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UBC Theses and Dissertations

Factors affecting post-logging debris flow initiation in steep forested gullies of the Southwestern Canadian cordillera, Fraser Valley Region Brayshaw, Drew Devoe


This thesis examines the factors which influence debris flow initiation in gullies. The analysis covers 53 gullies located in two neighboring, but separate, regions: the Chilliwack Valley region of the Cascade Mountains, and the Norrish Creek area of the southern Coast Mountains, respectively located to the south and north of the Fraser Valley in southwestern British Columbia. Lithology varies greatly between the two regions, but climate and logging histories are similar. All gullies studied had been logged during the period 1981 to 1990. Gullies were selected on the basis of their having experienced slope failures which potentially could have caused debris flows to occur. A wide range of geotechnical and morphometric terrain variables was measured in each gully system, at three different spatial scales: that of the entire gully system, that of the individual gully reach, and that of the individual slope failure. Gullies were also evaluated by the method of assessing Debris Flow Initiation Potential (DFLP) used in the Gully Assessment Procedures (GAP) section of the B.C. Forest Practices Code. Analysis of data showed that the most important variables influencing debris flow initiation in gullies were surficial material, gully wall gradient and length, and channel gradient, although not exactly in the manner which the DFLP assessment had predicted. Volume of the initial failure, amount of sediment stored in the gully channel, and angle of entry of the failure into the channel were other parameters which proved to be important. Regional variation in lithology seemed mainly to affect gully morphology, and not to influence debris flow initiation directly. Revisions were suggested for the DFLP assessment based on the observed behaviour of the most important and most easily and accurately measured variables. The current method uses the parameters of gully wall slope angle and surficial material type to qualitatively estimate the chance that a wall failure will occur, and the parameters of gully wall length and channel gradient to qualitatively estimate the chance that a slope failure entering the channel will initiate a debris flow. The two estimates are then combined into an overall estimate of the qualitative probability of debris flow initiation, using a three-class system of high, medium and low hazard. The revised method of DFLP evaluation devised in this thesis is similarity qualitative. The revised estimate of the chance of slope failure is determined using gully wall length in addition to wall slope gradient and surficial material type, and the chance of debris flow initiation from failure is determined using channel gradient only. The resulting estimate of debris flow initiation potential uses only two qualitative hazard classes, high and low.

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