Two public information sessions held on Oct. 30 laid out the findings of the Level 2 “Post-Wildfire Geohazard Risk Report” conducted by the province in the wake of the Shetland Creek wildfire.
The fire — which resulted from the merger of two fires near Spences Bridge which were first discovered around July 12 — grew to more than 28,000 hectares and covered an area from Spences Bridge north to Oregon Jack Road (Hat Creek Road) and east to Highway 1. Some 20 structures — including at least six permanent residences — were destroyed in the Sarangati Village at Venables Valley.
Gareth Wells — a geomorphologist with the Ministry of Forests — said that two air reconnaissance sessions were followed by on-the-ground assessments of areas that were not clearly visible from the air or which were closer to homes. He described the effect of fire on tree canopies — which soak up rainfall — and on the ground, where vegetation and intact soil act as a sponge to absorb rainfall and snowmelt before slowly releasing it.
Fire degrades or destroys these elements, meaning that after a fire more water reaches the ground more quickly, and the soil is not able to absorb it. This leads to overland flooding, as well as to debris flows, which pick up ash or other debris left by the fire and leads to further erosion of the soil.
“It’s important to recognize these hazards are fairly imminent and pressing,” said Wells. “They’re a threat now with rain we could be experiencing into the winter, and they’re a threat through the winter if we have a warmish winter with a fair bit of rain and less snow. They will continue to be a threat into the spring when we see snow melt.”
Wells added that while these hazards will not last forever, they’re the most pressing in the first handful of years after a fire.
“Most of the post-wildfire natural hazards occur in the first year or two after the fire. Threats diminish over time as the vegetation begins to come back in the watershed, but in some of the most severely burned areas there might be considerable delay in vegetation returning, so those hazards might persist.”
Wells said that several areas were severely impacted, particularly Twaal Creek, Venables Valley, Oregon Jack/Three Sisters, and parts of Murray Creek. The dominant hazard causing concern was run-off, triggered by rainfall or rapid snowmelt.
In the Twaal Creek Valley, many slopes burned at moderate to high severity, and are subject to run-off related hazards like debris flows or floods. Wells noted that there was a likelihood of flooding at the outlet of Twaal Creek near the Hilltop Gardens Campground north of Spences Bridge. There is also concern for the possibility of flooding and debris flows on Murray Creek Road.
At Venables Valley, west-aspect hillsides were severely burned, and there is concern for homes located at the outlets of fairly large fans where water has flowed significantly in the past. “Post-fire we might see substantially more water flowing down the hillside and/or where it hasn’t flowed before. We’ve seen some signs of overland flooding hazards already.”
Trevor Bohay, Director of All Hazard Response for the Ministry of Forests, said that field work on a Level 3 post-wildfire geohazard risk report was expected to start around the middle of November, with the report expected in January 2025. He added that if the consultant found something “really shocking or alarming,” that information would be made public before the final report is released.
He added that while the province can and would put up signage on roads warning of potential hazards from debris flows or flooding, there was very little that could be done to prevent these hazards.
“We tend not to go out on Crown land and construct deflection berms; by the time they’re built the worst of the hazard has probably passed, so that’s not typically something that happens. We’ve tried aerial seeding, but that’s not overly useful.
“There’s no magic bullet to reduce the risk on the landscape. Typically it’s around awareness about what could happen.”
The Thompson-Nicola Regional District’s (TNRD) emergency program coordinator Michael Knauff said there weren’t really any surprises in what he heard from the report.
“It was very much as expected. Values in the watersheds that were burnt have some risk associated with that, which is very common post-wildfire.
“The main concerns with Venables Valley are weather-driven, specifically rain. With post-wildfire hazards we see more water, more quickly mobilized through drainage channels where it used to run, or running overland where it never used to run. As it picks up more material the concern is that it won’t just be clear water, but will have debris as well.”
At the afternoon session on Oct. 30, a Venables Valley resident said they had already noticed water movement around the house they were now staying in, and asked what they should do.
“On private property it’s up to individual homeowners as to what the best approach will be to divert or mitigate water,” replied Kevin Skrepnek, manager of community and emergency services for the TNRD.
“Upslope is largely Crown land, and we’re working with the province to see if there are actions that can be done, but we’d likely recommend that you secure some sort of professional who can look at what can be done on individual properties. The broader picture will be ongoing with the TNRD and the province as this unfolds.”
Knauff says that other residents who are concerned about possible flooding and debris flows should reach out to the TNRD, adding that the Level 3 report will be more specific and granular on what those threats will be.
“Vigilance and awareness is the first step. Let people gain some awareness so they can look at their own properties, be aware of potential threats. If a rainstorm is causing significant misbehaviour of the land, or if there’s a threat to safety, we are prepared to help them however we can.
“The [Level 2] report talks a little about burn severity, and that will link into overall recovery for the land base. We’re looking at the same timeline as with the 2017 Elephant Hill fire. The first year saw some very significant [flooding and debris flow] events, the second year saw some events of lesser magnitude. Now we’re going on seven years, and I’m not aware of slides from Elephant Hill anymore. It’s really dependent on Mother Nature.”
Wells said that no one can say how long complete hydrologic recovery in the area will take.
“We could be talking decades, we might be talking hundreds of years. It’s challenging to know what sort of conditions we’ll experience, particularly under climate change.”
