25 October 2018
The Old Fort landslide in British Columbia
Earlier this month the small community of Old Fort, near to Fort St John in British Columbia, western Canada, was evacuated due to movement of the Old Fort landslide. In addition to potentially threatening some properties, the Old Fort landslide severed the only access road to the community. The landslide is large but quite slow moving. Alaska Highway News has a splendid gallery of images of the landslide released by the Peace River Regional District, which includes this image of the earthflow component of the landslide as it crosses the highway:-
This phase of movement of the landslide initiated on 30th September, although there is considerable evidence that it had been moving prior to this date. Local geologists, from Westrek Geotechnical Services, who have been studying the landslide, have diagnosed it as failure in bedrock in the area of a gravel pit operating on the hillslopes above Old Fort. The initial failure triggered a second failure in terrain to the west, allowing the development of an earthflow lobe that followed a pre-existing gully. The slide has a volume of about 8 million m³. This image, also from Peace River Regional District, gives an idea of the scale of the problem. Whilst the main earthflow is clear, the tension cracks and scarps extending across the hillslopes are also evident:-
Whilst this image shows the source area of the current main part of the Old Fort landslide:-
A temporary road is currently under construction across the landslide, and work continues to monitor and analyse its behaviour. As usual it is proving very challenging to forecast its likely future behaviour, as has been the case in the Mannen landslide in Norway. Winter is coming, and of course following that will be the snowmelt season in the spring, when groundwater levels could rise. Unfortunately this landslide could prove to be a challenge for some time yet, to the inevitable frustration of the inhabitants of Old Fort.