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CONSERVATION: Reproduced from vol. LX, No. 4, Dec. 1973, EXPLORERS JOURNAL, with persassion fro* The Explorers Club, New York, copyright 1973, by the FOREST SERVICE, U.S. Department of Agriculture, for official use Judging Landslide Potential in Glaciated Valleys of Southeastern Alaska A question of close examination followed by stratification into zones of varying stability RB1OVE FRE PLEASE DO NOT by Douglas N. Swanston Introduction helicopter was a welcome sight as it circled the small muskeg clearing in which T we stood. It was cold and rainy, a typical fall day in southeastern Alaska; and we were wet, bone tired, and full of devilsclub thorns from our last slope traverse. As the pilot settled his machine onto the muskeg surface and cut the HE Fig. 1—A typical landslide developed under natural conditions in this rugged terrain 214 I DECEMBER, 1973 engine, we hurriedly lashed our gear to the pontoons, anxious for a hot-buttered rum and a spot close to the campstove. It had been a good field session. In a little less than 10 days, we had completed an aerial and field reconnaissance of the stability characteristics and landsile potentials of a proposed timber harvesting area of approximately 165,000 acres, identifying and inspecting natural landslides and zones of potential landslide occurrence to determine how the land might slide and what might trigger the action. In the roadless area of rugged terrain in which we were working, this required extensive slope traversing with repeated helicopter pickups on the valley floor and lifts to the ridgetop (fig. 1) , a costly and time-consuming process; but by careful analysis of the data thus accumulated, the area could now be successfully stratified into zones of varying stability and effective land use recommendations for each zone developed. Steep-walled, glaciated valleys characterize this terrain. Careful reconnaissance is essential prior to any contemplated land use because of the natural instability of many of the slopes and the high probability of accelerated erosion following surface disturbance. Past investigations had demonstrated that landsliding, or erosion of the land surface by the downward movement of a soil mass, primarily under the influence of gravity, is a dominant form of erosion in coastal Alaska; and the area we had just surveyed was no exception. Slope gradients. were characteristically steep due to active and recent glacial and tectonic activity. Weathering processes were predominantly mechanical; and the soils, which are still in an early stage of development, were dominated by shallow, coarse-grained, highly permeable types Fig. 2—A surface view of the landslide shown In fig. 1 showing some of the important characteristics determining stability. Parent material is a glacially smoothed granite covered with less than 12 inches of soil. Slope at point of failure is approximately 40° overlying bedrock (fig. 2). The bedrock surfaces frequently paralleled the slope and provided a lower limit for ground water movement as well as readymade sliding surface. Because of high soil permeabilities, drainage was primarily below the surface with little or no surface flow except down established channels. Thus, during major storms, high soil-moisture levels with local areas of saturation greatly increased the natural potential for landslides. Clearly, any activity should be conducted with minimum environmental impact. This demands adequate identification of presently or potentially unstable terrain and determination of practical, usable methods for occupancy and use. An effective approach to this problem, and one we had chosen for the present investigation, is land stratification and qualitative stability rating using slope as a basic parameter. Over the past decade, my associates and I had applied this technique to a number of areas in southeastern Alaska, ranging from forested slopes designated for future logging to steeply sloping urban areas subject to increasing housing And commercial development pressures. Land stratification techniques Experience had taught us that stability stratification of an area for the purposes of adequate land use planning must first of all identify as closely as possible all areas of potential landslide hazard. It should then supply as much addiEXPLORERS JOURNAL j 215 Conservation tional information as possible within the limits of the survey accuracy desired. This additional information should include, at the very minimum, the accurate location and distribution of all active and potential landslides and snowslides, and the estimated or probable major variations in slope stability characteristics from one location to the next within the investigated area. Based on this information, a practiCal hazard rating system defining the principal zones of stratification can be developed. Because of logistics problems in this mountainous area, our stability analysis had to approached indirectly by contour mapping of unstable slopes identified from topographic maps and preliminary estimates of basic soil properties and probable sliding mechanics. The initial stability stratification was then supplemented and quantified by air photo interpretation and selected ground traverses. Air photo interpretation allowed accurate identification and location of active and dormant landslides prior to field investigations. Also identified were individual potentially hazardous situations, such as local bluff and cliff areas and steep-walled gullies, which may serve as origins of landslides or channels for landslide debris from upslope. The ground traverses we had just completed provided a means of field checking photo and map interpretations and of , measuring and estimating soil properties and characteristics, basic sliding mechanisms, and slide surface configurations. Stability mapping using slope as a controlling factor is relatively simple and effective, and its value in stability analysis under these terrain conditions is well established. 1,2 Essentially, this process involves contouring all slope areas above designated critical steepness, determined by the kind of soil involved and the individual requirements of the specific land use contemplated for the area. For this analysis, we chose to base our stratification on natural landslide hazard, determined 1 D. N. Swanston. "Geology report and landslide hazard analysis of northwestern Kuiu Island, southeastern Alaska" (unpublished interdisciplinary team report, northwestern Kuiu Island) , Petersburg Ranger District, North Tongass National Forest, Alaska, August 1971. 2 D. N. Swanston. "Report of the findings of the mass wasting inventory." In Geophysical' hazards investigation for the city and borough of Juneau, Alaska, Technical Supplement, October 1972, Daniel Mann, Johitson, and Mendenhall, Architects and Engineers, Portland, Oregon. 216 I DECEMBER, 1973 by the maximum steepness at which a slope will remain stable under natural conditions. This depends chiefly on the relative degree of cohesion (capacity of the soil particles to stick together due to cementation, bonding together of particles and capillary tension); friction along the potential sliding surface; and friction between individual particles (essentially the interlocking of soil particles) within the soil mass. The steep slope soils of our study area are typical of those found on similar terrain throughout southeastern Alaska. The soils are shallow, coarse grained, and permeable. Cohesion is absent or a minimal factor, and the major part of resistance to downslope movement is due to friction between soil particles and friction along the potential sliding surface. Engineering experience has indicated that the critical slope in soils of this type is seldom less than 26° or more than 36°. Thus, we have two slope values that can be used to designate demarcation points in estimating natural landslide hazard. A slope above 36° is highly unstable even under the most favarable of natural conditions. Slopes between 26° and 36° may or may not be stable depending on local variations in basic soil characteristics, soil moisture content and distribution, vegetation cover, and slope. Slopes below 26° were considered stable although local steep, hazardous areas not picked up in the initial survey may exist, and operations on them should be governed by the rules for more unstable areas. Interpretation Using the slope values generated by our investigations, we will be able to stratify landforms into three broad zones of low, medium, and high hazard. Such basic stratification can then be followed by a careful analysis of the factors contributing to the unstable conditions and a substratification of the medium and low hazard areas by activities and operations that can be safely performed within them. An application of this technique to a small watershed near Juneau, Alaska, primarily for timber management purposes, is shown in figure 3. In terms of good land management policy, areas within the highly unstable zone should be entirely withdrawn from developmental activities. Any disruption of surface cover or disturbance of the soil mantle in such areas is almost certain to cause or accelerate landslide Fig. 3—Stability stratification map of a small watershed near Juneau, Alaska Highly unstable slope >36° VIM Potentially unstable slope >26° <36° Recent debris avalanches and flows Active landslide zones occurrence, with little chance of effective control. Areas in the potentially unstable zone should be examined carefully for evidence of unstable conditions on a local basis, and operation and development criteria should be designed to fit each stability situation encountered. For example, in this zone, relative stability decreases with increasing slope. Wet spots on the slope, shallow intermittent drainages, and gullies are also excellent locales for increased instability. It is here that photo interpretation and additional ground checks kit' substratification purposes become essential. Areas in the low hazard zone can be developed with a minimum of special preparation as long as it is realized that local areas still may be subject to damage from periodic landsliding from more unstable slopes above. n1 Douglas N. Swanston, Ph.D., 1968 FNR Dr. Swanston is a geologist, currently working at the USDA Forest Service, Forestry Sciences Laboratory in Corvallis, Oregon, on stability problems on steep forested lands in the Pacific Northwest. For many years he served as Research Geologist with the Institute of Northern Forestry in Juneau, Alaska and has been primarily responsible for determining the nature and types of soil mass movements in southeast Alaska, the mechanism of their formation, and the relationship of their occurrence to logging practices. He is an active member of the academic staff of the Glaciological and Arctic Sciences Institute operating out of Juneau, Alaska and'Atlin, British Columbia and is a member of the Board of Trustees of the Foundation for Glacier and Environmental Research. EXPLORERS JOURNAL I 217