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Debris-flow risk assessment and mitigation design for pipelines in British Columbia, Canada

Gartner, Joseph E.
Jakob, Matthias
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Abstract
Pipelines in mountainous terrain in British Columbia, Canada often cross debris-flow fans and channels along valley bottoms and can be susceptible to various geohazard impacts, including debris flows. The design of new pipeline infrastructure and maintenance of existing pipelines necessitates debris-flow risk assessments and appropriate mitigation design. A methodology is presented for assessing debris-flow risk along pipeline routes that consists of estimating the probability of a debris flow causing a pipeline loss of containment or disruption in service. The methodology consists of estimating debris-flow frequency, scour potential, and the vulnerability of the pipeline to break if impacted. Debris-flow frequency is estimated based on field observations of debris-flow deposits, degree of vegetative growth on debris-flow deposits, evidence of debris-flow impacts on trees near the pipeline crossing, documented debris-flow events, review of historical air photos and terrain mapping based on LiDAR-generated topography. Debris-flow scour potential is estimated based on channel morphology, presence of bedrock and grain size distribution of channel bed material. Vulnerability is estimated based on flow width and velocity and can be modified for different pipe diameters and wall thicknesses. Mitigation options for buried pipelines include those intended to decrease the likelihood of bed and bank scour (e.g. rip rap bed and bank protection), decrease the likelihood of the pipeline being exposed (increasing the burial depth of the pipeline) and to increase the resiliency of the pipeline to debris-flow impacts if exposed, (e.g. increasing pipeline wall thickness, adding concrete coating to the pipeline). The final option is to prevent debris flows from reaching the pipeline by designing and installing debris-flow deflection berms or sedimentation basins. The methodology presented is embedded in risk-informed thinking where pipeline owners and regulators can define probability thresholds to pipeline exposure or rupture and the pipeline designer needs to show that the proposed mitigation measures achieve these threshold criteria in ways that honor the ‘as low as reasonably practicable’ (ALARP) principle.
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