Ash pits and weak trees: Wildfire dangers to watch for when returning home

As some wildfire evacuation orders end, thousands of Albertans are returning home to a different landscape: black and grey soot-covered ground and trees that officials say should be taken seriously. 

CBC News spoke with Bart Johnson, deputy fire chief of operations for the County of Grande Prairie Regional Fire Service, about potential risks after a fire.

The Town of Drayton Valley has also posted a video of dos and don'ts about the burn zone, with tips and comments from local firefighter and incident management team operations leader Cole Starling.

Answers have been shortened or condensed for clarity. 

Q: What is an ash pit? 

Johnson: Any depression in the ground or hole in the ground that pre-existed could be filled with ash. So a lot of the trees and the shrubbery, the deadfall would burn and turn into ash because the fire is burning quite hot and that ash will accumulate in those low spaces. … When bits of tree roots or that kind of material is burned underground, and because the roots burn out, it creates a void space and that void space now is filled with hot ash.

Q: What do you recommend people do if they encounter an ash pit?

Johnson: You might not even be able to see them. And so what I would recommend is that none of the residents returning should be walking around in what we would call the black area of the fire, because these types of ash pits may exist for a period of time, and quite frankly, they could be there for days or even weeks.

Q: How deep can ash pits get? 

Now the fire will burn down into the root system through the peat moss. And the peat moss itself might not burn at all, and it holds that heat down like a thermal blanket. How deep that goes would depend, I think, on how significant the root system was for that particular tree. I would say that it's unlikely to be much deeper than two or three feet deep, but if you weren't expecting it, that could be a significant fall hazard. 

Q: What can happen if someone steps in an ash pit? 

Because it's filled with hot ash, the potential for burns of the skin is significant. It's like any campfire that it has burned down to where you just have the white hot coals. ... So you know the part of your campfire where you get the best wiener-roasting coals or the best marshmallow-toasting part of the fire, where there's not a lot of flame, but it's very, very hot? That's what an ash pit would look like, and it could be two or three feet deep.

Q: What risks do trees pose? 

Johnson: What happens with some trees is that the fire burns up, and particularly with poplar [trees], they can burn three-quarters of the way up and they can burn in a three-foot space … in that one narrow spot. What happens is that area of the tree becomes very weak, as you can imagine, and then any kind of wind can make the top of it fall down. 

Even a small tree … one that's like perhaps 30 feet high, but maybe only six or eight inches in diameter at the base — that top part of that tree could still hurt significantly as it fell from that height.

Starling: Typically, conifer-style trees have root bases that spread out horizontally across the ground. A lot of the substrate and soil types we have in this area are peat moss. With this peat moss being burned off the trees, these root systems are now exposed, which makes the trees very susceptible to falling. We've seen a lot of this.

Q: When might it be safe for people to walk around their properties again? 

Johnson: These ash pits could smolder for a number of weeks. So, too, could some of these larger logs that may have fire that's been burning deep inside of it. We've tried to mop up most of these hot spots. We've flown drones with thermal imaging cameras in them multiple times, and continue to do so.