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Wind, embers and defensible space: The science of destruction in Laguna Niguel

Tony Briscoe, Alex Wigglesworth, Hannah Fry and Paul Duginski, Los Angeles Times on

Published in Science & Technology News

Years of persistent drought and limited rain had killed brush on the hillside, Fennessy said. Although the fire broke out on what he would call a “normal” day — it wasn’t a Santa Ana wind event and the humidity was high, around 70% — the fuel moisture is so low that even a normal coastal wind caused the fire to spread rapidly, he said.

“What we’re seeing that we haven’t seen in years past is these fires are starting and the vegetation is so dry that with any wind behind it — even a normal wind for that area — it’s going to spread faster than we’re used to and faster than we can get our units at the scene,” he said.

Fennessy said that while he hadn’t walked the edge of the vegetation, he was in backyards during the chaotic first hours of the blaze and did not see anything that would cause him to believe there was a real challenge with the defensible space — the amount of vegetation cleared around homes.

But once flames reached those homes, the fire began to spread from structure to structure. Fennessy said the homes that he saw burn caught fire as a result of embers that blew into the attic space or became wedged into the roofing material.

“It actually creates a condition where the homes themselves become the fuel,” said Max Moritz, a UC Cooperative Extension specialist in wildfire at UC Santa Barbara’s Bren School. Similar patterns could be seen when the Tubbs fire swept through the Santa Rosa subdivision of Coffey Park in 2017 and when the Camp fire destroyed the town of Paradise in 2018, he said.

Homes act as heavy fuels that give off a lot of heat and embers and can burn hotter due to the presence of synthetic, chemically treated materials. The result is a blaze that burns intensely and can be difficult for firefighters to control, he said.

 

“You can imagine a house is like many, many, many, many large trees’ worth of lumber, so it can take a long time for a home to actually burn,” he said. “And while that’s happening, it gives off a lot of radiant heat and it can produce a lot of burning embers that fly through the air to potentially ignite nearby structures.”

The effect is amplified when the homes are close together, said Crystal Kolden, a professor of fire science at UC Merced.

“Embers just go from house to house to house and find all the little nooks and crannies to get into the house, and in many cases burn it from the inside out,” she said.

Such urban conflagrations were more commonplace in the late 19th and early 20th centuries, when buildings were made of flammable wood, resulting in fires that would burn down entire cities, Kolden said. Then building codes changed the way cities were constructed, and these events became much more rare.

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