Michael J. Bovis

The role of debris supply conditions in predicting debris flow activity

Debris flow frequency and magnitude were determined for 33 basins in southwest British Columbia. Basins were first classifiedaseitherweathering-limitedortransport-limitedusingadiscriminantfunctionbasedondebris-contributingarea, an area-weighted terrain stability number, and drainage density. Multiple regression was used to predict magnitude, peak discharge, frequency and activity (frequency times magnitude) within each group of basins. Model performance was improved by stratifying the total sample of debris flow basins into weathering-and transport-limited groups. Explained varianceincreasedbyanaverageof15percentinthetransport-limitedsample,indicatingthatsedimentsupplyconditionsin themoreactivebasinsarefundamentalinpredictingdebrisflowactivity.Anindependenttestoftheregressionmodelswith 11basinsyieldedgenerallygoodresultsfordebrisflowmagnitudeandpeakdischarge.Predictionofdebrisflowfrequency provedproblematicalinweathering-limitedbasins.Themethodsdevelopedhereprovideestimatesofdebrisflowattributes in basins for which few data on past events are available. Copyright # 1999 John Wiley & Sons, Ltd.

The landslide response of alpine basins to post-Little Ice Age glacial thinning and retreat in southwestern British Columbia

Abstract The role of post-Little Ice Age (LIA) Neoglacial retreat on landslide activity is investigated in 19 alpine basins along the upper Lillooet River Valley, British Columbia. We examine how Neoglacial scouring and glacial recession have modified hillslope form and slope stability, and construct a decision-making flowchart to identify landslide hazards associated with glacial retreat. This work is based on field mapping, GIS analysis, statistical associations between landslides and terrain attributes, and a comparison between Neoglaciated and non-Neoglaciated terrain within each basin. The bedrock landslide response to glacial retreat varies appreciably according to lithology and the extent of glacial scour below the LIA trimline. Valleys carved in weak Quaternary volcanics show significant erosional oversteepening and contain deep-seated slope movement features, active rock fall, rock slides, and rock avalanches near glacial trimlines. Basins in stronger granitic rock rarely show increased bedrock instability resulting from post-LIA retreat, except for shallow-seated rock slides along some trimlines and failures on previously unstable slopes. In surficial materials, landslides associated with post-LIA retreat originate in till or colluvium, as debris slides or debris avalanches, and are concentrated along lateral moraines or glacial trimlines. Significant spatial association was also observed between recent catastrophic failures, gravitational slope deformation, and slopes that were oversteepened then debuttressed by glacial erosion. Eight out of nine catastrophic rock slope failures occurred just above glacial trimlines and all occurred in areas with a previous history of deep-seated gravitational slope movement, implying that this type of deformation is a precursor to catastrophic detachment.

Uphill-facing (antislope) scarps in the Coast Mountains, southwest British Columbia

Uphill-facing scarps are examined, at Affliction Creek, a glacierized area within the Meager Creek Volcanic Complex of southwest British Columbia. Scarps occur directly downslope of tension cracks and are considered to have developed from a combination of (1) antithetic movement between partly toppled slices of rock, defined by a master joint set dipping into the slope; and (2) erosion of the upslope face of each tension crack, leaving the downslope face prominent as an antislope scarp. A kinematic test for toppling failure is described. Glacial erosion and rapid glacial downwasting are probable causes of toppling. Lichenometric data indicate significant scarp development since Affliction Glacier attained its maximum Neoglacial dimensions about 140 yr ago. Dilatancy is cited as a possible mechanism for scarp stabilization.

Extensive deformations of rock slopes in southern Coast Mountains, southwest British Columbia, Canada

Extensive deformations of mountain slopes occur in crystalline intrusive rocks in the southern Coast Mountains of British Columbia. Typical morphological evidence of slope movement includes extensive systems of tension cracks, grabens, and antislope scarps (collectively referred to here as linears). These landforms involve displacements along penetrative joints observed in surface exposures. Kinematic tests on rock-structural data indicate that the observed patterns of linears are generally consistent with the feasible gravitational movements along the dominant discontinuities. Most sites indicate sliding as the most likely initial mode of movement, followed by or accompanied by toppling and toppling-induced sliding movements, and do not support the view that linears are the traces of active faults.

A model for debris accumulation and mobilization in steep mountain streams

Previous work on the initiation of debris flows has emphasized the roles played by material strength, stream gradient, and fluid pressure, but in most published models the friction angle (φ′) of the channel material is assigned some characteristic or constant value. The model presented here retains gradient and pressure as variables, and considers the probable changes in φ′ and hydraulic conductivity, K, of channel debris over time. Preliminary results from the Howe Sound area in southwest British Columbia suggest that stream reworking may lead to small increases in φ′ and large increases in K, rendering channel debris more stable with time. This is partially offset by a local increase in channel gradient as debris accumulates. These factors favour the growth of large, marginally stable debris deposits, and may lead to high-magnitude, low-frequency debris torrents in channels not steep enough to produce torrents directly from hillslope failure events.