Corinne S Kennedy, Elizabeth Weise and Alena Maschke

The (Palm Springs, Calif.) Desert Sun

PALM SPRINGS, Calif. — Iver Larson and his wife were halfway up the road to their home near Kenwood, Calif., when a fire truck coming down the hill stopped them.

“The fire’s just over the ridge, it’s close. You need to turn back. Now,” Larson was told by the captain in the truck, because the trees further up the road were in danger of coming down.

Commander Chris Childs of the California Highway Patrol in Napa county said that at least 50 people had been rescued from Napa hilltops by CHP officers flying in helicopters as multiple fires continue to rage.

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Fast-moving fires in Napa and Sonoma counties fueled by "devil winds" destroyed entire neighborhoods earlier this week, killing dozens . The wind, with gusts registering up to 79 miles per hour, combined with the late night timing and response are two elements of the fires' deadly pace, but the growth into fire-prone areas has increased the risk of wildfires consuming places where people live and work.

California wine country is famed not only for its vines but for the rolling hills and graceful valleys many vineyards sit in. Vineyards can be worth up to $400,000 an acre, making growing wine grapes one of the rare agricultural pursuits which is worth more than selling out to developers.

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The multi-million-dollar views, unobstructed by large-scale development, lend themselves not only to tasting rooms, but also to higher-end homes, whose owners have been willing to pay for the vistas of vines and rolling hills.

These homes tend to be sited on winding roads that go deep into canyons and up hillsides and are on several acres of land, though much of it is unbuildable because of the geography of the hills. These are at most risk in fires.

Rolling hills and defensible space

To combat the issues of proximity to areas at risk for wildfires, experts advocate for defensible space. Cal Fire defines this as “the required space between a structure and the wildland area” meant to stop the spread of wildfire. It's more difficult to create for homes on hillsides — those with views —because fire moves more quickly uphill.

Homeowners also loath to chop down the ancient oaks that give the area its character and given the topography it can be hard to clear 100 feet in each direction.

These homes are also difficult to protect because the roads leading up to them tend to be narrow, with oak and other trees on both sides. In a fire, the trees burn and lose branches and sometime fall over entirely when their trunks are damaged, making it impossible for residents to leave and for firefighters to get to.

Falling trees are an ongoing hazard. The trunks can smolder and become progressively weakened, then the tree can suddenly crash to the ground with no warning. One oak tree near Glen Ellen, well over 50 feet tall, fell within 10 feet of a USA TODAY reporter’s car parked by the side of the road on Tuesday.

Fire prevention experts say creating a defensible space around a house can help prevent the spread of fire from wildlands to residential areas.

California law demands any structures adjacent to forest, brush or grass-covered land or mountainous terrain maintain a defensible space of 100 feet from the structure in every direction. However, the state regulations lack teeth.

While Cal Fire inspectors enforce defensible space regulations in some rural parts of the state, in most parts of California the onus falls on the property owner to monitor and maintain potentially flammable plant life.

Cal Fire spokesperson Lynne Tolmachoff said wildfires were a fact of life in California. Following fire department guidelines can help, but can’t always stand up to the indiscriminate nature of wildfires.

“Any kind of defensible space, obviously gives your home a better chance of survival, but there’s no guarantee,” she said.

Experts say vineyards in the Sonoma and Napa counties helped create some extent of defensive space. "The vineyards actually helped, because there you have grapes, and grapes don't burn as fast," said Richard Minnich, professor of Ecology at the University of California Riverside.

But California's drought ending with a wet winter served as a double-edged sword.

“It seems like we can’t win when we have four years of drought and then we get all this rain and that fuels the plant growth that contributes to wildfires,” Cal Fire's Tolmachoff said.

California’s wet winter fueled a boom in plant growth, especially the types of tall grasses that provide perfect tinder once they dry out in the fall. Tolmachoff said Cal Fire saw more fires in lower elevations, where the tall grasses grow and the subdivisions flourish.

“Those type of fires typically would start, they would burn quickly and they would burn fast and generally are put out quickly,” she said. “But fires like that can be extremely dangerous, especially in residential areas, like the ones we’ve seen this week.”

Growing into fire-prone areas

A common question about areas that have burned north of the San Francisco Bay area has been whether increased development has contributed to the destruction.

"Just having more homes in fire-prone areas increases the potential for loss," said Dr. Max Moritz at the UC Extension Division of Agriculture and Natural Resources.

But increased development may not just increase actual loss, but also the risk of fires themselves.

"There is the potential that having more people in fire-prone areas, in addition to increased exposure, also feeds back to alter admission patterns for fires and increase the frequency of fires," said Moritz.

Moritz and his team have been creating models to assess the risk of wildfires for years. "Something that's been left out of a lot of these models is future developments," Moritz realized.

Just last year, they began including this factor in their models, Moritz said they came to an important conclusion. "The human development element in the models actually had just as much influence as the climate variables did," he concluded.

Some housing choices carry higher risk

In Napa and Sonoma counties, strict development rules designed to protect the agricultural character of the area have mostly kept out the kind of subdivision building typical in other parts of the country. However, there are several different development patterns in the area affected by the fires, each of which brings its own type of risk.

Cities like Santa Rosa have seen tremendous growth as people have left the San Francisco Bay area seeking more affordable homes and cost of living. Santa Rosa grew by more than 10% from 2000 to 2015, according to the city.

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Numerous residential developments of the type familiar to any suburban dweller have gone up in recent years. Some of these abut agricultural or wild lands, so when wildfires sweep through they can hit the edge of the development and rapidly destroy dozens of homes and sometimes hundreds of homes.

This meeting of rural and residential spaces – known in the fire and development worlds as wildland-urban interface – has brought new challenges in combating wildfires.

Tolmachoff said fires in these types of tightly-packed suburban neighborhoods are less common, but still happen. And they could become more frequent as California’s urban areas expand.

According to census data, 95% of California’s population lived in urban areas, the highest density of any state. As the state’s population has expanded, those urban areas have had to expand as well to accommodate the millions that have flocked to the Golden State over the last decade.

That puts more people in the wildland-urban interface area.

“In cases like the ones we saw this week with these type of wind driven fires ... with houses so close together, they can be susceptible because if one home catches fire, other homes around it can quickly catch fire as well,” Tolmachoff said.

She said people sometimes think that only the areas right at the edge of the wildlands are vulnerable, but the compact nature of suburban developments put many residents at risk.

Some suburban developments are more vulnerable than others, Richard Minnich points out. After reviewing footage of the destruction caused by wildfires in Santa Rosa, Minnich's assessment is that development choices were a factor: “When you see whole neighborhood that burn down, like Coffey Park, it’s because the construction’s bad."

It is especially wood-sided buildings like those common in Santa Rosa that are an invitation to destructive wildfires, Minnich pointed out. “We’ve had enough of that around here to know that we need really strong building codes. Santa Rosa needs to change their building codes. ASAP," he said.

According to Moritz, the issue was less with the current building codes, than with the age of some of the houses in the area. "If these were newer developments, they would have been subject to more stringent building codes," he noted. "Homes that had defensible space — and looked pretty fire-resistant and incombustible — burned," Moritz said.