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Morphometric and geotechnical controls of debris flow frequency and magnitude in Southwestern British Columbia

University of British Columbia

Morphometric and geotechnical basin characteristics are examined to assess their influence on the frequency and magnitude of debris flows. Frequency is assessed primarily from the dendrochronologic record. Magnitude is determined from a combination of field surveys of deposits and empirical methods, and calculations of probable magnitudes reconstructed from field evidence of peak discharge. Typical magnitudes are determined for basins in which sediment supply and debris transport limit the range of possible debris flow magnitudes. Sediment storage in channels is quantified by using an erodibility index in order to quantify the largest expected magnitude. Although this index requires refinement, it points to the sediment distribution pattern along channels, and in some cases allows extrapolation to unsurveyed channel reaches. Debris flow basins are classified into weathering-limited and transport-limited types according to their debris availability. This classification proves useful when estimates of total event volume are made from peak discharge evidence, and significantly improves the explained variance in the prediction of both frequency and magnitude. Discriminant functions are then developed to formalize the a priori identification of basin type. A sample of 12 basins is used to test the discriminant functions. All test basins were correctly classified which highlights the usefulness and regional applicability of this approach. Morphometric and geotechnical variables were measured on debris flow basins and subjected to regression analysis to identify the most important variables in accounting for the observed debris flow activity. Validation by an independent data set has shown some promising results. However, the overall performance of the regression model indicates the difficulties in reliably estimating debris flow activity. Large deviations between the predicted and observed values probably reflect the insufficient quality of the test data set. The study emphasizes that simple univariate and bivariate models can be used to draw geotechnical envelopes for debris flow activity, but lack higher predictive power. Prediction can be significantly improved by classifying basins as transport- and weathering limited prior to the regression analysis. Both model establishment and model testing require higher quality data than are presently available, which raises the need for more regular monitoring of debris flow channels Longer records of debris flow frequency and magnitude will benefit geomorphologists in understanding the tempo of geomorphic change, and will aid engineers concerned with the appropriate management of sites and structures threatened by debris flow.

Thesis/Dissertation

application/pdf

2009-03-17

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http://hdl.handle.net/2429/6147

Geography

University of British Columbia

UBCV

DSpace