Debris Flow

Risk: High

Debris flows are very fast-moving, liquefied landslides that carry soil and other debris including boulders, and vegetation. Debris flows are most often triggered during periods of intensive rainfall resulting in high soil saturation levels but can also be triggered by rockfall or avalanche on unstable slopes. Known debris flow hazards within Squamish include the Cheekye Fan and Stawamus River.

Concave slopes like gullies and ravines are particularly susceptible to debris flows because they tend to concentrate surface water runoff and accumulate surface water and loose soil. Debris flows can be the result of natural factors and man-made interventions (SLRD, 2015). BCG Engineering reports that due to the impacts of climate change, both the frequency and magnitude of debris flow events on the Cheekye Fan are expected to increase.

If you live near or visit areas prone to debris flows, you should become familiar with the terrain between your property and the creek channel and fan apex. During a major flood event, creeks may suddenly change course and flow along a new or abandoned flood channel, and debris flow material may run out onto the fan area. Terrain features to be aware of include: abandoned creek channels; levees; scarred trees; and local deposit features.

There are a number of vulnerabilities associated with the Cheekye Fan debris flow hazard in particular. This inlcudes the new residential development at the base of the Cheekye Fan, although this will be mitigated by the construction of a massive debris barrier. Within the current debris flow zone, there are several First Nations reserves, the neighborhood of Brackendale, a BC Hydro substation, Highway 99, and two schools. Most scenarios also show the debris flow running down the highway as the path of least resistance. This means the highway will be closed for many days impacting transportation to/from north of Squamish. The debris flow could also cause a dam outburst flood on the Cheakamus River, causing flooding as a secondary hazard.