Wildfires and Large Cities

Could an out-of-control wildfire enter a large city (like, say, Colorado Springs or Fort Collins or Boulder) and destroy it?
I know these fires have destroyed subdivisions, but apart from (presumably) seriously increased firefighting efforts, is there something about the composition of a large modern city, particularly its downtown, that would prevent this from happening?

Yes, I know about the Chicago Fire and Wallace Idaho and all that. I mean a large (<100,000) American city in 2012, with average greenspace, lots of roads and streets, tall buildings, the works.

I guess I’m just thinking that the core parts of a large city may be made of stuff that’s not especially flammable, unlike housing developments that are made out of wood and are surrounded by fuel (viz., trees, shrubbery, and grass).
I’m aware that large steel and glass skyscrapers, brick buildings, and the like occasionally catch fire but it seems that usually happens because of forces within (burst gas line, arson) rather than without (hot embers landing on the roof).

Most cities are not built in the forest…

My folks are always asking if “we are OK” when they hear about fire in Arizona. The reality is, there are plenty of little brush fires that occur within city limits every year, but there simply is not enough close-spaced fuel (trees) to allow a wildfire to propagate in the city.

I live in an urban forest. If the conditions were right, like a prolonged drought, we could easily lose much of the urban area. Here is a Satmap of Seattle you can see how green it is. A lot of the urban forest is evergreen, that, in dry conditions, are basicly torches. There is also lots of under growth that would lead a fire from house to house.
We, at least, have streets wide enough for fire trucks to access neighborhoods, unlike the Berkeley Hills fire in 1991.

I was in Canberra, Australia some years ago when significant parts of the city were burned to the ground.

The areas that burned were of low density housing, with sparse vegetation. In contemporary fire propagation theory, the areas that burned should have been “safe”.

The reality was somewhat different, and shocked all observers.

As a consequence of those fires, many of the established theories of fire propagation are under revision.

I understand that there is still a lot of confusion as to why those fires occurred in a manner so contrary to the accepted theories of propagation.

So I suggest that the answer to the OP question is: don’t assume you are “safe”; always assume you are in deep doggie doo, and act accordingly.

The Berkeley Hills / Oakland Firestorm was a good example of the problems with Eucalyptus in California. It’s not just that we have residences butted up against the forest, but that a lot of the forest in areas close to human habitation has been taken over by giant tiki torches that were introduced in the 19th century.

No kidding. I was in Oakland then. I still remember that day. It was terrifying. I wondered what would happen if the fire got into urban areas as so many of the buildings are two, three, four story wood frame, ie, fuel. Recently we had another small fire in the hills and I and probably a lot of other people had deja vu. Fortunately it turned out to be minor.

Yes, if the conditions are just right.

Large wildland fires can self-sustain themselves to a degree greater than anticipated by creating their own weather, using just one example. As long as the basics of the fire triangle remain intact (heat, oxygen, fuel) to allow the fire to create a micro-climate, that micro-climate can alter fire behavior to keep it self-sustaining longer than a break in the fire triangle would allow. In other words, the micro-climate can draw in more and more oxygen than ordinarily available. Simultaneously, that helps to sustain the higher heat levels. At the same time the increased oxygen and higher heat values may cause a poorer quality fuel supply to burn. And so on.

Portland, Oregon, may be one large city that can be devastated by such a fire. The topography, forest canopy and number of single/multi-family dwellings look to be sufficient to do the dirty work. Portland has a metro population in excess of two million. Add in the area’s strong conservation/environmental ethic, and the desire to bring as much “wildland” inside the city limits (Forest Park is a case in point) and the model for urban fire as you suggest is very, very strong. Yeah, downtown Portland is a concrete jungle, but there’s a hellofa lot of remaining in the city that is green and full of timber frame homes. picunurse is correct with respect to the Seattle metro area. I venture to state I think Portland has a more volatile mix than Seattle.

However, comparing Portland or Seattle to the 1991 Berkeley Hills / Oakland Firestorm offers a false comparison. The Berkeley Hills have an extensive introduced Eucalyptus tree problem. Australian eucalypts don’t really burn as much as the explode! The high oil content of eucalypts means a wildland fire causes the oil to vaporize in the air and set the stage for a natural BLEVE fire. The 1990s Canbera and Sydney fires were made worse because the eucalypt’s oil vaporizes before exploding in a BLEVE fire. In turn, they also generated huge fireballs that blew through neighborhoods with tremendous damage. That just does not happen at that level with native North American trees.

The Waldo Canyon Fire in Colorado is the fire in the news, and rightly so. But as of 530 am MDT today, there were 242 new fires reported in the previous 24 hours, 11 of them being large. In total there are 41 large fires burning across the country. Still, fire support is relatively low, and gearing up real fast. We got a callout request just today asking anyone with red cards who are unassigned of our availability status for the next two weeks. A callout request out like this, this early in the fire season (relatively speaking) is not unheard of, but ominous. The prognosticator national drought map should scare the crap out of everyone.

It depends on the town. Many bedroom communities are built in wooded areas. These get taken out. Some towns are intentionally designed to resist the spread of fire.

The Mormans lost a couple of towns to fire early in their history. They responded by laying out towns with extremely wide streets, creating fire breaks that would tend to limit destruction to a single block. Prescott, AZ (the old part) has residential streets you can U-turn on even when cars are parked on both sides, and there is still parking strips between the street and the sidewalk. I think there are some other places in the Southwest with Morman influanced wide streets, but Prescott is the one I am very familiar with.

well, heh, the house my family inherited is 1/2 mile from the mandatory evacuation zone and last I saw, certainly the plant life (bone dry most of the year) would sustain a fire…

On the other hand there is belt of development which most of the land is cleared, and I’d think that while things can catch on fire, once it enters an area with much lower density of combustible material that it would be much easier to control compared to say random fire bombing of a city.

I live in Mountain Shadows in Colorado Springs, I can answer the op in the affirmative, with the exception of a completely built up downtown. 5 houses within rock throwing distance of my house burned to the ground. What might be hard to realize is that there is no direct connection to the forest and my neighborhood. The natural terrain leading up to the forest is very rocky, with yucca, cactus and grass mixed with shrubs and the occasional tree. That material combined with 65 mph winds were sufficient to create a raging inferno. If you look at our landscape, you can see that the fire came out of of just a few natural “weakpoints” relatively low lying areas with more trees and shrubs than surrounding areas. Anywhere there was fuel within tens of yards was easily reached. The Flying W ranch was in the forest, though, and the worst of the damage came through its location.

If the fire had spread another mile or so, I’m not sure what would have stopped it. It would then have been in a very large, forested park from which it could easily have jumped the interstate and spread throughout town. What did stop it was 1200 skilled firemen with an endless supply of water. The firemen not only used the fire hydrant at the base of our property, but also the two hoses my wife left outside. (Yeah, I was gone until the worst was over.) It is amazing to look at intact houses surrounded by a moat of lawn and that surrounded by completely burned natural landscape. It is also humbling to look at streets for which every house is leveled.

If this had been a forested area and these were mountainous homes, you could dismiss it as unlikely to occur at other large cities. You can’t even blame Eucalyptus trees. In this case there is only naturally occurring, sparse pines, and planted trees. The well watered landscaping trees seem to have done ok. The only part of our yard that burned was yucca and cactus. I suspect the firemen prevented the naturally occurring brush from igniting, which might have endangered our house. Where I grew up in PA is much more heavily forested. It’s just that those trees are deciduous, and usually well watered.

People are talking about concrete buildings and streets preventing a fire from spreading. But wouldn’t this be mostly irrelevant in case of firestorm (I don’t know how common those are)? Wouldn’t anything that can burn in a large area actually burn in such a case?

A street wouldn’t do jack, as it wouldn’t take much wind to jump a street. Video of our fire seems to show the fire jumping at least 100 yards due to wind blown debris. Concrete won’t combust whatever you do, though. It seemed like many homes burnt once their windows shattered, and flames came inside, most once the roof caught. As long as there is something to burn inside, I could see a concrete building burning down. It is hard to see a fire spreading much once it was confined to the insides of buildings, though.

Are shake roofs still common on new construction?

With reference to my post about the Canberra fires, above:

Apparently in subsequent investigation of these fires, it was found that skylights were a significant contributor to the fire damage.

Apparently, embers settled on the plastic domes and set them alight, the subsequent melted and burning plastic fell down the light tubes and ignited the inside of the homes.

The heat and flames from the melted and burning plastic were so intense that people were unable to extinguish this source of ignition, and the houses burned.

But the big issue was, apparently and as noted by others above, that the firestorm occurred and was propagated despite the apparently low fuel loading.

According to contemporary theory, the firestorm should not have happened, and they are still trying to figure out why it did.

The best theory at the moment is that it was those eucalyptus oils once again. The fire was sufficiently hot to volatilise oils several hundred metres away from the fire front. Being significantly heaver than air the oils ran down waterways and slopes and pooled in flat areas.

The result was some truly bizare behaviour, such as fire burning with flames several metres high over ovals with lawn mown a few inches high, firefighters becoming trapped by fire fronts coming up behind them over breaks that had been burned bare just minutes before, fires literally blasting through culvert pipes under roads and fires described as “pouring” along tarmac roadways and carparks.

IOW a freakish set of circumstances basically filled thousands of acres of land with flammable gas, that then sat and waited until the fire front caught up with it. A truly frightening thought.

It’s worth noting that while eucalypts seem to be the worst plants the worst for this problem, there is evidence of it happening with pine trees under similar circumstances.

No. The houses that had shake roofs were left from the 80’s. The original developer actually made shake or tile a requirement. Most had been replaced: fortunately we get more hail than anywhere else, so the shake roofs would eventually leak and insurance would cover the cost. But even fire resistant materials burn, once the fire is hot enough, and hot embers were blowing incredible distances. Being down a steep hill was a actually a good thing, because embers blew over those areas.

Very interesting; your comments do seem to explain some of the things I saw while watching the fires in progress, particularly the fireballs I saw forming over seemingly bare ground.

Do you have any links to any further analysis of this that I can read up on?



Excellent! Thank you.

After reading these papers I now have a clearer understanding of some of the things that I observed at the time, and which have puzzled me ever since.

For one, I have been puzzled as to why the Mt.Stromlo Observatory telescope enclosures burned. Despite the fact that the telescopes were housed in poured concrete housings, and set back from the up-slope, their interiors were destroyed, and the telescopes melted to puddles on the floor.

Contemporary logic had it that they should have been shielded from radiation by their position and protected from flame by their construction, and therefore should not have suffered any more than blistered paint on their outside walls.

However, the papers referenced suggest a mechanism that may explain their destruction: the fire front in the surrounding pine forest distilled volatiles and formed pyrolysis off gases which would have been forced up-slope by convection, and which then accumulated on the level ground around the telescopes, and within the telescope housings. These then then flashed with the arrival of the flame front.

This is markedly different from other fires I have observed where volatile oils were distilled from trees ahead of a fire front, and the fire then flashed through the crowns of the trees, fuelled by these distilled oils.

The difference between the Mt. Stromlo Observatory fires and conventional crown fires would be in the time interval between the distillation and the flashing; in the case of crown fires, I have observed that ignition usually takes place within seconds.

On Mt.Stromlo, I speculate that there must have been an extended time of accumulation of volatile fuels due to the topography of the terrain; when the flame front reached the top of the slope, it ignited the accumulated gases, of which there were sufficient to melt the telescopes.