Once constructed, the Cheekeye barrier will be among the largest debris flow mitigation structures in the world, according to a staff report to council. A detailed design option was presented to the council yesterday. The District of Squamish will manage the mammoth structure at an estimated annual cost of $919,106.
BGC Engineering has prepared a design on behalf of the Sqomish Sea to Sky Developments LP, a company that has applied to develop 1,215 homes and commercial development in the Ross Road area on the Cheekeye Fan, an area that is subject to debris flow hazards.
Past studies and expert opinion have concluded that up to a 5.5 million cubic metre of debris can pour from the flanks of Mt. Garibaldi, then flow along Cheekeye River and then envelop areas through Brackendale and even Garibaldi Estates. The barrier will guard against this nightmarish scenario, reducing debris flow risk to a level that is ‘tolerable’ to the District of Squamish.
The barrier will be located near the upper Cheekeye Fan apex, close to the northeast corner of Alice Lake Provincial Park. This structure will be approximately 24 metre tall and can manage 2.8 million cubic metre of debris flow in a 3,000 to 10,000-year debris flow event. Composed of roller compacted concrete, the barrier is a symmetrical gravity dam with 45-degree inclined slope. A six-metre wide vertical slit outlet will allow routine Cheekeye River flows to pass but will restricts debris flow. Periodic maintenance will be required to manage sediment within the basin, including removal of boulders and logs that block the outlet.
The District independently completed an assessment and concluded that a barrier at the apex of the fan funded through development is the preferred mitigation option as it protects the greatest extent of the Cheekeye Fan and can potentially save the District tens of millions of dollars in capital expenditures through funding by a third party. The developer will pay for the barrier and District will manage it.
The BC Inspector of Dikes has policy that requires all new dikes (and barrier is considered a dike) to be owned and operated by a local or regional government to avoid issues with funding and orphaned structures. The District will be the owner and operator of the barrier, carrying significant responsibility.