20 ms·

Strong winds fan flames in a hurry, but for a typical forest fire, the problem isn't actually the flame front - it's the firebrands (burning pieces of wood toss

by Syonyk 5y ago

Strong winds fan flames in a hurry, but for a typical forest fire, the problem isn't actually the flame front - it's the firebrands (burning pieces of wood tossed ahead of the fire by the wind). They get lofted by the rising hot air, catch a breeze, and extend typically half a mile or more in front of the actual flame front. So you end up with a ton of spot fires that, if not addressed, do things like "light the whole town on fire." Buildings burning create a ton more of these, and you're screwed. If you look at the aftermath, you're almost certain to see buildings mostly burned from the top down, with intact landscaping around them - that's a strong sign that the spread was firebrand based. Unfortunately, if you've not built for it ahead of time (and sometimes even if you have), there's literally nothing you can do when the fire hits in high winds. I mostly deal with grass fires locally, and even those can get going in a hurry with a strong wind. Of course, the only days they ever seem to burn is in a strong wind...

monkeybutton 5y ago
Here's a terrifying example from fort McMurray where it's raining embers: https://youtu.be/7E_OLgC4nV0 https://youtu.be/7E_OLgC4nV0
- Syonyk 5y ago
  Yeah, that's a great example. You can see some spot fires starting in a few places in the video along the side of the road. If you know what you're doing, have backup power, and some water storage, someone staying behind to deal with the spot fires before they grow can have a large impact on the survivability of a house or neighborhood. If you're somewhere prone to wildfires and you have a pool (and are the sort to stay behind...), a good propane powered pump and enough hose to run good lines around the house will have a huge impact - and even if you don't have that, a few thousand gallons of water storage isn't that expensive (compared to a house). If you've got a properly set up wildfire rated house without much debris around it, a good Class A rated roof, and some water, you can do a lot. Of course, you can always roll a 1, and certain houses (shake roof) will simply go up like a candle in an ember storm like this, but in that situation, I'm not sure a car stuck in traffic is the better option. Houses can handle an awful lot of radiant heat, and they'll survive perfectly fine a radiant heat flux far beyond what a human can tolerate. You just can't let direct flame impinge on them for very long before it spreads.
LorenPechtel 5y ago
Yup. The only defense against firebrands is to build such that your house won't catch fire in a rain of firebrands. Locally, a combination of stucco and concrete tile roofs can make a house basically immune to fire rain--but concrete tile roofs are not an option in most places.
- Syonyk 5y ago
  You don't actually need non-flammable roofing - just a roof rated for the fire type. Check Class A roof tests. A lot of asphalt shingles, properly installed, actually are Class A fire rated - which is a 12x12 2kg grid. https://www.builddirect.com/learning-center/building-materials/roofing/roof-fire-ratings-guide/ https://www.builddirect.com/learning-center/building-materia... Interior corners on the roof or house tend to cause problems, though. Debris builds up and ignites, and just impinges on the siding until you light something behind it. You can also have firebrands entering through attic vents and bypassing the roof - there are vent types that are resistant to this, but tend to cost a good bit more. Why we don't do this in urban wildland interfaces... no idea.
- viraptor 5y ago
  Or keep the roof wet. As long as your water supply exists, roof sprinklers work. They're popular in Australia with people living in bushfire prone areas. https://build.com.au/roof-sprinklers https://build.com.au/roof-sprinklers (It also helps that bush properties have large water tanks attached to them)
davidbanham 5y ago
A firefront moves a lot faster through grass than through forest fuels given the same moisture content. Totally different strategies required to fight them. They don’t throw embers like a crowning bushfire, though.
davidbanham 5y ago
Out of curiosity I plugged Lytton’s weather readings into the Mccarthur tables we use to calculate forest fire danger in Australia. I’m using guesstimates for humidity and wind speed based on the spotty data available from the local government website. Also just totally fudging the fuel load number. I’m reading spotting distances between 1.5 and 2km. Overall FFDI of 41 so a bad day but not extreme by our standards. Would have been a lot worse with lower humidity.
- moistly 5y ago
  Parts of the province were down to 11% humidity. Lytton is renown for being extremely dry.

by Syonyk 5y ago

monkeybutton 5y ago

Here's a terrifying example from fort McMurray where it's raining embers: https://youtu.be/7E_OLgC4nV0 https://youtu.be/7E_OLgC4nV0

Syonyk 5y ago
Yeah, that's a great example. You can see some spot fires starting in a few places in the video along the side of the road. If you know what you're doing, have backup power, and some water storage, someone staying behind to deal with the spot fires before they grow can have a large impact on the survivability of a house or neighborhood. If you're somewhere prone to wildfires and you have a pool (and are the sort to stay behind...), a good propane powered pump and enough hose to run good lines around the house will have a huge impact - and even if you don't have that, a few thousand gallons of water storage isn't that expensive (compared to a house). If you've got a properly set up wildfire rated house without much debris around it, a good Class A rated roof, and some water, you can do a lot. Of course, you can always roll a 1, and certain houses (shake roof) will simply go up like a candle in an ember storm like this, but in that situation, I'm not sure a car stuck in traffic is the better option. Houses can handle an awful lot of radiant heat, and they'll survive perfectly fine a radiant heat flux far beyond what a human can tolerate. You just can't let direct flame impinge on them for very long before it spreads.

LorenPechtel 5y ago

Yup. The only defense against firebrands is to build such that your house won't catch fire in a rain of firebrands. Locally, a combination of stucco and concrete tile roofs can make a house basically immune to fire rain--but concrete tile roofs are not an option in most places.

Syonyk 5y ago
You don't actually need non-flammable roofing - just a roof rated for the fire type. Check Class A roof tests. A lot of asphalt shingles, properly installed, actually are Class A fire rated - which is a 12x12 2kg grid. https://www.builddirect.com/learning-center/building-materials/roofing/roof-fire-ratings-guide/ https://www.builddirect.com/learning-center/building-materia... Interior corners on the roof or house tend to cause problems, though. Debris builds up and ignites, and just impinges on the siding until you light something behind it. You can also have firebrands entering through attic vents and bypassing the roof - there are vent types that are resistant to this, but tend to cost a good bit more. Why we don't do this in urban wildland interfaces... no idea.
viraptor 5y ago
Or keep the roof wet. As long as your water supply exists, roof sprinklers work. They're popular in Australia with people living in bushfire prone areas. https://build.com.au/roof-sprinklers https://build.com.au/roof-sprinklers (It also helps that bush properties have large water tanks attached to them)

davidbanham 5y ago

A firefront moves a lot faster through grass than through forest fuels given the same moisture content. Totally different strategies required to fight them. They don’t throw embers like a crowning bushfire, though.

davidbanham 5y ago

Out of curiosity I plugged Lytton’s weather readings into the Mccarthur tables we use to calculate forest fire danger in Australia. I’m using guesstimates for humidity and wind speed based on the spotty data available from the local government website. Also just totally fudging the fuel load number. I’m reading spotting distances between 1.5 and 2km. Overall FFDI of 41 so a bad day but not extreme by our standards. Would have been a lot worse with lower humidity.

moistly 5y ago
Parts of the province were down to 11% humidity. Lytton is renown for being extremely dry.