California Burning: Lessons from the Cedar Fire of 2003

By Tammy Viggato

california-burning2.jpgIn the annals of California’s wildfires, 2018 marked the 15th anniversary of the Cedar Fire. Spread by Santa Ana winds, this wildland-urban interface (WUI) fire wreaked havoc across San Diego County, burning more than 273,000 acres and destroying more than 2,800 structures.

Until the ravages of this year’s wildfires, the Cedar Fire was the largest to hit California and was the third largest and third most destructive in the state’s historical record (Tables 1 and 2). It demonstrates the devastation that can occur in the WUI, an area where human development meets or intermingles with undeveloped land.

Using the AIR Wildfire Model for the United States, AIR estimates that the Cedar Fire would result in insured losses of about USD 3.5 billion if it were to recur today.

The Tubbs and Thomas Fires at the end of 2017 and the Carr, Mendocino, Camp, and Woolsey Fires this year have served as reminders of how large and destructive wildfires can be (Tables 1 and 2)—especially in the WUI, the area most at risk.

Table 1. California’s six largest wildfires by acres burned as of December 3, 2018
(data source: CAL FIRE)

Event

Date

County

Acres Burned

Mendocino Complex

July 2018

Colusa, Lake,
Mendocino, and Glenn

459,123

Thomas

December 2017

Ventura and Santa Barbara

281,893

Cedar

October 2003

San Diego

273,246

Rush

August 2012

Lassen

CA: 271,911 
NV: 43,666

Rim

August 2013

Tuolumne

257,314

Zaca

July 2007

Santa Barbara

240,207

 

Table 2. California’s eight most destructive wildfires by structures destroyed as of December 3, 2018
(data source: CAL FIRE)

Event

Date

County

Structures Destroyed

Camp

Tubbs

November 2018

October 2017

Butte

Napa and Sonoma

18,700+

5,636

Tunnel

October 1991

Alameda

2,900

Cedar

October 2003

San Diego

2,820

Valley

September 2015

Lake, Napa, and Sonoma

1,955

Witch

Woolsey

October 2007

November 2018

San Diego

Los Angeles and Ventura

1,650

1,643

Carr

July 2018

Shasta and Trinity

1,604

 

On October 25, 2003, a lost hunter lit a small signal fire at the bottom of a canyon in the Cleveland National Forest. A federal rule prohibiting the flight of firefighting aircraft later than 30 minutes before sunset prevented aerial suppression efforts and left ground crews to look for the flames of the growing fire. Due to overgrown brush and the propensity of fires to blaze uphill quickly, firefighters could only wait for flames to approach the dirt road half a mile away.

Initially, the fire held at 20 acres. Less than a day later, however, that small signal fire exploded to 62,000 acres, traveled almost 30 miles, jumped the San Diego River, and crossed into the city of San Diego. A major contributing factor was the Santa Ana winds—strong, hot, extremely dry winds that descend to the Pacific Coast from inland desert areas. But the Santa Ana winds were just one environmental condition that exacerbated the Cedar Fire. Low humidity and warm temperatures had dried fuels and likely contributed to the spread.

The first house caught fire in San Diego Country Estates, just eight hours after the fire was first reported at 1:16 a.m. Later that same day, the Cedar Fire approached the nearby community of Ramona in San Diego County (Figure 1). Flames as high as 100 feet were reported near Scripps Ranch. By 6:00 p.m. on October 27, the Cedar Fire had grown to more than 200,000 acres, surpassing the Laguna Fire of 1970 to become the largest fire San Diego County had ever experienced. On October 28, smoke and ash filled the air in surrounding towns, making breathing difficult and daily life almost impossible; schools, courts, and businesses were forced to close. When it reached more than 250,000 acres on October 29, the Cedar Fire set a record for the largest wildfire in California history.

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Figure 1. The Cedar Fire approaching Ramona in San Diego County on October 26, 2003 (source: San Diego Fire-Rescue Department)

Firefighters achieved two-thirds containment (that is, the establishment of a control line around a fire and inhibition of fire spread) on October 31, when the Cedar Fire had reached its maximum size. But not until November 3 was 100 percent containment achieved.

The wildland-urban interface: Where the largest insured losses have occurred

The Cedar Fire quickly escalated from a wildland fire to a wildland-urban interface (WUI) fire. The WUI, an area where human development meets or intermingles with undeveloped wildland, is the zone where the largest insured losses from wildfire in the United States have occurred. Approximately one-third of U.S. homes are in the WUI, with 4,000 acres per day converting from wildland to WUI through residential development—even in the most fire-prone regions. Several hundred thousand businesses and an estimated population of more than 120 million people are located in the WUI.

The proximity of WUI developments to wildland full of fuels puts them at increased risk, as was the case with the Cedar Fire. Fires may start burning in less-populated areas, but they can travel great distances and potentially into more densely populated areas when strong winds blow embers beyond the flames. Wind-borne embers landing on roofs and setting structures on fire is known as fire branding/spotting and is the primary way fire moves through urban areas.

Continued expansion of the WUI is the main driver of the increases in insured losses from wildfire. More exposure exists in previously undeveloped areas, and today’s structures have higher overall costs—including construction materials, finishes, and floor plans.

Human activity and wildfire risk

Human activity can change the distribution of fire ignitions. While wildfires caused by lightning tend to follow natural patterns and long-term climatic conditions, ignitions caused by humans follow road networks—especially near the WUI. These road networks give people easy access to wildland where fires can be ignited, and fires often follow road networks as they spread. And while one homeowner may be diligent at managing fuels on their property (such as removing dead shrubs or cutting back trees and branches too close to the house), the community’s ability to mitigate wildfire risk is contingent upon similar efforts from all neighbors.

In an ever-changing landscape with a growing population, depending on historical experience alone won’t help insurers manage their wildfire risk; the types and distributions of exposure and fuels today vary from those of the past (Figure 2).

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Figure 2. Satellite imagery showing the differences in exposure between 1993 (left) and 2016 (right) within the burn perimeter of an unnamed California wildfire that occurred on July 18, 1993 (red outline). (data source: Monitoring Trends in Burn Severity; map source: Google Earth)

The AIR Wildfire Model for the United States captures the wildfire risk in the 13 western U.S. states where the most significant wildfire losses have occurred. The most comprehensive wildfire model available for the region, it explicitly accounts for all the ways a fire can spread, including to and through urban settings—most commonly due to fire branding/spotting, as was the case in the Cedar Fire. The model uses a probabilistic approach that considers today’s environment and conditions, such as the Santa Ana winds that exacerbated the Cedar Fire, to provide a realistic view of wildfire risk not achievable with historical experience alone.

The combination of circumstances that lead to urban conflagration events—large, destructive fires that move beyond the WUI and penetrate an urban area—are also explicitly modeled. AIR’s innovative U.S. wildfire model enables companies to develop a realistic understanding of their wildfire risk so they can make more informed decisions, more effectively manage portfolios and exposure concentrations, consider risk financing strategies, and enhance resilience.

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Tammy Viggato is a senior scientist at AIR, a Verisk (Nasdaq:VRSK) business.