Undersøgelse finder risikoen for skovbrand i beboelseshegn og mulchbede

Når man bygger hegn og anlægger deres ejendomme, bør husejere holde brandsikkerheden på toppen af ​​deres sind, især hvis de bor i en naturbrand-udsat region, ifølge en ny undersøgelse.

På tværs af næsten 200 brandeksperimenter brændte forskere ved National Institute of Standards and Technology (NIST) bolighegn og mulchbede for at undersøge den rolle, de spiller i at sprede ild. De fandt ud af, at brandfaren generelt var uforholdsmæssigt højere, når brændbare genstande brændte sammen. Flammer susede langs mulch langs bunden af ​​hegn, og infernos slugte hurtigt par af hegn, da de brændte tæt på hinanden. I modsætning hertil brændte selvstændige hegn fri for barkflis eller snavs i et meget langsommere tempo. Baseret på deres resultater anbefalede forfatterne af rapporten, at husejere ikke placerer to hegn ryg mod ryg, holder andre brændbare overflader langt fra hinanden og tager vigtige andre handlinger.

Naturbrande kan spredes hurtigt gennem og potentielt overvælde samfund, der støder op til vildmarken - den såkaldte vildland-urban interface (WUI). Undersøgelser efter naturbrande, herunder NISTs undersøgelse af lejrbranden i 2018, har udpeget hegn og barkflis som syndere i at sprede ild.

"Hegn, der brændte, var ofte helt væk. Hvis man kiggede godt efter, kunne man se nogle søm og skruer tilbage. Dem, vi så delvist brændt, blev stående, fordi de blev forsvaret af nogen," sagde rapportens medforfatter Alexander Maranghides, som leder NIST's Camp Fire-undersøgelse. "Disse ting går ikke ud af sig selv."

Hegn og mulch kan fungere som broer for flammer til at nå bygninger og som affyringsramper for luftbårne gløder til at antænde brande langt væk, men brandkoder i USA omhandler ikke, hvordan de skal installeres og vedligeholdes, og der findes kun lidt vejledning til at hjælpe husejere.

For at hjælpe med at bygge et teknisk grundlag for udviklingen af ​​retningslinjer i fremtiden, forsøgte forfatterne af den nye rapport at studere hegns- og muldbrande under forhold, der er tættere på det virkelige liv, end hvad der er blevet brugt i tidligere undersøgelser.

Forskerholdet brændte hegn, mulchbede og kombinationer af begge udendørs, og antændte materialerne flere meter medvind fra en vindmaskine, der blev brugt til at simulere reelle brandspredningsforhold. I modvind af bålet satte holdet et skur eller et mulchbed op som mål.

Gennem 187 eksperimenter afbrændte de brændsel alene og i kombination, herunder hegn af flere almindelige designs lavet af træ, vinyl eller træ-plast kompositter og barkflisbede sammensat af revet hårdttræ, fyrrebark-klumper, fyrrehalm eller gummi.

Forskerne fangede optagelser af flammerne for at måle hastigheden og mønsteret af flammespredning og for at registrere, hvor ofte gløder antændte målskuret. They also conducted research in the lab to measure how quickly samples of fence raw materials released heat.

Taking all the data together, the authors categorized the relative fire threat level for the various test conditions.

"In the highest hazard category, the fences and mulch are going to carry the fire along toward your house in a matter of a few minutes, not hours," said NIST physicist Kathryn Butler, co-lead author of the report.

The most dangerous fires observed were those that had multiple sources of fuel burning at the same time. In the tests where mulch lined the bottom of a fence, fire tended to swiftly advance across the beds of fine combustibles, which served as rich sources of embers and allowed flames to quickly ignite the fence along its entire length.

The researchers learned that the fires could get much worse as well.

When two fences made of combustible materials were placed back to back—mimicking the scenario where two neighbors each put up a fence along their property lines—the most intense flames of the entire project erupted. After the fire was established, long-reaching flames quickly shot up, completely engulfing fence panels 2.4 meters (8 feet) long and 1.8 meters (6 feet) tall in as little as four minutes. Embers sparked fires on the target in almost every one of these tests.

"It's very well known that when you confine a fire on the sides, it's bad news. Keeping those hot gases between the fences with surfaces radiating intense heat to each other leads to explosive fire behavior," Butler said.

When fence panels burned alone, it was a different story. Flames slowly chewed away at the fences and did not spread very far during these experiments, with some progressing less than a meter (3.3 feet) in an hour. Although these fires burning slowly on a sole fuel source were less hazardous and more manageable, high winds that may accompany a wildfire could blow debris toward the fence, adding fuel to the fire.

Butler, NIST mechanical engineer Erik Johnsson and the rest of the authors crafted seven recommendations for homeowners living in WUI zones based on their analysis. The first recommendation is to avoid doubling up on fences completely, as the study found that fences as far apart as 91 centimeters (3 feet) still produced large flames. The second says that combustible fences should be placed where they will not interfere with exit routes.

Other recommendations advise keeping combustibles as far away from each other as possible, even between property lines, and clearing yards—and especially the space near or between fences—of debris, such as leaves or fallen branches.

The report also indicates that homeowners should replace combustible landscape features with those consisting of noncombustible material such as stone, steel or cement, when possible.

Because of the ever-present danger of embers during wildfires—even when there is a large distance between a structure and the fire—the researchers also urge homeowners to enhance their homes to resist ember ignition through a process called hardening. The detailed steps for hardening are described in a separate NIST report on mitigating wildfires.

The team members intend to keep pushing forward to cover new ground in wildfire research.

With additional experiments, they plan to offer additional insights on risk mitigation and eventually lay a groundwork for new guidance, standards and fire codes that could curb the real wildfire hazards that endanger life and property in the U.S.

The research was published as NIST Technical Note 2228 . + Udforsk yderligere

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