Highway 97 – Dale/Dragon Drainage

250px-Dendroctonus_ponderosae Highway 97 – Dale/Dragon The spread of Mountain Pine Beetle (MPB) infestation in the forests of BC in recent years has raised concerns about the effect on hydrology and as a consequence, the adequacy of highway drainage structures in MPB affected areas.

The Dale Lake Road to Dragon Lake Road (Dale/Dragon) segment of Highway 97 is located immediately south of Quesnel, BC. The pine beetle effect concern was raised in conjunction with the project, because a part of the watershed draining towards the highway is covered by pine forests, therefore suspect for MPB infestation. The BC Ministry of Transportation and Infrastructure (MoT) requested recommendations for a methodology for considering the “pine beetle effect” on the design of highway drainage structures.

Several reports have described the effect of MPB infestation on hydrology, primarily due to the removal of vegetation by fire or salvage logging. However a BC-specific method that would help to quantify the effect for use with design projects was not available at the time.

The effect of fires on hydrology is well documented in California. Based on several decades of experiences, the County of Los Angeles had developed a hydrologic method, called the “Burned Watershed Hydrology”, for estimating the increase in runoff rates after fires, and the effect of debris load on flows due to the increased erosion on the de-vegetated watershed. While the geography, climate and the reasons for fires are very different in California, the effects on hydrology are similar to those in British Columbia. Therefore, the Los Angeles County’s Burned Watershed Hydrology was adapted to the topography, soils and the climate of the Quesnel area and used in the design of the drainage structures for the Dale/Dragon project. Hydrologic and hydraulic analyses for normal watershed, burned watershed and for debris loaded runoff were completed by using the XPSWMM model and GIS analyses.

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