(Williams Lake) A landslide has piled debris 30 metres high and 600 metres long, damming the Chilcotin River in central British Columbia and creating a lake that officials warn could burst its banks and trigger dozens of evacuation orders and alerts downstream.
Margo Wagner, chair of the Cariboo Regional District, says water building up behind the debris “jam” south of the town of Williams Lake could build up to a level where it could start flowing over the dam. All that water could also undermine the debris mound and cause the jam to break.
“As the water continues to flow and accumulate behind this landslide, the pressure is increasing, but we have no idea when exactly it will all give way,” she admitted at a press briefing.
Authorities estimated it could happen Friday or Saturday. “We don’t know exactly when this landslide will give way. We hear it’s expected in 24 to 48 hours,” she added.
The landslide is “significant”, has a quantity of Mme Wagner, adding that authorities were not aware of any injuries except for a man who broke his leg while fleeing the slide, which happened late Tuesday or early Wednesday.
State of emergency and evacuations
The nearby Tsilhqot’in National Government declared a local state of emergency on Thursday and warned people to stay away from the river.
“At this point we don’t know what each day will bring, but we are prepared and have dealt with emergencies for thousands of years, including several landslides blocking the river in my lifetime,” Chief Joe Alphonse said in a statement.
Emergency Management Minister Bowinn Ma said at a separate news conference Thursday that if the dam were to break, dozens of evacuation orders and alerts could be issued along both banks and “people need to be prepared.”
“Let’s be clear, the risk to people and communities along the river is unknown at this time, but it could be significant,” she warned, adding that engineers, geologists and hydrologists were on site and in the air assessing the landslide and working to understand potential impacts downstream.
The minister ordered that people living along the Chilcotin and Fraser rivers for hundreds of kilometres to Hope, at the eastern end of B.C.’s Fraser Valley, are encouraged to gather their belongings and prepare their homes for possible flooding.
The evacuation orders extend across 107 square kilometres along the Chilcotin River, which the district says affects 60 properties, including 12 homes with about 13 residents.
Landslide-prone region
Mme Wagner recalled that the landslide occurred in an area that had been “burned” during the 2017 wildfires.
“There are many areas where trees are dead. They are still standing, but their root system is totally destroyed and they are not absorbing water, which is a problem.”
Dwayne Tannant, a professor of geotechnical engineering at the University of British Columbia Okanagan, noted that the region’s topography is “prone to landslides” and that while the 2017 fires were likely a factor, he doesn’t believe they were the primary cause.
“I think it’s the geology, the topography and the overall erosion that’s probably at fault here,” he said, stressing that the slide was not a preventable event.
“To do anything would not be an economical use of provincial resources, especially given the magnitude of this slide and where it is,” he said when asked about mitigation measures.
Mr Tannant said a landslide blocking a river was “relatively rare” but there was concern that slopes upstream of the blockage could become destabilised once the dam was breached, potentially creating a “domino effect” leading to further landslides.
Flood warning
B.C.’s River Forecast Centre has issued a flood warning for the Chilcotin north of the slide, saying the debris is “creating a lake” that extends several kilometres upstream.
The forecast center says the eventual breakage of landslide debris could also lead to “flash flooding” with water rushing downstream.
A flood watch has been issued downstream on the Chilcotin and Fraser rivers to Hope, while a high flow advisory has been issued for the Fraser River to its mouth in Metro Vancouver.