than previous measurements," said Butler, "but no other measurements, at least to my knowledge, have been made in full-scale crown fires. Given the relatively low speed winds, marginal relative humidity, and vegetation moisture contents, it is likely that the conditions observed on these fires represent the lower end of the potential fire behavior spectrum."

The in-stand video images recorded during the test burns show promise for use in both training and fire research, and at least some of the footage will be included in the video under development at the Missoula Technology & Development Center (MTDC). They plan further testing of alternate shelter designs this summer, and both MTDC and Storm King Mountain Technologies are working on testing of shelters. Another study proposed for this summer will measure the consumption of live branches, and further work on identifying biomass combustion using remote sensing is planned.

Jack Cohen, research physical scientist at the Fire Lab's fire behavior research unit, last year tested heat flux and temperature data on wall sections set up 10 and 30 yards downwind of the burn plots. Cohen is evaluating the fire threat to structures from high-intensity wildland fire, particularly wildland/urban interface fires. "Understanding how structures ignite during wildland fires is critical for the development of effective, economically efficient, and aesthetically acceptable methods of building homes in or near wildlands," says Cohen. "Towards that end, the International Crown Fire Modeling Experiment offers an opportunity to experimentally examine wood structures exposed to forest crown fires."

Last summer Cohen collected data from two crown fires; he set up wood-paneled wall and roof sections of about 8 by 10 feet downwind of the fire. The experimental crown fires produced 500-foot flame fronts with flame heights of approximately 65 feet. During these fires, Cohen's results ranged from no ignition and no scorch to flaming ignition of the walls. "The wall sections at 98 feet did not ignite and had thermal exposures insufficient for scorch during both crown fires," said Cohen. "At 33 feet, one of the crown fires resulted in moderate scorch; the other produced ignition. The difference between the results was very evident in the data and the observed fire behavior. When the crown fires reached the plot edge, considerable turbulence and firewhorl generation resulted. Due to this turbulence, flames would extend 30 to 35 feet beyond the forest edge in various locations along the flame front. The wall section that ignited was exposed to direct flame contact; the section that did not ignite had no flame contact."

The most interesting result of Cohen's testing, at least from a zoning and prevention perspective, is the possible fire-shielding effect of trees. "The heat flux data also indicated a radiant shielding effect due to the tree crowns," added Cohen. "The wall sections at 98 feet �saw' the fire before the wall sections at 32 feet. This suggests that a thinned tree canopy (one that will not support crown fire spread) can be better than no tree canopy for reducing a structure's thermal exposure." This summer's experiments will include further testing with structures. Two of the burn plots will have 24 x 24-foot complete structures instrumented and sited within a 30-foot clearing centered inside the plot -- and Cohen says the tests will be videotaped. "These buildings will offer the opportunity to observe complete structure exposed to and surrounded by a crown fire."

For more information on this year's burn experiments, contact Brian Stocks at (705)949-9461 or Marty Alexander at (403)435-7346.