A wildfire expert says a blizzard of blazing embers that were blown by the wind over fireguards and a river sparked the flames that destroyed homes in Fort McMurray last spring and then spread the destruction deeper into neighbourhoods.
Alan Westhaver says the embers ignited combustible material such as dry grass, leaves, pine needles, fences, patio decks, wood piles, evergreens and ornamental shrubs that were too close to homes in the northern Alberta city.
The smaller blazes set homes on fire and flames then crept to nearby houses in the closely built subdivisions.
"Mass ember production and long-distance transport by strong winds subjected neighbourhoods to intense ember showers," Westhaver wrote in a report for the Institute for Catastrophic Loss Reduction.
"Wind-driven embers from the forest fire likely caused the majority of home ignitions near the urban perimeter which, in turn, likely triggered the massive urban conflagration and losses that followed."
The wildfire forced more than 80,000 people from the area and destroyed more than 2,400 buildings. Insured losses are estimated at about $4 billion.
The report says few homes caught fire due to direct contact with the flames or heat from the burning forest.
"No, this fire was not, at least initially, an insurmountable force that rolled into, and over, an entire community like a smashing tidal wave," said Westhaver.
"Primarily, it was millions of raisin-sized firebrands searching for places to carry on with combustion, and succeeding all too often."
Homes on the edges of neighbourhoods with yards that had less combustible material, including landscaping that kept trees and bushes further away, suffered less damage.
The report suggests that plans to protect communities from wildfires should include preventative action by homeowners.
Westhaver, a former Parks Canada wildfire manager, said building fireguards and clearing trees and bushes outside of communities isn't enough.
More attention must be paid to raising awareness among homeowners and governments about landscaping and building practices that will reduce the risk.
"This progression can only be broken, and disaster avoided, by substantially increasing the proportion of homes that are resistant to ignition — especially by embers," Westhaver said.
"The ignitability of homes and properties is really the weak point."
Other researchers are using computer technology to study the disaster to better understand how wildfires — and embers — behave.
Canadian Forest Service scientists are feeding data about the fire into a computer model known as FIRETEC.
The program allows scientists to set virtual wildfires to study how flames grow and move. It factors in the effects of weather, topography and the types of trees and shrubs burned.
Westhaver's report is providing some data and context for the work, said wildfire scientist Dan Thompson.
The results of the computer program are to be analyzed this fall.
FIRETEC was developed at the Los Alamos National Laboratory in New Mexico to model fire behaviour in the dry pine forests of the U.S. southwest.
Researchers began modifying the program for use in Canada's boreal forest following the 2011 wildfire that destroyed parts of Slave Lake, Alta.