Download Judging Landslide Potential in Glaciated Valleys of Southeastern

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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.
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