Trapping large magnitude debris flows induced by massive rock avalanche collapses: Cheekye debris barrier, Squamish (British Columbia, Canada)

Near Vancouver, on the west coast of Canada, the Cheekye River is subject to episodic torrential debris flows of very high magnitudes induced by collapses at altitude. Such collapses involving volumes of several million m³ have occurred in the region (Mount Meager, 2010; Joffre Peak, 2019). Numerous scientific studies show that a major event of the same type could occur at any time in the Cheekye River. Modelling has shown that the phenomenon threatens the Brackendale district of the town of Squamish as well as Highway 99, which links Vancouver to tourist resorts such as Whistler. The risk is considered too high by British Columbia standards, and mitigation measures are required.

Some fifteen different strategies were studied with a view to mitigating the risks associated with such an event. A multi-criteria analysis showed that the most appropriate solution would be a filter dam designed to trap almost all of the 10,000-year return period debris flow. The structure in question would be the largest debris flow trap in North America.

Our support was sought because of the extraordinary dimensions: 2,400,000 m3 of debris flow to be trapped in a structure around 30 m high. Sedimentation structures of the same type in France are generally 100 to 1,000 times smaller, and the largest French structures are still 10 times smaller than the Cheekye barrier. Filter dams designed to trap debris flows are rare in North America, so North American specialists lack experience in designing such structures. However, such structures are regularly used in Europe and Japan. BGC Engineering, the consultancy in charge of designing the structure, therefore sought the support of researchers from INRAE’s ETNA research unit, who had published some noteworthy research on the subject in recent years and who lead international working groups on these issues.

ETNA scientists helped to refine the functional design of the barrier: its type, shape and main dimensions. A specific modelling tool was developed for the occasion in order to characterise the operation of the structure in terms of storage and release as a function of the inflows and the obstruction rates of the various hydraulic components. In addition to their knowledge of the operation of these structures, this development provided an opportunity to apply possibilistic uncertainty propagation methods, which were particularly well-suited to the small amount of data available and the many doubts that persisted.

The mission to support BGC Engineering, a major North American consultancy firm, focused on the design of this protection structure against torrential risks on the Cheekye river.

The results of this appraisal, carried out mainly in conjunction with BGC Engineering and various local bodies, remain confidential and may not be published.