The science of fire and destruction in Laguna Niguel

Weather conditions are unremarkable for coastal Orange County, and even mildly pleasant: Mild temperatures, relatively humid air, and seasonal inshore winds.

But as firefighters struggled to contain the 50 by 50 foot blaze on a sere hillside in Laguna Niguel on Wednesday afternoon, officials became concerned. Within hours, many homes were on fire and spewing hot embers as the Coastal Fire methodically burned through an upscale development overlooking the Pacific Ocean. By the time the fire’s spread slowed, at least 20 homes had been burned, many overlooking the canyon where the fire started.

The sudden and severe devastation caused many to wonder how such a fire could break out in mundane conditions. However, experts say preliminary reports suggest the devastation is due to an unfortunate combination of factors. Moderate winds, steep terrain and drought-damaged vegetation have worked together to fan the flames into a community where homes were built before the fire-resistant building rules went into effect. .

Captain Sean Doran, spokesman for the Orange County Fire Department, said: “That’s not typical, a fire will happen when those pieces start to align – not in our favor. for us but to unite against us”. “As they start to stack, it becomes a stronger self-propagating force.”

After the fire broke out on a hillside west of the South Orange County Wastewater Administration’s treatment plant, county firefighters arrived within minutes and found the blaze had spread for a little while. least an acre, Doran said.

Firefighters trudged up the hillside with equipment and lined up on either side in hopes of extinguishing the blaze, he said. However, a sea breeze pushed the fire downhill and eastward. According to the National Weather Service in San Diego, the winds peaked at up to 25 mph around 4 p.m.

The fires then burned the hills to the east of the treatment plant, burning hay and shrubs. The wind-driven blazes continued westward, and the heat generated and the flames naturally grew higher and higher, quickly reaching homes on the hilltops, Doran said.

OCFA director Brian Fennessy said: ‘Once the fire hit the foothills below the houses, it was like an arrow had just shot to the top.

On hillsides, fire creates a chimney effect as a column of hot air, smoke, ash, and debris rises, potentially crowding the surrounding wind direction and creating a blast of oxygen. Features such as gulls, ravines, or trenches gather this hot, rising air, known as convection columns. The hot gas preheats the flame as the flame rises steeply. Hot embers can be carried by this rising air and wind, hitting smaller fires in front or behind the main flame.

The graphic shows the chimney effect when a forest fire occurs on the slopes.

(Paul Duginski / Los Angeles Times)

“A match – or any flame – does the same thing,” said Issac Sanchez, battalion commander in charge of communications at the California Department of Forestry and Fire Protection. He said: Hot air rises and because nature restricts the vacuum, air from the surrounding environment fills the void.

Fennessy says years of persistent drought and limited rainfall have killed hillside brooms. Although the fire broke out on what he called a “normal” day – it wasn’t a Santa Ana gale and the humidity was high, around 70% – the fuel humidity was so low that even a gust of wind Coastal normals also cause fires to spread quickly, he said.

“What we’re seeing that we haven’t seen in years is these fires are starting and the vegetation is so dry that with any wind in the background – even a wind normal for the area.” that area – it will spread faster than us. used and faster than we could get our units to the scene,” he said.

Fennessy said that while he did not walk beyond the edges of the vegetation, he was in the backyard during the tumultuous first hours of the fire and did not see anything that would lead him to believe there was a The real challenge with defensive spaces – the amount of vegetation cleared around the house.

But once the fire reached those houses, the fire started to spread from one structure to another. Fennessy said the homes he’s seen have caught fire from embers blowing into attic spaces or becoming roofing materials.

“It really creates a condition for homes to become their own fuel,” said Max Moritz, a UC Cooperative wildfire extension specialist 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 ravaged the town of Paradise in 2018, he said.

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

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

This effect is increased when homes are close together, says Crystal Kolden, a professor of fire science at UC Merced.

She said: “The embers kept going from house to house and looking for every little nook and cranny to get into the house, in many cases still burning from the inside out.

Such urban fires were more common in the late 19th and early 20th centuries, when buildings were made of combustible wood, resulting in fires that consumed entire cities, Kolden said. . Later, building rules changed the way cities were built, and these events became much rarer.

“That has changed in the last decade or so,” she said. “What we’ve seen is urban conflicts that start with fires in the wilderness and then move into more crowded areas.”

Kolden attributes this resurgence to fires burning closer to the built environment, as development continues to push into wilderness areas. Meanwhile, drought pressured by climate change has made it more difficult for authorities to prevent fires from reaching these densely populated areas, she said.

“They’ve got a huge amount of fuel on those hillsides, which this year is very, very dry, consistent with the California drought, and it basically can’t hold it before it spills over into the neighborhoods,” she said. close,” she said. “And that’s what we’ve seen, especially over the last five to 10 years, not just in California, but in many places across the West and across the United States.”

On top of that, Kolden said, it’s common for homes in areas burned by these types of fires to be built before modern building codes are enacted to minimize the spread from house to house. other house. That was the case with the Coastal Fire, which is burning in the Cal Fire’s highest fire severity zone, she said. At least 18 burned homes were built in the late 1980s or early 1990s; two more were built in the late 1990s.

Chapter 7A of the California building code sets out regulations for newly built homes in designated fire hazard zones including the use of non-combustible roofing materials, partition walls and eaves; vents in the attic prevent embers from entering the house; and double layer tempered glass window. The rules went into effect in 2008 and only apply to new construction and have no requirement for homeowners to perform a retrofit.

Even if some homes have been retrofitted, Kolden added, if others don’t, entire neighborhoods could still be vulnerable, with embers able to jump onto stiffened houses and burn. fire the next unhardened structure, or attempt to penetrate even already hardened homes.

“We’ve had so many destructive fires in recent years that Cal Fire has actually been working overtime to track what’s happening across the state: which houses are on fire and which ones are on fire,” she said. any house,” she said. “And the pattern that starts to emerge is that when entire neighborhoods are built to these higher standards, they are so much more resilient that they essentially act as a barrier to the crowd. on fire.”

On Thursday, a day after flames consumed large swaths of the canyon, a gray fog blanketed the hillside charred black as residents returned to the Coronado Pointe community.

As a firefighter drills his hose into the remains of a stately home in a covered estate overlooking the canyon, 20-year resident Aimee Larr steps out of her SUV and hesitantly steps forward. a yellow safety tape. She recognized the stone fountain in the front yard.

“This is my house!” Larr said, voice trembling with emotion. “I can not believe this. It is completely leveled. There’s nothing left.”

She recalled how Orange County Sheriff deputies urged neighbors to leave their homes on Wednesday. Wearing only sweatpants and a T-shirt, Larr passed away without medication or a personal memento.

When she returns on Thursday afternoon, she hopes that her house will last. After all, it was several thousand feet from the original fire, and the canyon her house overlooked was fitted with a sprinkler system that kept the plants cool and green.

“I don’t know how quickly it got to my house,” she said. “It’s on the other side of the canyon, that’s why I don’t have a big concern. i thought [firefighters] will drop something on it and it will be done. “

As of Thursday night, the fire was 200 acres with a 15% containment capacity. Its cause is still under investigation, but Southern California Edison issued an initial report to state regulators saying they recorded “circuit activity” around the time the fire started. .

Times staff photographer Raul Roa and staff writer Luke Money contributed to this report. The science of fire and destruction in Laguna Niguel

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