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Debris avalanche and debris torrent initiation, Whatcom County, Washington, U.S.A. Buchanan, Peter


Heavy rainfall on the evening of January 9 and morning of January 10, 1983 triggered debris avalanches and debris torrents at Smith Creek, western Whatcom County, Washington, USA. Nine debris avalanches are back analyzed in detail. Conclusions are drawn concerning, 1) climatic controls on debris avalanches and debris torrents; 2) debris avalanche characteristics; 3) hillslope hydrology; 4) slope stability. Rainfall data show that the January 9-10, 1983 storm had a 71-year recurrence interval in the 12-hour duration, with less than 6-year recurrence intervals in 1, 2, and 3-hour durations. In contrast, rainfall during a torrent event on January 29-30, 1971 had recurrence intervals of less than 2 years in all durations, but snowmelt was a contributing factor. The types of debris torrents produced by these contrasting storms are discussed. Four distinct failure geometries are defined, based on avalanche descriptions: 1) wedges; 2) drainage depressions; 3) logging roads; 4) discontinuity surfaces. Three scour zones are also distinguished, based on slope segment types observed. To model storm water table levels a one-dimensional, vertical, transient, saturated-unsaturated finite difference infiltration program is linked to a kinematic wave equation. Rainfall duration and intensity, initial conditions, soil hydraulic conductivity, and soil depth are factors controlling vertical soil discharge rates. January, 1983 discharges are clearly distinguishable from comparison storm discharges at all avalanches. Kinematic wave results help differentiate Coulomb shear and washout type failures, and provide pore pressures for stability analyses. The modified Mohr-Coulomb strength equation is used to outline factors controlling debris avalanche initiation. The factors are: 1) slope angle; 2) soil depth; 3) soil density; 4) vegetative cover; 5) bedrock surface characteristics; 6) snow. These factors are quantitatively assessed. Infinite slope analyses show limiting slope angles of 29.7° for Group I vegetation, and 24.6° for Group III vegetation. Vegetative cover and soil depth are the two controlling factors that change significantly over the short term. A root cohesion parameter, Cr, is used to assess the shear strength provided by vegetation. Four vegetative covers are distinguished, three of which were logged between 1918 and 1950: Group I - relatively weak understory vegetation (Cr range: 1.6 -2.0 kPa); Group II - understory plus stunted trees (Cr range: 2.3 - 2.6 kPa); Group III - understory plus mixed, regenerating forest (Cr range: 2.6 - 3.0 kPa); Group IV - old-growth forest of higher root strength.

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