DESIGN AND PERFORMANCE OF A DEBRIS FLOW PROTECTION STRUCTURE

In resource management, it is sometimes not possible to reduce the likelihood of debris flow initiation at a site. However, it may be possible to reduce the potential for impact to resources by constructing protection works in the deposition zone. Different types of structures can be used to stop, contain, deflect, or maintain the flow of potential events. The selection of a structure (or a combination of structures) depends on the gully system characteristics such as the magnitude of previous and expected events, the fan geometry, and the types of resources at risk.

In the Escalante River on western Vancouver Island British Columbia, a large gully system posed a high debris flow risk to a mainline logging road and a large fisheries stream. A berm was designed to deflect and contain potential debris flow events. Design considerations included the fan geometry, the design magnitude of a potential debris flow event, the streamflow activity at the site, the available materials for construction, the need to maintain access along the road, and the vulnerability of fish habitiat. The design for the berm structure was based on an expected maximum magnitude of 10,000 cubic metres.

The work was completed using natural, nearby materials with two excavators and a large all-wheel drive dump truck. Forest operation considerations led to relocation of part of the berm structure. The construction cost was about $65,000.

In the next two years, the berm structure was impacted by three debris flow events. The first two events were contained within the structure, while the third overtopped the berm. Comparison of these events with past events suggests the berm was effective in protecting the stream and the much of the mainline. The performance of this structure demonstrates that: 1) it is easier to deflect a potential flow than contain it; 2) debris flow structures need to be maintained; and 3) debris flow protection for resource management can be achieved at a reasonable cost.