Title: Mass wasting in coastal Alaska
Identifier: CAT92272723 (find matches)
Year: 1969 (1960s)
Authors: Swanston, Douglas N
Subjects: Mass-wasting Alaska
Publisher: Juneau, Alaska : Pacific Northwest Forest and Range Experiment Station, U. S. Dept. of Agriculture, Institute of Northern Forestry
Contributing Library: U.S. Department of Agriculture, National Agricultural Library
Digitizing Sponsor: U.S. Department of Agriculture, National Agricultural Library

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approximately one-fourth inch per 12/ 1 "3 / year. -±-£' Such movement un- doubtedly produces a stress buildup in the soil over a period of time and may considerably increase slide sus- ceptibility of the soil in critical areas. Debris Torrents Debris avalanches on extremely steep (40°-60°) sideslopes of deeply incised, steep-gradient stream chan- nels or V-notch ravines are common to both soil series and occasionally produce massive debris flow and torrent flow deposits in the valley bottoms (Bishop and Stevens 1964). The debris avalanches begin in shallow soils developed in colluvium or glacial till. The slope gradients involved are far greater than the angle of internal friction of the particular soil, greatly intensifying any factors tending to reduce stability. The slopes are extremely unstable and on the verge of failure at all times. Downslope stress on the soil is in- creased due to the weight of added water during periods of high rainfall. Increased pore-water pressure or destruction of stabilizing root systems aggravates the condition and may initiate sliding, as may bank under- cutting near a stream. Usually these slides have short runs, beginning at the tops of valley sideslopes and end- ing at the streams (figs. 7 and 8). (a) Barr, D. J. Report of activities and results of the soil creep study (FS-NOR-l604:13), June-November 1965, Hollis, Alaska, (b) Barr, D. J. Report of the movement-measurement phase of of the Landslide Triggering Study (FS-NOR-l604:17). Both reports on file at the Pacific Northwest Forest and Range Experiment Station, Institute of Northern Forestry, Juneau. 13/ Barr, D. J. , and Swanston, D. N. Meas- urements of creep in a shallow, slide-prone till soil. (In preparation for publication, Pacific North- west Forest and Range Experiment Station, U. S. D. A. Forest Serv. , Portland, Oregon. )
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Figure 8.—Debris avalanches in steep-walled V-notch drainages. A, In till soil, Maybeso Creek valley. B, In bedrock-derived soil, Neets Bay. Note the masses of small roots overhanging the headwall of the slide. Many still remain firmly attached to the fractured and jointed bedrock substrata. 12

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