Researchers evaluate how fences can spread fires and ignite structures

Tests fire hazard fences
Examples of very high hazard fences and mulch: A) parallel privacy fences, B) double
lattice fences, C) wood-plastic composite #1 fence, D) good neighbor fence, E) rubber mulch. (From the NIST research)

Three weeks ago a television station in Dallas, FOX 4, streamed live video as a grass fire spread across a field and reached densely-packed houses, eventually destroying 9 and damaging another 17. There were several reasons why the structures were so vulnerable, including medium-heavy vegetation in the field adjacent to the property lines, very little impact from efforts of the fire department to stop the spread, no air support, and wood fences that connected all of the properties on the rear and sides of the homes.

fences burn fire Texas homes destroyed
Parallel fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.

On August 10 the National Institute of Standards and Technology (NIST) published results of 187 experiments that examined how fire spread toward a structure is affected by combustible fences and mulch under conditions that may be encountered in a wildland-urban interface (WUI) fire. They looked at mulch only, fence only, fence plus mulch, parallel fences, and long range firebrands. The materials studied were western red cedar, California redwood, pine, vinyl, and wood-plastic composites. Fence styles included privacy, lattice, and good neighbor (board on board).

A small structure was located between zero and up to six feet downwind of the fence or mulch bed as a target for flames and firebrands. A target mulch bed at the base of the structure tested the ability of firebrands produced by the burning fence and mulch bed to ignite spot fires that threatened the structure.

While not many homeowners place store-bought mulch below their fences, it can be common for debris to build up along the base on one or both sides. The amount and flammability can be extremely variable, but the researchers’ use of mulch gave them control over the flammable material which could be consistently duplicated in their experiments.

The study found that firebrands capable of igniting spot fires downwind were generated by nearly all combinations of fence and mulch tested in this study. All wood fences with mulch at the base caused spot fires in the target mulch bed. Spot fires were often ignited within a few minutes of mulch and fence ignition. Shredded hardwood mulch and pine bark mulch burned and emitted firebrands for longer than an hour. Ignition of spot fires was also demonstrated from firebrands transported by the wind over distances as far as 156 feet from the burning item under high wind conditions and over a paved surface.

The research generated a number of recommendations:

  • Avoid parallel fences, to reduce exposure to large flames. Parallel fences can result in highly hazardous fuel accumulation corridors that are difficult to access and maintain. Spacing of 3 feet between fences is not sufficient.
  • Avoid proximity to other combustible fuels, to reduce fire intensity and limit fire spread. This includes fuels above the fence and fuels across parcel boundaries. Avoid mulch at base of fence.
  • Avoid proximity of combustible fences to residence, including neighboring residence, to prevent direct ignition.
  • Fire spread is more likely with wood and wood-plastic composite fences than with fences made of vinyl or noncombustible materials such as stone, brick, or steel.
  • Keep fence and yard clear of debris, to reduce the amount of fuel and potential pathways for fire.
  • Harden structures against firebrands to prevent structure ignition from embers produced by fences or other combustible sources.

The video from the July 25 fire in Texas does not clearly show the fences between the residences, but it is likely that they directly connected to the structures. An example is seen in the photo below of two homes in Superior, Colorado, one of the communities devastated by the Marshall Fire that destroyed 991 structures south of Boulder December 30, 2021.

Fence connecting houses fire spread hazard
File photo of fence connecting houses in Superior, Colorado. July, 2012 Google image.

The images below are from the FOX 4 Dallas-Fort Worth video shot July 25, 2022.

fences burn fire Texas homes destroyed
Fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.
fences burn fire Texas homes destroyed
Fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.
fences burn fire Texas homes destroyed
Fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.

Most structures that burn in a wildfire are not ignited by direct flame impingement, but by burning embers that are lofted and carried downwind ahead of the fire.  At Wildfire Today we first covered the role of embers in igniting structures in 2010, a concept brought into the public consciousness by Jack Cohen, a researcher at the Missoula Fire Science Lab. To reduce the chances of a home burning in a wildfire, the most bang for the buck is to concentrate on the Home Ignition Zone. The flammable material near the structure needs to be modified, reduced, or eliminated to the point where multiple burning embers landing in the zone will not propagate the fire and spread to the structure.

More information: Six things that need to be done to protect fire-prone communities

Thanks and a tip of the hat go out to Gerald.

Grass fire burns into Dallas suburb, destroys 9 homes

Another 17 were damaged

11:53 p.m. CDT July 25, 2022

Balch Springs Fire
Grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.

A grass fire burned into a suburb of Dallas, Texas Monday afternoon destroying 9 homes and damaging another 17, according to city officials. It occurred in Balch Springs when a mower struck an object in a field, creating a spark which ignited dry grass near Interstate 20 and South Beltline Road.

A steady breeze pushed the fire into a row of houses adjacent to the field. One by one the fire ignited house after house, aided by a fence that ran behind and between all of the homes which contributed to the fuel load and the continuous spread.

Balch Springs Fire
Grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.

In the 30-minute video below very few firefighters are seen for the first 20 minutes. Balch Springs, with an estimated population of about 25,000 in 2019 has about eight firefighters working on any given day, Fox 4 news reported. The fire was well established when the video began, with at least one home already burning.

Looking at the video from a firefighter’s perspective, it is interesting to see how the fire progresses as the fence and outbuildings burn intensely, structures ignite, police gather in the street, a dog in a backyard looks worried (at 17:48), and little is seen in the video to initially stop the spread through the field or the neighborhood. However we don’t see the street side of the homes except at the very beginning; there may have been more firefighter activity on that side. There was a tower/ladder truck in the street that looked like it kept about four houses from being destroyed.

Our hearts go out to the residents who lost their homes.

Photos show devastation after the 6,000-acre Marshall Fire in Colorado

The surviving homes in subdivisions that were destroyed had a common feature

Marshall Fire, Louisville, Colorado, by WxChasing/Brandon Clement
Marshall Fire, Louisville, Colorado. Photo by WxChasing/Brandon Clement, Dec. 31, 2021.

Early Friday morning, about 20 hours after the Marshall Fire ignited, a drone operated by Twitter user WxChasing/Brandon Clement flew over subdivisions that were devastated by the December 30 fire. It found block after block of ash piles, some still smoldering. In many scenes there was scarcely a structure still standing. (Scroll down to see the video.)

To see all articles on Wildfire Today about the Marshall Fire, including the most recent, click here:

All of the reasons why some houses did not burn even though dozens around them were consumed could not be determined from the video, but there was one common feature — the survivors were more distant from the neighboring homes. Many houses in the subdivisions were only 10 to 20 feet apart based on archived imagery in Google Earth.

The fire was driven by very strong winds gusting at 60 to 100 mph, extremely dry conditions after months of drought, and relative humidity in the mid-20s. These are the very worst fire conditions. The weather paired with the nearly back to back structures led to the fire spreading through a continuous human-made fuel bed. When one house burned the convective and radiant heat easily ignited its neighbor, which ignited its neighbor, etc.

The fire in the vegetation and structures lofted burning materials far downwind, creating distant spot fires in the home ignition zone on bone dry lawns, mulch beds around ornamental plants, and on structures. It is unknown at this point how many had been designed and built to be fire resistant, such as the characteristics of the roof, vents, siding, doors, windows, foundation, fences, eaves, and decks. A FEMA publication (13 MB) has excellent detailed recommendations. Headwaters Economics found that the cost of building a fire-resistant home is about the same as a standard home. Local building codes could regulate these features. But if the lot size is so small that residences are only 10 to 20 feet apart, if one becomes fully involved, the neighbors also burn, especially during windy conditions.

So far we have listed some factors that affect the vulnerably of structures during a wildland-urban interface fire: home spacing and lot size, the envelope of the structure itself, fire codes, and the home ignition zone. Others are:

  • Evacuation capability and planning;
  • Safety zones where residents can shelter in place;
  • Road and driveway width, wide enough for large fire trucks;
  • Turnarounds at the end of roads;
  • Signage, and;
  • Emergency water supply.

The video below of the Marshall Fire devastation was shot by WxChasing/Brandon Clement at first light on December 31, 2021, the day after the fire started. Not long after, snow began falling. The National Weather Service in nearby Boulder recorded an accumulation of eight inches.

Wildfire risk reports available for communities in the U.S.

Reports can be customized for every city, county, and state

Wildfire risk, Yreka, CA
Homes at risk from wildfire in Yreka, CA. An excerpt from a report by Headquarters Economics.

Headwaters Economics has developed new Wildfire Risk report system for communities. Reports can be customized for every U.S. city, county, and state which provide information about wildfire risk and potentially vulnerable populations. It uses public data from the USDA Forest Service, US Census Bureau, and other sources and is one of several reports available in their Economic Profile System.

Headwaters Economics explains that they created this new report because wildfire risk is more than a physical hazard. Economic, demographic, and social vulnerabilities put some people disproportionately at risk. Information in their reports can help elected officials, land use planners, fire personnel, and community health organizations to:

  • prioritize and direct resources to the people and places most at-risk;
  • customize and target outreach and education efforts; and
  • tailor wildfire response and operational plans.

The graphic at the top of this article is a portion of the 15-page report for Yreka, California, which can be downloaded here.

Arizona Central also has a system for generating data about wildfire risk for individual communities in the Western United States.

Studying a wildland urban interface maintained by Native Americans from 1100 to 1650

Ancestors of the Jemez Pueblo used fire 27 different ways

Conceptual map of landscape zones Hemish people
Figure 2 from the publication. Conceptual map of landscape zones and 27 fire and wood uses for Hemish people.

A paper being published this week looks at an area in Northern New Mexico that was populated by Native Americans living within a wildland urban interface (WUI) that was sustainable from approximately 1100 to 1650.

Below are excepts from the publication.

Researchers combined ethnography, archaeology, paleoecology, and ecological modeling to infer intensive wood and fire use by Native American ancestors of Jemez Pueblo and the consequences on fire size and intensity. Initial settlement of northern New Mexico by Jemez farmers increased fire activity within an already dynamic landscape that experienced frequent fires. Wood harvesting for domestic fuel and architectural uses and abundant, small, patchy fires created a landscape that burned often but only rarely burned extensively.

Depopulation of the forested landscape due to Spanish colonial impacts resulted in a rebound of fuels accompanied by the return of widely spreading, frequent surface fires. The sequence of more than 500 years of perennial small fires and wood collecting followed by frequent “free-range” wildland surface fires made the landscape resistant to extreme fire behavior, even when climate was conducive and surface fires were large. The ancient Jemez WUI offers an alternative model for fire management in modern WUI in the western United States, and possibly other settings where local management of woody fuels through use (domestic wood collecting) coupled with small prescribed fires may make these communities both self-reliant and more resilient to wildfire hazards.

Policy implications

The Jemez ancient wildland urban interface (WUI) obviously contrasts with modern WUI in the American West in ways that make the ancient WUI an imperfect analog for modern conditions. The economic, technological, and political differences are irreconcilable but they do not obviate the relevance of the ancient WUI for modern problems. The cultural contrasts between ancient and modern WUI highlight opportunities to cultivate more resilient communities by supporting particular cultural values.

Two of the important characteristics of the Jemez ancient WUI are: 1) That it was a working landscape, in which properties of the fire regime were shaped by wood, land, and fire use that supported the livelihoods of the residents; and 2) that there was much greater acceptance of the positive benefits of fire and smoke.

We emphasize that these are malleable cultural features, because reshaping western United States culture by learning from indigenous cultural values may be critical for building adaptive and transformative resilience in modern communities. Learning to value the positive benefits of fire and smoke and to tolerate their presence will undoubtedly be critical to WUI fire adaptations.

Furthermore, the ancient WUI highlights two key processes that may make modern WUI more resistant to extreme fires: 1) Intensive wood collecting and thinning, particularly in close proximity to settlements; and 2) using many small, patchy fires annually (approximately 100 ha) rather than using larger burn patches (thousands of hectares) to restore fire and reduce fuel hazards, particularly closer to settlements.

Many WUI communities—especially rural and Indigenous communities—rely on domestic biomass burning for heat during the winter. Public/private–tribal partnerships to thin small diameter trees and collect downed and dead fuel for domestic use could have dual benefits for the community by meeting energy needs and reducing fuel loads. Tribal communities that have deep histories in a particular forested landscape may be ideal partners for supervising such a program. Lessons from the Jemez ancient WUI also suggest that federal and state programs to support prescribed burning by Native American tribes, WUI municipalities, and private land owners would provide equal benefit to modern communities.

It is imperative that we understand the properties and dynamics of past human–natural systems that offer lessons for contemporary communities . The Jemez ancient WUI is one of many such settings where centuries of sustainable human–fire interaction offer tangible lessons for adapting to wildfire for contemporary communities.

Authors of the paper: Christopher I. Roos, Thomas W. Swetnam, T. J. Ferguson, Matthew J. Liebmann, Rachel A. Loehman, John R. Welch, Ellis Q. Margolis, Christopher H. Guiterman, William C. Hockaday, Michael J. Aiuvalasit, Jenna Battillo, Joshua Farella, and Christopher A. Kiahtipes.

Data shows the worsening trend of California wildfires

Cumulative Acres Burned, 1979-2018, for all Fires in CaliforniaIt’s clear to most citizens of California that wildfires have become more intense over the last few years. Researchers at UC Santa Barbara and the Nature Conservancy have compiled a new dataset of damage caused by wildfires in California in areas protected by the state of California. (Some of the data does not include fires on lands protected by federal agencies, such as the U.S. Forest Service, National Park Service, and the BLM). The report illustrates how the recent set of severe fires fits into a broader trend of increasing burn area and damage over the past 40 years.

The report was written by: Hanna Buechi (Environmental Market Solutions Lab, UCSB), Dick Cameron (The Nature Conservancy), Sarah Heard (The Nature Conservancy), Andrew J. Plantinga (Environmental Market Solutions Lab, UCSB), and Paige Weber (Environmental Market Solutions Lab, UCSB).

The researchers studied data on fire perimeters and estimates of damages for each fire and used the information to calculate trends involving the number and timing of fires throughout the state by time of year. They also calculated the total area burned and specifically identified the amount of wildland urban-interface burned. These are areas where houses intermingle with wildland vegetation, and are of particular concern to those studying wildfire.

Andrew Plantinga
Andrew Plantinga. UCSB photo.

“The main finding is that the recent severe fires in California — including the Thomas fire in 2017 and the Camp fire in 2018 — are part of a trend in California over the past four decades,” said Andrew Plantinga, an economics professor at UC Santa Barbara’s Bren School of Environmental Science & Management. “The trend is toward more wildfires that burn larger areas and cause more damage.”

The number of acres burned per year has not only been increasing, the report found, it is also accelerating. And this increase isn’t only during the season’s peak, from June through October. The state is also seeing a longer fire season, with more acres burned in late fall than in the past. And while greater burn areas don’t automatically translate to greater damages, the researchers found that these, too, have been on the rise.

“I expected the recent severe fires to be outliers, and they are,” said Plantinga, “but it’s also clear that they represent part of a trend toward larger and more damaging fires.”

Cumulative WUI Acres Burned in California, 1979 – 2018

The report is part of a larger effort to estimate the costs associated with a business-as-usual approach to development in California, when considering the potential impacts of climate change. The team had previously found that interventions on natural and working lands — like forests, farms and rangelands — can contribute 2.5 times the emissions reductions by 2050 as residential and commercial sectors combined.

What’s more, for every dollar spent on implementing land-use strategies, close to fifty cents would be recouped in economic benefits. And that’s without accounting for other positive impacts, the previous report states.

California Civilian and Firefighter Deaths
Civilian and Firefighter Deaths on wildfires in California that occurred on State Responsibility Area lands, 1979 – 2017.
California Estimated Value of Structure Losses
Estimated Value of Structure Losses (in 2018 dollars) for State Responsibility Area Fires, by Year, 1979 – 2018.

Acres Burned by Month and Decade for all Fires in California

California Map of Structure Losses and Fire Perimeters
Map of Structure Losses and Fire Perimeters for State Responsibility Area Fires, 1979 – 2018.