Check water management around Whitehorse bluffs, advises landslide expert

An expert who studies landslides says last Saturday's incident in Whitehorse wasn't the first and likely won't be the last.

John Clague is a retired professor of earth sciences at Simon Fraser University who specializesin geological hazards such as earthquakes, tsunamis, landslides and floods. He saw pictures of the landslide and said he was surprised at the amount of water that was in the sliding material.

He was also surprised by how far it went, crossing Robert Service Way, the Millennium Trail and spilling into the Yukon River.

Whitehorse Mayor Laura Cabott said Tuesday the city didn't know when Robert Service Way, one of two thoroughfares into downtown Whitehorse, would reopen. She said the area was too unstable to have crews clean up the debris. She added the city engaged the services of a landslide expert to help them determine the cause of the slide. That expert, who is not Clague, determined an excess of moisture, or water, in the soil caused the slide.

Clague spoke with Elyn Jones on CBC's Yukon MorningThursday. Their interview has been edited and condensed for clarity.

You've seen the photos [of Saturday's landslide] online. What do they tell you?

There was an initial slide of material and it was very, very wet. You can tell this because the material mobilized and flowed and bulked up as it came off that slope and crossed Robert Service Way and actually traveled a surprising distance over the ice on the river, about almost 100 meters.

There's evidence that you can see, on that slope, of events in the past. Just to the south of the river on that slope, you can see some scars that come down there, like avalanche tracks that indicate that there's been another failure off that same slope. So this wasn't a one-off event. But what really surprised me was the travel distance.

Why was that surprising to you?

Well, it just indicated that it was flowing, that there was a lot of water involved. There was a lot of water coming out of that slope at that level, just about 10 meters below the top of the bench, below the airport level, and [it] told me that there was a lot of water within that uppermost package of soils. And that is largely sands there so they're very permeable or porous they can store a lot of water. It appears that water was coming out of that slope over a larger distance along Robert Service Way. There's a lot of little tracks, but this one was very, very large, so it engaged a lot more of the material on the slope.

Would something like this happen because of all the water in the soil, the saturation?

Yeah. Normally, those types of sediments, they're old ancient ice age lake silts. But at the top they're sands. And so they can hold water. You've got a big bench on top that allows water to penetrate down through the soil. It moves easily through that uppermost sandy layer and then kind of pools on the silts below. The silts can't transmit the water as easily.

You can get a build up of water that you don't see in the subsurface below that bench. And it kind of made me wonder, why is that? This doesn't happen every year. Why did this event occur? Why was there so much water coming out along the slope there?

You've had a couple of heavy years in which you've had more snow than usual. So that could be a factor.

But I also wondered about how the airport manages snow removal. Where does that snow go? Do they pile up anywhere on that bench? There was so much water coming out of that slope, it surprised me.

We've seen sloughing in the area at the bottom in that immediate area, that covers the train tracks. There was a slide just south of that area just last year. When you see that movement, what does that tell you?

It tells me it's unstable. Normally, sediments like that, if they're dry, not a problem. Even on a steep slope, you just don't have a problem. So the water is the bad actor in all this.

My big question would be... somebody needs to look at the water management on that bench above [in the airport area] because that is what kind of driving a failure like that and probably has way back in time.

What's the possibility of it happening in another area of downtown?

That, I don't know. I know around the airport, you've got this thick sheet of sand that sits at the top and I think that's a factor in this.

But as you go around the corner, you can see there are houses pretty much at the base of the slope. I don't see quite the evidence for instability in the past because most of those slopes are covered by trees and they don't seem to be reasonably active. On the other hand, if I were the city, I would look at the entire slope.

You just don't want to take a chance of somebody being impacted. I don't want to scare anybody, but you know, living at the bottom of a very steep slope is always something you're kind of wondering about. You want to be sure that that slope is stable.

I think the biggest problem is right along Robert Service Way before you enter the city. That's a very bare slope and it shows all this evidence of this process having operated in the past.

What sort of things can be done to try to stop this from happening again?

You could engineer barriers to the bottom, but I don't think that's very practical. There's not much room there. First of all, a geoscientist needs to look at the issue and confirm that water is the bad actor and then you need to think, how can we manage that water better?

Was it just the fact that there was a lot of snow or does the way the airport-managed snow removal come into play at all? I don't know but that would be the tact I would take.

If it turns out thatthere is an issue with water getting into the slope at the top, you can begin to think about how we deal with that.How do we remove snow and move it somewhere else or those types of solutions?

Over the long term, what should we expect if we continue to get more snow in our winters, as we have the last two years?

Well, that won't help. You don't want more water in your landscape. Water in these still steep silt bluffs don't mix well.

We don't know how climate is going to change in terms of whether you get more precipitation or less. I mean, there are forecasts, but there's a lot of uncertainty in those forecasts.

What would you do now if you were the city?

I would ask the Yukon Geological Survey to have somebody look carefully at that water management issue.