Unstable material remains at Johnsons Landing slide
After a geotechnical team visited Johnsons Landing this month, experts have more insight into what led to the slide and what is going on at the site.
While it may take months for the team to release their findings as to the cause, geomorphologist Peter Jordan said there is a large unstable area remaining near the source.
“There is an area that could conceivably slide again in the future,” said Jordan about the unstable area. “That doesn’t mean that it will slide, it just means that it is possible.”
Mapping by the geotechnical team will allow them to calculate what the volume of a future slide might be and how far it might travel.
“We can’t really predict when or if it would slide or whether all the unstable area would slide at once which isn’t that likely really, or whether a smaller area would,” said Jordan. “We do have to figure out just what is up there. It also gives us clues as to what caused it and what kind of materials are involved.”
Jordan has been onsite at Johnsons Landing since the slide in July and said compared to other slides he’s worked on, this one was “exceptionally big.”
“This is kind of a one in a million landslide,” he said.
“It was certainly impossible to predict a landslide that big could happen or even that a landslide could happen in that area. No big landslide has ever happened in that area before from what we can tell from our field work. There weren’t any apparent indicators a landslide of that size would happen.”
Jordan said the maximum volume of the unstable area is comparable to what came down in July.
“That doesn’t mean it will all come down at once,” he said. “None of it may come down or it might not come down for thousands of years, or it might come down next spring. It’s really hard to tell at this point.”
Throughout the fall and spring, Jordan and the geotechnical team will continue to watch the slide site to see how the coming weather affects stability of the area.
“We do need to see how the area responds to wet weather because it pretty much hasn’t rained since the slide,” he said.
“If there is wet weather in fall we want to see whether things move or shift and more importantly next spring what happens with the next snow melt.
“We don’t know if it is going to be a wet spring like it was this year, but certainly if there is some shifting of the ground that we can observe and measure it will tell us what is likely to happen up there.”
As experts watch the site in the coming months, residents will continue to rebuild.
Jordan said they will give residents and local government the approximate probability of a landslide affecting any location in the Gar Creek valley.
“Safe is a relative term,” said Jordan about rebuilding at Johnsons Landing.
“There are lots of hazards when you live out in the bush. There is wildfire and animals and any number of things, so safe is relative.
“Landslides can happen anywhere in the mountains, even in Nelson. It’s just a case of probabilities. With the information provided, local government and residents will have to look at the risk and decide whether it is acceptable.”